1 00:00:00,090 --> 00:00:02,590 NARRATOR: The following content is provided under a Creative 2 00:00:02,590 --> 00:00:04,030 Commons license. 3 00:00:04,030 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,720 continue to offer high-quality educational resources for free. 5 00:00:10,720 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:17,280 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,280 --> 00:00:20,400 at ocw.mit.edu. 8 00:00:20,400 --> 00:00:23,160 MARK HARTMAN: All right, so light as a wave. 9 00:00:23,160 --> 00:00:28,320 We have, for so long, been talking about the particle 10 00:00:28,320 --> 00:00:30,810 model of light. 11 00:00:33,665 --> 00:00:43,560 And we're going to contrast that with the wave model of light 12 00:00:43,560 --> 00:00:46,770 because the Doppler effect-- this thing that we're going 13 00:00:46,770 --> 00:00:47,970 to learn about-- 14 00:00:47,970 --> 00:00:52,470 only shows up when you think about light is a wave. 15 00:00:52,470 --> 00:00:55,440 The model of light as a wave helps 16 00:00:55,440 --> 00:00:58,170 us to make interesting and useful predictions. 17 00:00:58,170 --> 00:00:59,610 Well, this particle model of light 18 00:00:59,610 --> 00:01:01,800 has also helped us to make interesting and useful 19 00:01:01,800 --> 00:01:03,480 predictions too. 20 00:01:03,480 --> 00:01:06,270 We were able to predict things about measuring luminosity 21 00:01:06,270 --> 00:01:07,952 and how luminosity and flux are related 22 00:01:07,952 --> 00:01:09,660 because we're thinking of these particles 23 00:01:09,660 --> 00:01:11,890 coming out in all directions. 24 00:01:11,890 --> 00:01:14,410 So here's what the particle model of light looked like. 25 00:01:14,410 --> 00:01:16,290 Here's our light bulb again. 26 00:01:16,290 --> 00:01:17,890 We have particles that are moving 27 00:01:17,890 --> 00:01:22,020 outward in straight lines in all directions. 28 00:01:22,020 --> 00:01:23,395 They're moving at the same speed. 29 00:01:27,430 --> 00:01:32,140 We know that each one of those photons has a different energy. 30 00:01:32,140 --> 00:01:37,800 So photons have different energies. 31 00:01:43,590 --> 00:01:46,110 And we experience those different energy photons 32 00:01:46,110 --> 00:01:47,610 as different colors. 33 00:01:47,610 --> 00:01:49,170 When those photons hit the detector 34 00:01:49,170 --> 00:01:52,280 at the back of our eyes, which we call the retina, 35 00:01:52,280 --> 00:01:54,900 a signal is transferred up to our brain. 36 00:01:54,900 --> 00:01:59,760 And our brain says that looks blue, or that looks red. 37 00:01:59,760 --> 00:02:05,880 Now in this case, in the wave model of light, what we have is 38 00:02:05,880 --> 00:02:11,340 here is our light bulb, and we have essentially 39 00:02:11,340 --> 00:02:14,880 disturbances in the electromagnetic field 40 00:02:14,880 --> 00:02:21,480 that travel outward from this source. 41 00:02:21,480 --> 00:02:22,950 So we are going to say we're going 42 00:02:22,950 --> 00:02:29,030 to represent that as this. 43 00:02:29,030 --> 00:02:37,550 We're going to draw a couple of circles around this object. 44 00:02:37,550 --> 00:02:42,020 And these circles are ripples. 45 00:02:42,020 --> 00:02:44,940 We're going to look at the surface of water in a minute. 46 00:02:44,940 --> 00:02:49,310 But we're going to say that-- 47 00:02:49,310 --> 00:02:51,380 you'll understand this in just a minute-- 48 00:02:54,502 --> 00:02:56,960 here, just let me just say so photons have different energy 49 00:02:56,960 --> 00:02:58,820 we're going to say here that-- 50 00:03:03,400 --> 00:03:09,480 well, this was light is photons that have different energies. 51 00:03:09,480 --> 00:03:21,150 In here, light is a traveling disturbance 52 00:03:21,150 --> 00:03:23,475 in the electromagnetic field. 53 00:03:31,567 --> 00:03:33,150 And the reason we don't spend too long 54 00:03:33,150 --> 00:03:38,494 on the wave model of light is because it's not intuitive. 55 00:03:38,494 --> 00:03:40,410 I don't think most people really understand it 56 00:03:40,410 --> 00:03:42,930 until they've taken like a second or third college 57 00:03:42,930 --> 00:03:44,989 course in physics about it. 58 00:03:44,989 --> 00:03:47,280 I, personally, don't really understand it all that well 59 00:03:47,280 --> 00:03:48,870 myself. 60 00:03:48,870 --> 00:03:51,490 So there's a lot more that goes into this. 61 00:03:51,490 --> 00:03:54,390 You have to understand how magnetism and electricity are 62 00:03:54,390 --> 00:03:55,440 related. 63 00:03:55,440 --> 00:03:58,890 You have to understand how sources and moving charges 64 00:03:58,890 --> 00:04:01,704 produce currents and currents produce magnetic fields. 65 00:04:01,704 --> 00:04:03,120 It's not impossible, but it's just 66 00:04:03,120 --> 00:04:05,740 a little bit beyond what we want to get into right now. 67 00:04:05,740 --> 00:04:08,590 So we're going to simplify it a little bit. 68 00:04:08,590 --> 00:04:11,280 But if we have two models that are 69 00:04:11,280 --> 00:04:13,770 describing the same phenomenon, describing 70 00:04:13,770 --> 00:04:15,630 the same observations that we make, 71 00:04:15,630 --> 00:04:18,089 there's got to be correspondence between these two. 72 00:04:18,089 --> 00:04:20,399 There's got to be something that's similar. 73 00:04:20,399 --> 00:04:22,890 In this case, our photons are traveling 74 00:04:22,890 --> 00:04:26,370 outward in all directions at the same speed. 75 00:04:26,370 --> 00:04:29,880 Here we have waves or disturbances 76 00:04:29,880 --> 00:04:31,830 in the electromagnetic field that 77 00:04:31,830 --> 00:04:37,440 are still traveling outward in all directions 78 00:04:37,440 --> 00:04:38,295 at the same speed. 79 00:04:41,040 --> 00:04:46,920 And I want everybody to come up and take a look at a couple 80 00:04:46,920 --> 00:04:49,220 of things over here. 81 00:04:49,220 --> 00:04:51,189 So can we have the balloons? 82 00:04:51,189 --> 00:04:52,230 Oh, you guys are writing. 83 00:04:52,230 --> 00:04:53,146 I'll get the balloons. 84 00:05:00,000 --> 00:05:08,270 So I want everybody to come to the middle [? Shakib, ?] 85 00:05:08,270 --> 00:05:10,770 What's the best way to do this? 86 00:05:10,770 --> 00:05:15,180 Let's each have one of us do these up here. 87 00:05:15,180 --> 00:05:16,620 So everybody put their arms out. 88 00:05:24,620 --> 00:05:27,100 So what we're doing right now is we're charging up. 89 00:05:27,100 --> 00:05:29,800 We are putting extra electrons from our head 90 00:05:29,800 --> 00:05:31,990 onto these balloons. 91 00:05:31,990 --> 00:05:36,590 Extra charges create an electric field. 92 00:05:36,590 --> 00:05:39,490 The electromagnetic field is electric and magnetic fields. 93 00:05:39,490 --> 00:05:41,710 But what is it field? 94 00:05:41,710 --> 00:05:48,070 A field is just how you sense or interact with-- 95 00:05:48,070 --> 00:05:50,130 ooh, let's see this. 96 00:05:50,130 --> 00:05:51,320 Is that working? 