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What is light? Scientists have been trying
to answer this question for hundreds of years.

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The process goes a little like this: we set
up an experiment and make observations about

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what light does. Then, using the data from
our experiment and all the previous experiments,

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we make a model—or models—of what we think
light is. This process continues as more experiments

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are done and the models are refined. In this
video, we'll use everything we know about

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what light does to explain the different models
of what light is.

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This video is part of the Representations
video series. Information can be represented

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in words, through mathematical symbols, graphically,
or in 3-D models. Representations are used

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to develop a deeper and more flexible understanding
of objects, systems, and processes.

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Hello. My name is Paola Rebusco. I am a physics
instructor in the Experimental Studies Group

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at MIT. Today I'll be talking with you about
different models of light.

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In order to understand this video, you'll
need to be familiar with the electromagnetic

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spectrum—particularly, the idea that there
are varieties of light that our eyes cannot

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see. You should also be familiar with the
behavior of waves, especially diffraction

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and interference. Finally, you should have
some experience solving problems with collisions,

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so that you better understand how particles
interact with each other.

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After watching this video, you should be able
to explain the major models of light, and

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should also be able to explain the different
facets of light in terms the models that best

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describe them. This video should also help
you to understand how models are used and

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verified in science.

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Let's start by looking at the things that
light does. We cannot expect to describe what

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light is without having a good picture of
its behavior. We are going to make a list

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and refer to it later, so be sure to take
notes. First, we know that light moves very

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quickly.

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When it reaches an object, light can interact
in many ways. It might be transmitted through

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the object, be reflected off, or be absorbed.

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However, beams of light that pass through
each other come out unchanged.

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Light can refract, changing directions as
it moves from one material to another.

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It can also diffract as it passes through
small openings.

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When light passes by a charged particle, it
can make that particle oscillate back and

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forth.

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There are many kinds of light, some of them
invisible to us.

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It can strip electrons away from atoms, ionizing
them almost instantly.

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Light can collide with particles.

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Finally, light can transfer energy and momentum
almost instantaneously, but it can also do

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so in a gradual way. Some of the atomic-scale
phenomena listed are actually the reason for

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the macroscopic phenomena. For instance, light
that strips away electrons from atoms is absorbed

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in the process.

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Take a moment to make sure you have all of
this in your notes. These observations can

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help us create a model for light. When we
talk about "what light is," what we're really

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doing is creating a model. A model in physics
is a representation -- a mental analogy that

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we can use to better understand the physical
world. Once we have a model, we can predict

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the behavior of an object or phenomenon based
on the ways that the model would behave.

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It's important to understand that no model
is perfect -- all models go through cycles

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of testing and refinement. There are typically
conditions under which a model can break down,

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giving incorrect results. However, because
models are just analogies, we often create

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a new and improved one to better describe
how something works. A single phenomenon can

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be modeled in many different ways, with different
assumptions or simplifications. Therefore,

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we can even use more than one model for the
same thing. To give an example, imagine describing

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gravity as an invisible rope holding objects
together. There are several good things about

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this model. Ropes can only pull, not push,
and this is true for gravity. Ropes can be

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used to swing objects in curved paths, the
same way that gravity makes the planets move

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in curved paths.

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Unfortunately, the model breaks down quickly.
The amount of force a rope exerts does not

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change with the length of the rope, whereas
gravity's force definitely does depend on

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distance. Ropes can become tangled, whereas
gravity has nothing to tangle.

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We can see that this model works for some
purposes, but not for others. Let's consider

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some of the things we might want to use to
build our model for light. We know from our

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list that light can pass through other light,
can diffract and refract, and that it can

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make charged particles oscillate.

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You may have heard light described in terms
of rays of light, or waves, or particles called

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photons. Which one of these models seems to
fit these observations? Pause the video to

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consider.

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If we consider waves, we know that waves can
pass through each other, that they diffract

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and refract, and that waves can make particles
oscillate. These match the properties of light

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that we chose very well.

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The fact that light specifically makes charged
particles oscillate is an indication that

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we are likely dealing with an electromagnetic
phenomenon. Therefore, we can model light

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as an electromagnetic wave. As you probably
know, this is a common model for light. We

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imagine a situation similar to the animation
shown to the right, where electromagnetic

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waves propagate in the direction that the
light is shining. This animation shows the

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electric field vectors as the wave moves to
the right. Below is another image that shows

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the electric field in red and the magnetic
field in blue, perpendicular to each other.

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However, there is a problem. Light cannot
be just a wave. Let's look at the reasons

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why.

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In addition to the aspects that are wave-like,
light has other properties. Light can also

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collide with particles of matter. Waves, on
the other hand, move through or around particles.

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Light can transfer momentum and energy almost
instantaneously through these collisions;

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waves must transfer energy and momentum gradually.
Finally, light can ionize atoms almost instantly,

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but electromagnetic waves would need more
time to do this because of that gradual energy

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transfer.

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It is clear that light cannot be just a wave.
Waves model some aspects of light well, but

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not others. How else might we model light?
Consider the aspects that we just discussed,

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and try to come up with an alternative model.
Pause the video here to discuss.

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The other major model for light is the particle
model. Particles can collide with each other,

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and in the process, they transfer momentum
and energy almost instantly. Through these

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collisions, they can quickly knock electrons
loose from their atoms.

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It is because of this that a beam of light
can also be modeled as a large number of tiny

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particles, called photons. If we return to
our list, we can see that some of the items,

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shown in red, are described well by the wave
model. Others, shown in blue, are the items

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we just discussed for the particle model.
We cannot deny that light does all of these

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things. Experiments show us that it behaves
like a particle in some ways, and like a wave

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in other ways. This leads to a phenomenon
called "wave-particle duality." In some situations

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light seems to behave in a manner predicted
by the wave model. At other times it behaves

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in a particle-like way. We cannot rely just
on the wave model of light, or just on the

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particle model. Neither of our models works
in every possible situation. Scientists and

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engineers typically set up situations to take
advantage of one model or the other, using

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the particle-like or wave-like behaviors of
light to their advantage.

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We should also mention here that it's not
just light that has the properties of both

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particles and waves. All things -- electrons,
protons, any form of matter -- can be made

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to behave like particles or like waves. This
was one of the driving forces behind the development

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of quantum physics in the early 19th century.
One of the most striking experiments in this

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area is the double-slit experiment, in which
light is directed towards a pair of thin slits

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and a pattern appears on a detecting screen.

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The screen is capable of detecting individual
photons one at a time, which can be explained

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by the particle model.

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However, the pattern is created through the
interference and diffraction of light, which

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can only be explained through the wave model.
This experiment gives the same results with

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matter as with light. If we use an electron
source rather than a laser, we can create

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a beam of electrons forming the same pattern.
This shows that matter can also be represented

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as a particle and as a wave. In the modern
view of quantum physics, objects are often

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represented as "probability wave packets."
These are localized collections of waves that

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describe where those objects may be found.
The wavepacket approach is more accurate than

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the wave model or the particle model, but
is also difficult to fully understand and

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to make calculations with. The separate wave
and particle models are both still in common

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use today. Models are useful for many purposes.
Two of the most common are describing