97 00:05:51,320 --> 00:05:55,505 So if we put this on your arm, what is it that you feel? 98 00:05:58,752 --> 00:05:59,460 AUDIENCE: Tingly. 99 00:05:59,460 --> 00:06:00,790 AUDIENCE: Yeah, it's kind of-- 100 00:06:00,790 --> 00:06:02,331 MARK HARTMAN: What does it feel like? 101 00:06:04,642 --> 00:06:05,610 AUDIENCE: Tingly. 102 00:06:05,610 --> 00:06:07,068 MARK HARTMAN: Feels kind of tingly? 103 00:06:10,170 --> 00:06:13,570 So why does it feel tingly? 104 00:06:13,570 --> 00:06:15,086 AUDIENCE: Because the hair rises. 105 00:06:15,086 --> 00:06:18,720 MARK HARTMAN: Yeah, so your hair moves around a little bit. 106 00:06:18,720 --> 00:06:21,810 And depending on where the charge is, 107 00:06:21,810 --> 00:06:25,860 your hairs kind of stand up, and they move towards that charge. 108 00:06:25,860 --> 00:06:29,510 It's not that there's one continuous force. 109 00:06:29,510 --> 00:06:32,280 It's not that there's one force between this and your hairs, 110 00:06:32,280 --> 00:06:36,860 but it depends on where the balloon is. 111 00:06:36,860 --> 00:06:40,680 And you can kind of feel that there's changes 112 00:06:40,680 --> 00:06:45,080 when this source moves around. 113 00:06:45,080 --> 00:06:48,030 When it's down here, you can feel the-- ooh, 114 00:06:48,030 --> 00:06:50,880 it's kind of weird. 115 00:06:50,880 --> 00:06:56,250 So what you're doing is the hairs on your arms 116 00:06:56,250 --> 00:06:58,830 are responding to the electric field. 117 00:06:58,830 --> 00:07:01,020 They're kind of like being attracted towards it. 118 00:07:01,020 --> 00:07:04,350 But when you move around, if you move further away, 119 00:07:04,350 --> 00:07:06,340 you can't feel it as much. 120 00:07:06,340 --> 00:07:08,400 That's what the idea of a field is. 121 00:07:08,400 --> 00:07:10,770 A field is just describing how does 122 00:07:10,770 --> 00:07:14,130 one thing influence another thing, 123 00:07:14,130 --> 00:07:16,060 and it's not always the same. 124 00:07:16,060 --> 00:07:19,920 It depends on how far away you are. 125 00:07:19,920 --> 00:07:21,330 What is this? 126 00:07:21,330 --> 00:07:22,370 More? 127 00:07:22,370 --> 00:07:22,870 All right. 128 00:07:22,870 --> 00:07:24,710 AUDIENCE: Are you going to shock somebody? 129 00:07:24,710 --> 00:07:26,710 MARK HARTMAN: No, I don't want to shock anybody. 130 00:07:26,710 --> 00:07:29,430 But yeah, that does work. 131 00:07:29,430 --> 00:07:31,470 So what you're doing-- 132 00:07:31,470 --> 00:07:37,290 a field is a way to describe how things interact. 133 00:07:37,290 --> 00:07:39,840 It's not that there's one number that can describe this. 134 00:07:39,840 --> 00:07:42,550 But if you put it close, then it pulls harder. 135 00:07:42,550 --> 00:07:43,600 If you put it over here. 136 00:07:43,600 --> 00:07:47,550 It pulls harder on these hairs, pulls less hard on those hairs. 137 00:07:47,550 --> 00:07:50,070 So the idea of a field is just a description 138 00:07:50,070 --> 00:07:54,840 of how this object interacts with other objects, 139 00:07:54,840 --> 00:07:56,100 and it depends on distance. 140 00:07:56,100 --> 00:07:57,984 It depends on where you are. 141 00:07:57,984 --> 00:07:59,400 And when you move this around, you 142 00:07:59,400 --> 00:08:03,600 can feel the field changing because the amount of pull 143 00:08:03,600 --> 00:08:07,200 on each one of these hairs is different. 144 00:08:07,200 --> 00:08:11,070 So the same idea happens with light. 145 00:08:11,070 --> 00:08:14,580 That's an electric field. 146 00:08:14,580 --> 00:08:17,160 When you have those bouncing charged particles 147 00:08:17,160 --> 00:08:20,760 that are bouncing around that we talked about producing photons, 148 00:08:20,760 --> 00:08:25,320 when those move up and down, their charge and they 149 00:08:25,320 --> 00:08:27,240 cause a changing field. 150 00:08:27,240 --> 00:08:31,410 So everything else around them changes in response to that. 151 00:08:31,410 --> 00:08:34,950 And that's what causes these disturbances 152 00:08:34,950 --> 00:08:36,600 in the electromagnetic field. 153 00:08:36,600 --> 00:08:38,640 Now we're talking about big disturbances, 154 00:08:38,640 --> 00:08:41,520 like moving one or two centimeters, three 155 00:08:41,520 --> 00:08:43,080 or four centimeters back and forth, 156 00:08:43,080 --> 00:08:47,160 and our hairs can kind of tell the difference. 157 00:08:47,160 --> 00:08:50,560 But light is talking about moving up and down really, 158 00:08:50,560 --> 00:08:53,070 really fast and moving up and down really, really small 159 00:08:53,070 --> 00:08:56,280 amounts, moving those charges. 160 00:08:56,280 --> 00:09:01,470 That disturbance travels outward. 161 00:09:01,470 --> 00:09:03,030 We just like felt the influence. 162 00:09:03,030 --> 00:09:07,860 Our hairs didn't start moving in response to that necessarily. 163 00:09:07,860 --> 00:09:11,880 But let's have everybody come over here. 164 00:09:11,880 --> 00:09:15,450 If we have this pan of water that's describing-- 165 00:09:15,450 --> 00:09:18,105 if we say this is like the electromagnetic field. 166 00:09:20,730 --> 00:09:24,480 If we put a disturbance into it, you can see-- 167 00:09:24,480 --> 00:09:27,570 actually, it looks a little bit better if you can see a light. 168 00:09:27,570 --> 00:09:29,280 So get so that you can see reflected 169 00:09:29,280 --> 00:09:31,630 light in between these things. 170 00:09:31,630 --> 00:09:35,580 So if I put a disturbance or move something around here, 171 00:09:35,580 --> 00:09:37,290 you notice what? 172 00:09:37,290 --> 00:09:39,010 AUDIENCE: Ripples. 173 00:09:39,010 --> 00:09:40,260 MARK HARTMAN: Ripples-- right. 174 00:09:40,260 --> 00:09:44,040 So the energy of me dipping this into the water 175 00:09:44,040 --> 00:09:46,030 is carried outwards in all directions. 176 00:09:46,030 --> 00:09:49,230 And if you look what shape are the ripples. 177 00:09:49,230 --> 00:09:50,650 AUDIENCE: Circular. 178 00:09:50,650 --> 00:09:53,100 MARK HARTMAN: They're circular-- right. 179 00:09:53,100 --> 00:09:55,770 And they start out small, and they travel outward 180 00:09:55,770 --> 00:09:57,940 in all directions. 181 00:09:57,940 --> 00:10:01,110 So in the same way that by me dipping this in and out, 182 00:10:01,110 --> 00:10:05,820 I'm producing waves in this water, 183 00:10:05,820 --> 00:10:08,880 electrons or ions or charged particles 184 00:10:08,880 --> 00:10:11,010 are like bouncing around, and they're 185 00:10:11,010 --> 00:10:13,805 producing ripples in the electromagnetic field. 186 00:10:16,980 --> 00:10:20,790 We think, in our particle model, that each time you 187 00:10:20,790 --> 00:10:23,930 have a bounce, you release one photon. 188 00:10:23,930 --> 00:10:28,170 Well, in the wave model of light, 189 00:10:28,170 --> 00:10:30,870 when you have that disturbance moving up and down, 190 00:10:30,870 --> 00:10:34,320 you're actually sending out these waves 191 00:10:34,320 --> 00:10:36,780 or these disturbances in the electromagnetic field. 192 00:10:36,780 --> 00:10:40,950 It's not that something actually physically goes outward, 193 00:10:40,950 --> 00:10:43,260 but there's a disturbance there. 194 00:10:43,260 --> 00:10:46,410 And that disturbance is what travels to us. 195 00:10:46,410 --> 00:10:48,330 Now you can't really think of how much of that 196 00:10:48,330 --> 00:10:51,180 disturbance do I collect with my detector? 197 00:10:51,180 --> 00:10:52,560 You can think of how many photons 198 00:10:52,560 --> 00:10:53,910 do I collect with my detector? 199 00:10:53,910 --> 00:10:55,890 And that makes sense in terms of counts 200 00:10:55,890 --> 00:10:57,980 because you're just saying I got 20 photons, 201 00:10:57,980 --> 00:10:59,580 or I got 50 photons. 202 00:10:59,580 --> 00:11:01,740 You can't say, well, how much of this wave 203 00:11:01,740 --> 00:11:03,480 did I get in my detector? 204 00:11:03,480 --> 00:11:05,760 25. 205 00:11:05,760 --> 00:11:07,720 You could say a number like that. 206 00:11:07,720 --> 00:11:11,000 But it's that different models help 207 00:11:11,000 --> 00:11:13,617 us to predict different things. 208 00:11:13,617 --> 00:11:15,200 And in the case of the particle model, 209 00:11:15,200 --> 00:11:17,610 it helped us to figure out flux and luminosity. 210 00:11:17,610 --> 00:11:20,712 In the case of the wave model, it's 211 00:11:20,712 --> 00:11:22,170 going to help us to figure out what 212 00:11:22,170 --> 00:11:24,150 happens when these sources of light 213 00:11:24,150 --> 00:11:26,779 are actually moving in one direction or another. 214 00:11:26,779 --> 00:11:28,320 So one observation I want you to make 215 00:11:28,320 --> 00:11:31,920 is if I do this, you can see these circles because 216 00:11:31,920 --> 00:11:33,510 from the side, if you were to look 217 00:11:33,510 --> 00:11:37,270 at the surface of the water, it goes up and down like this. 218 00:11:37,270 --> 00:11:38,820 So when you see those circles, that's 219 00:11:38,820 --> 00:11:40,920 saying that there's one part of the wave that's 220 00:11:40,920 --> 00:11:43,110 up, which we call the crest, and then one 221 00:11:43,110 --> 00:11:46,140 part of the wave that's down, which we call the trough. 222 00:11:46,140 --> 00:11:48,300 Everybody's heard of the crest and the troughs. 223 00:11:48,300 --> 00:11:50,670 In this case, we have circular waves 224 00:11:50,670 --> 00:11:52,290 that are traveling out on the surface 225 00:11:52,290 --> 00:11:55,160 of a two-dimensional object. 226 00:11:55,160 --> 00:11:57,887 AUDIENCE: [INAUDIBLE]. 227 00:11:57,887 --> 00:11:58,720 MARK HARTMAN: Right. 228 00:11:58,720 --> 00:11:59,803 So that's the other thing. 229 00:11:59,803 --> 00:12:02,040 When you send waves out, they bounce off 230 00:12:02,040 --> 00:12:05,170 of stuff in the same way that photons bounce off of stuff. 231 00:12:05,170 --> 00:12:06,900 So if we have a source over here, 232 00:12:06,900 --> 00:12:08,700 we can watch that source from here, 233 00:12:08,700 --> 00:12:12,340 and we see that it bounces out that way. 234 00:12:12,340 --> 00:12:15,300 So there's still some of that correspondence between the two. 235 00:12:15,300 --> 00:12:18,030 But what you notice here is these circles-- 236 00:12:18,030 --> 00:12:20,790 each circle represents the crest of a wave. 237 00:12:23,400 --> 00:12:24,900 So let's all head back to our seats. 238 00:12:30,030 --> 00:12:33,990 If I were to draw a wave from the side, I would see-- 239 00:12:33,990 --> 00:12:35,610 here was my pencil. 240 00:12:38,670 --> 00:12:43,570 If I were to draw it from the side, it would look like that. 241 00:12:43,570 --> 00:12:45,930 Here's the regular surface of the water. 242 00:12:45,930 --> 00:12:47,370 This is the crest. 243 00:12:47,370 --> 00:12:49,830 That's the trough-- the crest, the trough. 244 00:12:49,830 --> 00:12:52,290 And that's what we're seeing from a top view here. 245 00:12:55,770 --> 00:12:58,800 So let's write down a couple of these things before we 246 00:12:58,800 --> 00:13:01,050 forget them. 247 00:13:01,050 --> 00:13:11,060 The first thing that we want to say is-- 248 00:13:11,060 --> 00:13:11,560 oops. 249 00:13:14,660 --> 00:13:17,270 So we said wave mild light-- light 250 00:13:17,270 --> 00:13:20,070 is a traveling disturbance in the electromagnetic field. 251 00:13:20,070 --> 00:13:22,122 So what is a field? 252 00:13:22,122 --> 00:13:26,330 A field-- how would you guys describe it? 253 00:13:26,330 --> 00:13:27,425 AUDIENCE: Area surface. 254 00:13:27,425 --> 00:13:28,300 MARK HARTMAN: A what? 255 00:13:28,300 --> 00:13:29,000 AUDIENCE: Area surface 256 00:13:29,000 --> 00:13:30,380 MARK HARTMAN: An area surface? 257 00:13:30,380 --> 00:13:31,810 AUDIENCE: Like a surface of it. 258 00:13:31,810 --> 00:13:33,560 MARK HARTMAN: It's kind of like a surface. 259 00:13:33,560 --> 00:13:37,430 In that case, we said that the field would 260 00:13:37,430 --> 00:13:39,830 be the surface of the water. 261 00:13:39,830 --> 00:13:42,860 Over here, when we put the balloon next to our arms, 262 00:13:42,860 --> 00:13:49,490 we said it was a way to describe how one object interacts 263 00:13:49,490 --> 00:13:52,260 with another object. 264 00:13:52,260 --> 00:13:53,420 So we're going to say-- 265 00:13:53,420 --> 00:13:57,020 let's say how should we say this? 266 00:13:57,020 --> 00:14:18,870 Field is a description of how charged particles interact 267 00:14:18,870 --> 00:14:22,500 with other matter. 268 00:14:25,470 --> 00:14:35,670 And this is going to be the electromagnetic field. 269 00:14:35,670 --> 00:14:39,180 When you have a source that is a charged particle, 270 00:14:39,180 --> 00:14:41,730 like we charged up those balloons, 271 00:14:41,730 --> 00:14:43,950 the field is a description of how it's going 272 00:14:43,950 --> 00:14:46,350 to interact with other stuff. 273 00:14:46,350 --> 00:14:48,750 When you hold the balloon farther away, 274 00:14:48,750 --> 00:14:50,460 your hairs don't stand up. 275 00:14:50,460 --> 00:14:53,290 When you hold the balloon closer, your hairs do stand up. 276 00:14:55,767 --> 00:14:57,600 Now this isn't a perfect definition of this, 277 00:14:57,600 --> 00:14:59,130 but it's good enough for us to use. 278 00:15:01,950 --> 00:15:04,310 AUDIENCE: [INAUDIBLE]. 279 00:15:04,310 --> 00:15:06,064 MARK HARTMAN: Say that again. 280 00:15:06,064 --> 00:15:06,980 AUDIENCE: [INAUDIBLE]. 281 00:15:06,980 --> 00:15:07,896 MARK HARTMAN: OK. 282 00:15:07,896 --> 00:15:08,812 AUDIENCE: [INAUDIBLE]. 283 00:15:08,812 --> 00:15:11,730 MARK HARTMAN: So in your physics class, 284 00:15:11,730 --> 00:15:15,920 what would you say when somebody says a field? 285 00:15:15,920 --> 00:15:18,420 AUDIENCE: Exactly what that is really [INAUDIBLE] just said. 286 00:15:18,420 --> 00:15:20,450 MARK HARTMAN: OK, one way that we can 287 00:15:20,450 --> 00:15:25,190 describe a field for a single-- 288 00:15:25,190 --> 00:15:29,170 you can also use this to think about the gravitational field. 289 00:15:29,170 --> 00:15:30,950 A gravitational field is the description 290 00:15:30,950 --> 00:15:34,280 of how a massive particle, or any mass, 291 00:15:34,280 --> 00:15:36,590 interacts with other mass. 292 00:15:36,590 --> 00:15:38,180 We had described that, and we said 293 00:15:38,180 --> 00:15:40,040 that the force between two masses 294 00:15:40,040 --> 00:15:42,860 depends on the product of the masses, 295 00:15:42,860 --> 00:15:45,050 as well as the difference-- or I'm 296 00:15:45,050 --> 00:15:48,060 sorry-- as well as the distance in between them. 297 00:15:48,060 --> 00:15:50,570 So you can represent a field mathematically, 298 00:15:50,570 --> 00:15:52,280 and there is a mathematical description 299 00:15:52,280 --> 00:15:56,150 of what all of this looks like or how you could describe it. 300 00:15:58,670 --> 00:16:00,380 Oops, where did my notebook go? 301 00:16:00,380 --> 00:16:06,170 The other important point is that when 302 00:16:06,170 --> 00:16:09,140 you have a source that's moving-- 303 00:16:09,140 --> 00:16:10,215 so moving charges. 304 00:16:12,800 --> 00:16:14,860 Let's do-- blue's OK? 305 00:16:14,860 --> 00:16:16,070 AUDIENCE: Yeah. 306 00:16:16,070 --> 00:16:22,980 MARK HARTMAN: Let's do moving charges 307 00:16:22,980 --> 00:16:29,660 create a disturbance or waves. 308 00:16:33,120 --> 00:16:34,130 And we say disturbance. 309 00:16:34,130 --> 00:16:35,114 We really mean waves. 310 00:16:35,114 --> 00:16:35,780 That's horrible. 311 00:16:40,490 --> 00:16:42,560 Creates a disturbance-- that's better-- 312 00:16:42,560 --> 00:16:50,390 or waves in the electromagnetic field. 313 00:16:50,390 --> 00:16:53,000 So light is a traveling disturbance, 314 00:16:53,000 --> 00:16:56,270 but moving charges or moving charged particles create 315 00:16:56,270 --> 00:16:59,000 that disturbance just like my pen 316 00:16:59,000 --> 00:17:03,050 created the disturbance on the surface of the water. 317 00:17:03,050 --> 00:17:06,740 Except there's a big difference between the waves on the water 318 00:17:06,740 --> 00:17:10,220 there, and the light that comes out from a source. 319 00:17:10,220 --> 00:17:13,490 Those only traveled outwards in two-dimensions. 320 00:17:13,490 --> 00:17:17,839 These waves travel outward in all three dimensions. 321 00:17:17,839 --> 00:17:23,490 So towards us, that way, up, down, back in all directions. 322 00:17:23,490 --> 00:17:28,280 OK, so you got that. 323 00:17:28,280 --> 00:17:32,180 Now the problem is we need to have a connection 324 00:17:32,180 --> 00:17:35,480 between these two models. 325 00:17:35,480 --> 00:17:41,790 And right now, how does a photon relate to a wave? 326 00:17:41,790 --> 00:17:43,820 Well, we don't really know. 327 00:17:43,820 --> 00:17:48,470 And we also, when we're looking at these waves-- 328 00:17:48,470 --> 00:17:53,390 I'm sure everybody has seen this before-- but if you want 329 00:17:53,390 --> 00:17:55,220 to measure something about these waves, 330 00:17:55,220 --> 00:17:57,740 you can measure a thing called the wavelength. 331 00:17:57,740 --> 00:18:01,610 How far is it between one crest and the next crest? 332 00:18:01,610 --> 00:18:05,840 How far is it between one trough and the next trough? 333 00:18:05,840 --> 00:18:15,210 So on this diagram, the wavelength 334 00:18:15,210 --> 00:18:18,990 is the distance from crest to crest or from trough to trough. 335 00:18:22,678 --> 00:18:25,170 Yeah, trough. 336 00:18:25,170 --> 00:18:28,560 Well, lots of people farm, but nobody in this room 337 00:18:28,560 --> 00:18:29,820 has probably farmed before. 338 00:18:29,820 --> 00:18:31,740 So you maybe not have seen a pig trough 339 00:18:31,740 --> 00:18:33,480 but like something that an animal would eat out of. 340 00:18:33,480 --> 00:18:34,050 It's a trough. 341 00:18:34,050 --> 00:18:35,508 It's a place that goes down, so you 342 00:18:35,508 --> 00:18:37,560 can fill in the animal's food. 343 00:18:37,560 --> 00:18:39,270 So that's a wavelength. 344 00:18:39,270 --> 00:18:44,700 This distance between here and here is also a wavelength. 345 00:18:48,960 --> 00:18:51,600 If we took a picture real quick of when 346 00:18:51,600 --> 00:18:55,104 I was dipping the pen in the water, 347 00:18:55,104 --> 00:18:56,520 you can take a picture from above, 348 00:18:56,520 --> 00:18:58,860 and you could actually measure the wavelength. 349 00:18:58,860 --> 00:19:00,390 You can look at the angular size you 350 00:19:00,390 --> 00:19:01,810 can convert it into a linear size 351 00:19:01,810 --> 00:19:05,910 if you knew how far away it was from the camera. 352 00:19:05,910 --> 00:19:07,230 So here's the next part. 353 00:19:12,660 --> 00:19:28,370 The connection between the wave and particle model of light 354 00:19:28,370 --> 00:19:32,420 came in the early 1920s with the development 355 00:19:32,420 --> 00:19:35,200 of quantum mechanics. 356 00:19:35,200 --> 00:19:42,800 So we're going to say that we've already 357 00:19:42,800 --> 00:19:46,070 seen that quantum mechanics helps us to understand 358 00:19:46,070 --> 00:19:50,060 those electron transitions that we saw in elements 359 00:19:50,060 --> 00:19:52,880 or atoms of elements that lead to the peaks 360 00:19:52,880 --> 00:19:57,890 that we saw in our supernova remnant spectrum. 361 00:19:57,890 --> 00:19:59,690 And we said that quantum mechanics 362 00:19:59,690 --> 00:20:01,340 said that those electrons can only 363 00:20:01,340 --> 00:20:03,630 be in certain energy levels. 364 00:20:03,630 --> 00:20:06,110 Remember, electrons can have energy levels. 365 00:20:06,110 --> 00:20:09,560 Photons just have an amount of energy. 366 00:20:09,560 --> 00:20:11,180 Quantum mechanics is the study-- 367 00:20:11,180 --> 00:20:23,645 or it's a model of the behavior of really small things. 368 00:20:31,530 --> 00:20:34,020 We saw that it tells us about how electrons move, 369 00:20:34,020 --> 00:20:35,010 and electrons are tiny. 370 00:20:35,010 --> 00:20:36,840 It tells us things about photons, 371 00:20:36,840 --> 00:20:39,390 and photons are very tiny. 372 00:20:39,390 --> 00:20:43,730 I mean, you really have a size that's just kind of an idea. 373 00:20:43,730 --> 00:20:51,630 So from quantum mechanics, there was a relationship that kind of 374 00:20:51,630 --> 00:20:54,570 served as a bridge between the particle model of light 375 00:20:54,570 --> 00:20:57,390 and the wave model of light, and that bridge 376 00:20:57,390 --> 00:21:11,594 is that the energy of a photon was equal to-- 377 00:21:11,594 --> 00:21:24,860 well, let me just say the constant some number-- 378 00:21:24,860 --> 00:21:27,860 a constant is like a constant in mathematics-- 379 00:21:27,860 --> 00:21:37,865 divided by the wavelength of the wave. 380 00:21:44,670 --> 00:21:46,090 So what that means is if we think 381 00:21:46,090 --> 00:21:48,130 about things in terms of photons, 382 00:21:48,130 --> 00:21:50,330 we have energies of each photon. 383 00:21:50,330 --> 00:21:54,710 But if we think of things in terms of the wave model, 384 00:21:54,710 --> 00:21:58,270 we don't have any photon energy to measure, 385 00:21:58,270 --> 00:22:01,030 but we can measure wavelength. 386 00:22:01,030 --> 00:22:05,950 So what this says is if we have a high energy photon, 387 00:22:05,950 --> 00:22:07,750 what does that mean about its wavelength? 388 00:22:13,850 --> 00:22:17,780 So what I want you to do, think to yourself, 389 00:22:17,780 --> 00:22:23,240 if you had a large energy photon, what 390 00:22:23,240 --> 00:22:25,370 would be the wavelength? 391 00:22:25,370 --> 00:22:28,040 OK, so Peter and then Steve and then Chris. 392 00:22:28,040 --> 00:22:28,738 Peter? 393 00:22:28,738 --> 00:22:30,934 AUDIENCE: High energy the wavelength will 394 00:22:30,934 --> 00:22:35,570 be smaller than the low energy would be [INAUDIBLE] 395 00:22:35,570 --> 00:22:38,342 wavelength would be longer and [INAUDIBLE].. 396 00:22:38,342 --> 00:22:39,050 MARK HARTMAN: OK. 397 00:22:39,050 --> 00:22:40,394 Steve? 398 00:22:40,394 --> 00:22:48,266 AUDIENCE: [INAUDIBLE] but it is smaller [INAUDIBLE] 399 00:22:48,266 --> 00:22:52,700 smaller wavelength the energy [INAUDIBLE] would be bigger. 400 00:22:52,700 --> 00:22:54,080 MARK HARTMAN: OK, great. 401 00:22:54,080 --> 00:22:56,440 Chris? 402 00:22:56,440 --> 00:22:58,300 AUDIENCE: If the wavelength was like two, 403 00:22:58,300 --> 00:23:03,230 you have to multiply by the reciprocal [INAUDIBLE] 404 00:23:03,230 --> 00:23:05,828 bigger number like [INAUDIBLE]. 405 00:23:08,822 --> 00:23:10,614 So it will be bigger. 406 00:23:10,614 --> 00:23:12,030 MARK HARTMAN: OK, so you're saying 407 00:23:12,030 --> 00:23:15,460 if we took this wavelength and then put it up on this side, 408 00:23:15,460 --> 00:23:20,130 that wavelength times energy is just some number. 409 00:23:20,130 --> 00:23:23,700 So if wavelength gets smaller, energy has to get bigger 410 00:23:23,700 --> 00:23:25,685 to multiply to give us the same number. 411 00:23:25,685 --> 00:23:26,560 That's actually good. 412 00:23:26,560 --> 00:23:29,580 That's three different ways of looking at this idea. 413 00:23:29,580 --> 00:23:33,780 Let's see if it actually plays out in what we see. 414 00:23:33,780 --> 00:23:35,850 Do these people have it right or were they crazy? 415 00:23:38,800 --> 00:23:42,930 So if we look at our chart of the different telescopes 416 00:23:42,930 --> 00:23:48,510 that we were looking at and then what is the energy of photons? 417 00:23:48,510 --> 00:23:50,370 We said that visible light has like two 418 00:23:50,370 --> 00:23:53,460 to three electron volts of energy per photon. 419 00:23:53,460 --> 00:23:53,960 I'm sorry. 420 00:23:53,960 --> 00:23:55,410 Yeah, visible light and then x-ray 421 00:23:55,410 --> 00:24:01,710 had about 1,000 to 100,000 electron volts per photon. 422 00:24:01,710 --> 00:24:03,810 But we saw that as the energy goes up, 423 00:24:03,810 --> 00:24:06,897 what happened to the wavelength of light? 424 00:24:06,897 --> 00:24:07,980 AUDIENCE: It gets smaller. 425 00:24:07,980 --> 00:24:10,438 MARK HARTMAN: As energy goes up, the wavelength gets small. 426 00:24:10,438 --> 00:24:13,680 Gamma rays-- or gamma ray light, I'm sorry-- 427 00:24:13,680 --> 00:24:15,600 has high energy photons, which you 428 00:24:15,600 --> 00:24:17,820 can think of as waves with really, really 429 00:24:17,820 --> 00:24:20,150 short wavelength. 430 00:24:20,150 --> 00:24:22,919 And just like Peter said, if you make the energy-- 431 00:24:22,919 --> 00:24:23,460 or I'm sorry. 432 00:24:23,460 --> 00:24:27,180 If you make the wavelength really long like here 433 00:24:27,180 --> 00:24:30,000 is five billion nanometers, which 434 00:24:30,000 --> 00:24:33,600 is about a meter-- maybe a half a meter, 435 00:24:33,600 --> 00:24:37,604 then the radio light has photons. 436 00:24:37,604 --> 00:24:39,270 Or you can think of it as having photons 437 00:24:39,270 --> 00:24:41,940 with very, very low energy-- 438 00:24:41,940 --> 00:24:46,170 less than a millionth of an electron volt. 439 00:24:46,170 --> 00:24:48,180 If I rewrite this and actually put 440 00:24:48,180 --> 00:24:55,650 in the numbers, when you get is the energy of a photon 441 00:24:55,650 --> 00:25:03,600 is equal to this number h times c over wavelength, 442 00:25:03,600 --> 00:25:09,510 which lots of people write as this Greek letter lambada. 443 00:25:09,510 --> 00:25:12,750 So this is how you say that. 444 00:25:12,750 --> 00:25:20,290 You say L-A-M-B-D-A. Lambada-- 445 00:25:20,290 --> 00:25:21,667 can everybody say that? 446 00:25:21,667 --> 00:25:23,040 AUDIENCE: Lambda 447 00:25:23,040 --> 00:25:26,880 MARK HARTMAN: Lambda or lambda. 448 00:25:26,880 --> 00:25:29,850 That's a Greek letter, and that stands for the wavelength. 449 00:25:29,850 --> 00:25:35,954 So wavelength equals lambda. 450 00:25:35,954 --> 00:25:38,070 Oh, wow, OK. 451 00:25:41,200 --> 00:25:46,096 So this number h is a constant called Planck's constant. 452 00:25:46,096 --> 00:25:48,220 Planck was the name of the guy who figured this out 453 00:25:48,220 --> 00:25:49,990 or who came up with this idea. 454 00:25:49,990 --> 00:25:53,600 C is just the speed of light. 455 00:25:53,600 --> 00:25:56,290 So this is speed of light. 456 00:25:59,434 --> 00:26:07,050 This is Planck's constant. 457 00:26:07,050 --> 00:26:08,800 Yeah, a question? 458 00:26:08,800 --> 00:26:11,478 AUDIENCE: Is this like c [INAUDIBLE]?? 459 00:26:11,478 --> 00:26:12,686 MARK HARTMAN: Say that again. 460 00:26:12,686 --> 00:26:14,310 AUDIENCE: C equals m over v. 461 00:26:14,310 --> 00:26:18,160 MARK HARTMAN: C equals m over v? 462 00:26:18,160 --> 00:26:19,630 AUDIENCE: [INAUDIBLE]. 463 00:26:24,040 --> 00:26:26,190 MARK HARTMAN: OK, so there's another equation 464 00:26:26,190 --> 00:26:28,440 that just describes waves. 465 00:26:28,440 --> 00:26:31,590 That is, the speed of a wave is equal to the frequency 466 00:26:31,590 --> 00:26:33,780 times the wavelength. 467 00:26:33,780 --> 00:26:34,920 That's the one. 468 00:26:34,920 --> 00:26:38,760 This is a description of just waves. 469 00:26:38,760 --> 00:26:42,870 And it says that if you know how fast they're going, 470 00:26:42,870 --> 00:26:46,290 that is equal to the wavelength times the frequency, which 471 00:26:46,290 --> 00:26:50,130 is how many waves per second do you have. 472 00:26:50,130 --> 00:26:52,290 So this is just for waves only. 473 00:26:52,290 --> 00:26:56,490 We don't actually have to worry about this because for light-- 474 00:26:56,490 --> 00:27:00,149 light waves or light particles-- always have the same speed. 475 00:27:00,149 --> 00:27:01,440 It's always the speed of light. 476 00:27:04,650 --> 00:27:07,299 So I'm going to go ahead and take that off just, 477 00:27:07,299 --> 00:27:08,340 so we don't get confused. 478 00:27:08,340 --> 00:27:11,742 You don't have to worry about working with that. 479 00:27:11,742 --> 00:27:13,200 I'm just going to give this to you. 480 00:27:13,200 --> 00:27:16,180 We're not actually going to use it, unless you really want to. 481 00:27:16,180 --> 00:27:18,540 But this number h times c-- 482 00:27:18,540 --> 00:27:20,700 you can turn into the value. 483 00:27:20,700 --> 00:27:22,700 I mean, if you actually calculate what it is. 484 00:27:22,700 --> 00:27:32,800 It's 1,240 electron volts times nanometers 485 00:27:32,800 --> 00:27:34,570 because most of the time, wavelength 486 00:27:34,570 --> 00:27:36,310 is measured in nanometers. 487 00:27:36,310 --> 00:27:39,520 The wavelength of these light waves is about 10 488 00:27:39,520 --> 00:27:43,090 to the minus ninth, as large as one meter. 489 00:27:43,090 --> 00:27:45,500 So they're really, really small. 490 00:27:45,500 --> 00:27:48,500 So this is our bridge that gets us back and forth. 491 00:27:48,500 --> 00:27:51,370 So if we have long wavelength, that means we have-- 492 00:27:51,370 --> 00:27:53,440 we can think of a long wavelength light wave 493 00:27:53,440 --> 00:27:57,070 as having a low energy photon. 494 00:27:57,070 --> 00:27:58,510 And if we have a short wavelength, 495 00:27:58,510 --> 00:28:00,385 then we can think of the energy of the photon 496 00:28:00,385 --> 00:28:01,747 as being very large. 497 00:28:01,747 --> 00:28:03,066 AUDIENCE: [INAUDIBLE]? 498 00:28:03,066 --> 00:28:04,190 MARK HARTMAN: Say it again. 499 00:28:04,190 --> 00:28:05,106 AUDIENCE: [INAUDIBLE]? 500 00:28:05,106 --> 00:28:10,660 MARK HARTMAN: Yeah, this is Planck's constant, P-L-A-N-C-K. 501 00:28:10,660 --> 00:28:13,400 Max Planck or Max Plunk, if you pronounce it the right way. 502 00:28:17,030 --> 00:28:20,544 Yeah, you don't have to know exactly what that is. 503 00:28:20,544 --> 00:28:21,960 But what you do need to understand 504 00:28:21,960 --> 00:28:25,650 is longer wavelength means or larger wavelength 505 00:28:25,650 --> 00:28:29,940 smaller or higher energy, smaller wavelength, 506 00:28:29,940 --> 00:28:31,344 larger energy. 507 00:28:31,344 --> 00:28:33,120 AUDIENCE: What's the unit of wavelength? 508 00:28:33,120 --> 00:28:34,510 MARK HARTMAN: The unit of wavelength. 509 00:28:34,510 --> 00:28:35,259 Let's take a look. 510 00:28:35,259 --> 00:28:38,340 If we want our energies to come out in electron volts, 511 00:28:38,340 --> 00:28:42,840 we need to have a unit of wavelength be nanometers. 512 00:28:42,840 --> 00:28:50,550 And this is for wavelength in nanometers. 513 00:28:50,550 --> 00:28:52,320 All right-- [INAUDIBLE] this together 514 00:28:52,320 --> 00:28:55,560 with what we learned before. 515 00:28:55,560 --> 00:29:00,150 If you had a source that was emitting light 516 00:29:00,150 --> 00:29:04,580 and that source was emitting light at a certain-- 517 00:29:04,580 --> 00:29:08,645 it was emitting photons of only one color. 518 00:29:08,645 --> 00:29:09,645 It was emitting photons. 519 00:29:12,720 --> 00:29:19,320 Energy of the photon emitted is equal to let's 520 00:29:19,320 --> 00:29:24,190 say 2.5 electron volts. 521 00:29:24,190 --> 00:29:28,620 If that source was moving toward you, 522 00:29:28,620 --> 00:29:34,500 what would be the energy of the observed photon? 523 00:29:40,480 --> 00:29:43,810 If that source was moving away from you, 524 00:29:43,810 --> 00:29:50,170 what would be the energy of the observed photon? 525 00:29:50,170 --> 00:29:52,000 And if that source is moving perpendicular 526 00:29:52,000 --> 00:29:55,320 to you, what would be the energy of that observed photon? 527 00:29:58,140 --> 00:30:00,200 So in this case, this source is emitting light. 528 00:30:00,200 --> 00:30:01,200 We're thinking about it. 529 00:30:01,200 --> 00:30:04,560 We're looking at it in terms of the wave model of light. 530 00:30:04,560 --> 00:30:06,600 I want you to think about, well, now, what if it 531 00:30:06,600 --> 00:30:08,010 was a particle model of light? 532 00:30:10,930 --> 00:30:13,560 So with your groups, for just a minute or so, 533 00:30:13,560 --> 00:30:18,300 think about if a source was emitting only 2.5 electron volt 534 00:30:18,300 --> 00:30:21,420 light, that's a yellow. 535 00:30:21,420 --> 00:30:26,790 And a flat spectrum-- it was only emitting yellow light. 536 00:30:26,790 --> 00:30:29,430 If it was moving towards you, what 537 00:30:29,430 --> 00:30:33,439 would be the energy of the photons that you would collect? 538 00:30:33,439 --> 00:30:34,230 Would it be yellow? 539 00:30:34,230 --> 00:30:36,229 Would it be exactly 2.5 electron volts? 540 00:30:36,229 --> 00:30:37,020 Would it be bigger? 541 00:30:37,020 --> 00:30:39,476 Would it be smaller or what? 542 00:30:39,476 --> 00:30:40,600 So I think with your group? 543 00:30:40,600 --> 00:30:42,150 If the energy of the emitted photon 544 00:30:42,150 --> 00:30:46,110 was 2 and 1/2 electron volts, if that source is 545 00:30:46,110 --> 00:30:52,112 moving towards us, what energy of photon do we observe, Jalen? 546 00:30:52,112 --> 00:30:54,932 AUDIENCE: We would like see green or blue. 547 00:30:54,932 --> 00:30:55,640 MARK HARTMAN: OK. 548 00:30:55,640 --> 00:30:57,880 AUDIENCE: The lower the wavelength, [INAUDIBLE] 549 00:30:57,880 --> 00:31:00,450 have to get higher-energy protons. 550 00:31:00,450 --> 00:31:04,290 MARK HARTMAN: To lower the wavelength of the light, 551 00:31:04,290 --> 00:31:07,710 the higher the observed energy of the photon. 552 00:31:07,710 --> 00:31:09,210 So the energy of the observed photon 553 00:31:09,210 --> 00:31:15,510 is going to be greater than 2.5 electron volts, which 554 00:31:15,510 --> 00:31:18,000 we see as not yellow. 555 00:31:18,000 --> 00:31:21,900 So let's say emitted photon is yellow. 556 00:31:21,900 --> 00:31:26,730 Here we're going to see it as blue or something 557 00:31:26,730 --> 00:31:28,200 that's green or blue-- 558 00:31:28,200 --> 00:31:31,590 green or blue. 559 00:31:31,590 --> 00:31:34,770 What about when the source is moving away from the observer? 560 00:31:34,770 --> 00:31:36,120 Chris is raising his hand. 561 00:31:36,120 --> 00:31:36,940 AUDIENCE: I was stretching. 562 00:31:36,940 --> 00:31:37,550 MARK HARTMAN: Looked like you were 563 00:31:37,550 --> 00:31:39,392 raising your hand-- same thing. 564 00:31:39,392 --> 00:31:41,320 AUDIENCE: It would be less. 565 00:31:41,320 --> 00:31:44,950 MARK HARTMAN: OK, what would be less? 566 00:31:44,950 --> 00:31:46,830 AUDIENCE: The one we would see. 567 00:31:46,830 --> 00:31:49,860 MARK HARTMAN: The energy of the observed photon would be-- 568 00:31:49,860 --> 00:31:51,480 AUDIENCE: It would be less than 2.5. 569 00:31:51,480 --> 00:31:55,260 MARK HARTMAN: --less than 2.5 electron volts. 570 00:31:55,260 --> 00:31:57,580 What would we interpret that as? 571 00:31:57,580 --> 00:31:58,350 What color? 572 00:31:58,350 --> 00:31:59,035 AUDIENCE: Pink. 573 00:31:59,035 --> 00:32:01,410 MARK HARTMAN: If it's yellow, it would be more like maybe 574 00:32:01,410 --> 00:32:02,300 orange or red. 575 00:32:07,210 --> 00:32:10,771 OK, what about in the last case? 576 00:32:10,771 --> 00:32:12,240 Jalen, you want to go again? 577 00:32:12,240 --> 00:32:14,690 AUDIENCE: It would be the same as the stationary. 578 00:32:14,690 --> 00:32:15,814 It would be yellow. 579 00:32:15,814 --> 00:32:16,480 MARK HARTMAN: OK 580 00:32:16,480 --> 00:32:17,409 AUDIENCE: Well, near. 581 00:32:17,409 --> 00:32:18,700 MARK HARTMAN: It would be near. 582 00:32:18,700 --> 00:32:19,840 It would be close. 583 00:32:19,840 --> 00:32:23,810 So it would be about 2.5 electron volts-- 584 00:32:23,810 --> 00:32:27,040 so still yellow. 585 00:32:27,040 --> 00:32:28,030 So hang on. 586 00:32:28,030 --> 00:32:29,740 Here's our object. 587 00:32:29,740 --> 00:32:31,870 It's moving away fast. 588 00:32:31,870 --> 00:32:32,841 So it looks like-- 589 00:32:32,841 --> 00:32:33,340 whoops. 590 00:32:36,312 --> 00:32:37,770 Somebody throw me a marker, please. 591 00:32:44,960 --> 00:32:47,285 So if it's moving away fast-- 592 00:32:51,410 --> 00:32:52,530 oh, that's horrible. 593 00:32:55,340 --> 00:32:57,770 If it's moving away fast-- 594 00:33:03,640 --> 00:33:04,780 again, horrible. 595 00:33:04,780 --> 00:33:06,460 These are all still circles. 596 00:33:06,460 --> 00:33:09,520 They're not ovals, which is what I'm having trouble with. 597 00:33:09,520 --> 00:33:12,520 If they're moving away fast and you're over here, 598 00:33:12,520 --> 00:33:17,590 you're going to measure this distance as your wavelength. 599 00:33:17,590 --> 00:33:19,750 So you're saying that's really wide. 600 00:33:19,750 --> 00:33:22,510 So what is different about when you're moving away slow? 601 00:33:22,510 --> 00:33:25,900 AUDIENCE: It produces like a yellowish orange. 602 00:33:25,900 --> 00:33:28,330 MARK HARTMAN: Well, let's not get to that just yet. 603 00:33:28,330 --> 00:33:29,005 So that's fast. 604 00:33:31,510 --> 00:33:34,750 What does it look like when it's slow but still moving away 605 00:33:34,750 --> 00:33:35,662 from you? 606 00:33:35,662 --> 00:33:37,972 AUDIENCE: [INAUDIBLE] but they're 607 00:33:37,972 --> 00:33:39,684 not as wide as [INAUDIBLE]. 608 00:33:39,684 --> 00:33:42,100 MARK HARTMAN: They're not as wide as when it's going fast. 609 00:33:42,100 --> 00:33:44,470 But how do they compare to when it was stationary? 610 00:33:47,464 --> 00:33:50,334 AUDIENCE: It was even [INAUDIBLE].. 611 00:33:50,334 --> 00:33:51,750 MARK HARTMAN: Go ahead and finish. 612 00:33:51,750 --> 00:33:54,670 AUDIENCE: Well, they weren't moving at all [INAUDIBLE].. 613 00:33:58,050 --> 00:33:58,820 MARK HARTMAN: OK. 614 00:33:58,820 --> 00:34:00,236 Lauren? 615 00:34:00,236 --> 00:34:03,134 AUDIENCE: They have shorter wavelengths 616 00:34:03,134 --> 00:34:07,964 than if it was stationary, I think. 617 00:34:07,964 --> 00:34:09,710 If it was moving from you. 618 00:34:09,710 --> 00:34:11,074 MARK HARTMAN: It's moving slow. 619 00:34:11,074 --> 00:34:11,990 It's going to be this. 620 00:34:16,100 --> 00:34:17,805 You're still over here. 621 00:34:17,805 --> 00:34:21,270 AUDIENCE: Yeah, so you'd have-- 622 00:34:21,270 --> 00:34:22,755 oh, never mind. 623 00:34:22,755 --> 00:34:25,280 I just screwed something [INAUDIBLE] no. 624 00:34:25,280 --> 00:34:26,679 MARK HARTMAN: That's slow. 625 00:34:26,679 --> 00:34:29,530 This is stationary. 626 00:34:29,530 --> 00:34:32,426 They're all perfectly evenly spaced. 627 00:34:32,426 --> 00:34:37,400 AUDIENCE: So there'd be larger wavelengths for the slower one. 628 00:34:37,400 --> 00:34:42,144 But [INAUDIBLE]-- I don't know. 629 00:34:42,144 --> 00:34:43,060 MARK HARTMAN: So here. 630 00:34:43,060 --> 00:34:44,290 Let's go back what Jalen said. 631 00:34:44,290 --> 00:34:45,956 Somebody give me another marker, please. 632 00:34:49,310 --> 00:34:52,909 Maybe a staff person. 633 00:34:52,909 --> 00:34:54,770 Oh, no, I can't tell which one it is. 634 00:34:54,770 --> 00:34:59,390 OK, so in this case-- oh, good marker choice. 635 00:34:59,390 --> 00:35:03,230 So that's like dripping off the wall. 636 00:35:03,230 --> 00:35:07,100 So in this case, our observed wavelength 637 00:35:07,100 --> 00:35:11,520 was much, much longer than our emitted wavelength. 638 00:35:11,520 --> 00:35:15,230 Here this is our emitted wavelength. 639 00:35:15,230 --> 00:35:17,630 Here we're still moving away but slow. 640 00:35:17,630 --> 00:35:19,790 Here we're moving away fast. 641 00:35:22,510 --> 00:35:26,720 My diagrams get worse the further into the day we get. 642 00:35:26,720 --> 00:35:31,620 So is this wavelength here-- 643 00:35:31,620 --> 00:35:34,100 this is lambda observed. 644 00:35:34,100 --> 00:35:38,300 This is lambda observed slow. 645 00:35:38,300 --> 00:35:41,790 That's lambda observed fast. 646 00:35:41,790 --> 00:35:46,820 How does this wavelength compare to the emitted wavelength? 647 00:35:46,820 --> 00:35:47,760 Is it the same? 648 00:35:47,760 --> 00:35:49,400 Is it larger? 649 00:35:49,400 --> 00:35:51,050 Is it smaller? 650 00:35:51,050 --> 00:35:53,840 AUDIENCE: Smaller. 651 00:35:53,840 --> 00:35:54,945 AUDIENCE: Larger. 652 00:35:54,945 --> 00:35:56,570 MARK HARTMAN: What do you think, Chris? 653 00:35:56,570 --> 00:35:57,200 You saw it. 654 00:35:57,200 --> 00:35:59,000 AUDIENCE: I think it's larger. 655 00:35:59,000 --> 00:36:01,460 MARK HARTMAN: So this is still larger. 656 00:36:01,460 --> 00:36:06,380 Lambda observed slow-- is still greater 657 00:36:06,380 --> 00:36:08,960 than the emitted wavelength, but it's not 658 00:36:08,960 --> 00:36:12,150 quite as big as that one. 659 00:36:12,150 --> 00:36:14,480 So how could you say all of this in a sentence? 660 00:36:17,170 --> 00:36:19,440 What does speed have to do with what 661 00:36:19,440 --> 00:36:21,164 happens to your wavelength? 662 00:36:23,541 --> 00:36:24,040 Peter? 663 00:36:24,040 --> 00:36:25,664 AUDIENCE: The faster an object travels, 664 00:36:25,664 --> 00:36:27,910 the larger the wavelength. 665 00:36:27,910 --> 00:36:30,130 MARK HARTMAN: The faster an object travels, 666 00:36:30,130 --> 00:36:31,930 the larger the wavelengths. 667 00:36:31,930 --> 00:36:33,400 Well, what if what if my object was 668 00:36:33,400 --> 00:36:36,316 traveling this way towards me? 669 00:36:36,316 --> 00:36:40,870 AUDIENCE: [INAUDIBLE] coming towards you, it depends. 670 00:36:40,870 --> 00:36:44,766 The faster it goes, the shorter the wavelength. 671 00:36:44,766 --> 00:36:48,970 [INAUDIBLE] slow, fast-- kind of medium [INAUDIBLE] 672 00:36:48,970 --> 00:36:50,636 it will still have a shorter wavelength, 673 00:36:50,636 --> 00:36:54,534 but it would be small as the one [INAUDIBLE].. 674 00:36:54,534 --> 00:36:57,340 MARK HARTMAN: OK, so Steve? 675 00:36:57,340 --> 00:36:58,782 That's good. 676 00:36:58,782 --> 00:37:00,270 AUDIENCE: [INAUDIBLE]. 677 00:37:05,740 --> 00:37:08,800 MARK HARTMAN: OK, so if you're going fast away 678 00:37:08,800 --> 00:37:11,770 from an observer, the wavelengths get long. 679 00:37:11,770 --> 00:37:14,850 The faster you go, the longer, they get. 680 00:37:14,850 --> 00:37:18,440 If you're going towards an observer, the faster you go, 681 00:37:18,440 --> 00:37:20,860 the shorter they get. 682 00:37:20,860 --> 00:37:25,450 So scientists write this relationship as lambda 683 00:37:25,450 --> 00:37:35,087 observed minus lambda emitted over-- 684 00:37:35,087 --> 00:37:36,670 let's just make sure we do this right, 685 00:37:36,670 --> 00:37:39,910 and then we'll stop for today because we're already way late. 686 00:37:47,970 --> 00:37:49,440 Peter, is it over lambada emitted 687 00:37:49,440 --> 00:37:51,364 or over lambada observed? 688 00:37:51,364 --> 00:37:52,780 I think it's over lambada emitted. 689 00:37:58,260 --> 00:38:03,390 Yeah, so this relationship says that the difference 690 00:38:03,390 --> 00:38:06,750 between these two, which is really the shift 691 00:38:06,750 --> 00:38:08,010 that you're getting-- 692 00:38:08,010 --> 00:38:08,850 is it getting wider? 693 00:38:08,850 --> 00:38:10,740 Is it getting narrower? 694 00:38:10,740 --> 00:38:20,600 --is equal to the velocity that it's moving 695 00:38:20,600 --> 00:38:22,790 divided by the speed of light. 696 00:38:29,776 --> 00:38:31,240 Oops, I suppose I could do this-- 697 00:38:33,660 --> 00:38:34,160 sorry. 698 00:38:37,782 --> 00:38:39,240 AUDIENCE: I have a question. 699 00:38:39,240 --> 00:38:40,031 MARK HARTMAN: Yeah. 700 00:38:40,031 --> 00:38:45,939 AUDIENCE: If you do that right, [INAUDIBLE] cancel out? 701 00:38:45,939 --> 00:38:47,980 MARK HARTMAN: Oh, wouldn't the thing that made it 702 00:38:47,980 --> 00:38:49,570 in the bottom cancel out? 703 00:38:49,570 --> 00:38:52,750 It would if this was a multiplication sign. 704 00:38:52,750 --> 00:38:55,845 This is a subtraction. 705 00:38:55,845 --> 00:38:58,720 So this is the change in wavelength. 706 00:38:58,720 --> 00:39:01,570 So in this case, let's check this out. 707 00:39:01,570 --> 00:39:05,871 Here we said that lambda observed was less than lambda 708 00:39:05,871 --> 00:39:06,370 emitted. 709 00:39:10,120 --> 00:39:13,760 So this number is less than this number. 710 00:39:13,760 --> 00:39:16,270 So what do we get on top? 711 00:39:16,270 --> 00:39:17,155 A negative number. 712 00:39:19,850 --> 00:39:22,840 In that case, if we divide that by the lambda emitted, 713 00:39:22,840 --> 00:39:25,990 we get a negative velocity, which we kind of just 714 00:39:25,990 --> 00:39:28,360 choose which way is negative and which way is positive. 715 00:39:28,360 --> 00:39:32,110 Negative velocity says the source is moving towards you. 716 00:39:36,590 --> 00:39:40,850 Now, well, let's look at the one that's 717 00:39:40,850 --> 00:39:44,180 moving away that we were just talking about. 718 00:39:44,180 --> 00:39:46,520 If the source is moving away from you, 719 00:39:46,520 --> 00:39:48,930 the observed wavelength is longer. 720 00:39:48,930 --> 00:39:51,710 So this number is bigger. 721 00:39:51,710 --> 00:39:57,950 So bigger number minus a smaller number is a positive number. 722 00:39:57,950 --> 00:40:01,190 So a positive velocity is moving away from you-- 723 00:40:01,190 --> 00:40:04,410 away from the origin. 724 00:40:04,410 --> 00:40:06,330 That's why we call it positive. 725 00:40:06,330 --> 00:40:10,850 But if we move away faster, what does that do 726 00:40:10,850 --> 00:40:12,390 to our observed wavelength? 727 00:40:15,820 --> 00:40:16,800 AUDIENCE: It changes. 728 00:40:16,800 --> 00:40:18,270 MARK HARTMAN: Say that again. 729 00:40:18,270 --> 00:40:19,180 AUDIENCE: It changes. 730 00:40:19,180 --> 00:40:21,700 MARK HARTMAN: How does it change if the source moves 731 00:40:21,700 --> 00:40:22,892 away faster? 732 00:40:22,892 --> 00:40:24,308 AUDIENCE: [INAUDIBLE]. 733 00:40:30,450 --> 00:40:33,970 MARK HARTMAN: OK, the wavelength that we observe-- 734 00:40:33,970 --> 00:40:36,820 the wavelength we received-- is even bigger. 735 00:40:36,820 --> 00:40:40,520 So this number up here gets even bigger, 736 00:40:40,520 --> 00:40:44,230 and that's reflected by saying it's a higher velocity. 737 00:40:44,230 --> 00:40:45,460 It's moving away faster. 738 00:40:48,100 --> 00:40:52,150 OK last thing for the day, and then we're going to finish up. 739 00:40:52,150 --> 00:40:57,100 Astronomers call this measurement this value right 740 00:40:57,100 --> 00:40:59,840 here is called redshift. 741 00:40:59,840 --> 00:41:10,000 It is a quantity called redshift, 742 00:41:10,000 --> 00:41:24,240 and you represent it as the letter Z. Why would they 743 00:41:24,240 --> 00:41:25,465 call this redshift? 744 00:41:29,828 --> 00:41:32,725 AUDIENCE: Because the [INAUDIBLE].. 745 00:41:32,725 --> 00:41:33,850 MARK HARTMAN: Because what? 746 00:41:33,850 --> 00:41:40,169 AUDIENCE: Because the core of the particle itself [INAUDIBLE] 747 00:41:40,169 --> 00:41:41,960 moving away [INAUDIBLE] moving towards you, 748 00:41:41,960 --> 00:41:44,130 it emits different kind of light? 749 00:41:44,130 --> 00:41:46,090 MARK HARTMAN: As the source is moving away 750 00:41:46,090 --> 00:41:48,970 from you or towards you, it doesn't always 751 00:41:48,970 --> 00:41:51,480 emit the same color. 752 00:41:51,480 --> 00:41:54,580 If it's moving towards you, it will 753 00:41:54,580 --> 00:41:58,600 be shifted toward the blue end of the spectrum. 754 00:41:58,600 --> 00:42:02,800 It will be shifted towards shorter wavelengths, higher 755 00:42:02,800 --> 00:42:04,270 energies. 756 00:42:04,270 --> 00:42:06,220 And it's moving faster, it'll be shifted 757 00:42:06,220 --> 00:42:08,350 to even faster energies-- 758 00:42:08,350 --> 00:42:10,700 or I'm sorry-- even higher energies. 759 00:42:10,700 --> 00:42:13,570 It will be shifted toward the blue. 760 00:42:13,570 --> 00:42:18,670 If this number is negative, it's a negative redshift. 761 00:42:18,670 --> 00:42:22,930 So the light that we see is shifted towards higher 762 00:42:22,930 --> 00:42:26,500 energies-- bluer energies. 763 00:42:26,500 --> 00:42:29,650 If the source is moving away from us, 764 00:42:29,650 --> 00:42:35,800 then this number gets larger, and it's a redshift. 765 00:42:35,800 --> 00:42:39,128 What happens to the energy of the photons that are emitted? 766 00:42:39,128 --> 00:42:40,024 AUDIENCE: Gets red. 767 00:42:40,024 --> 00:42:41,370 AUDIENCE: Goes toward the red. 768 00:42:41,370 --> 00:42:42,911 MARK HARTMAN: It goes toward the red. 769 00:42:42,911 --> 00:42:43,960 They don't turn red. 770 00:42:43,960 --> 00:42:51,980 They just become smaller energy, longer wavelength.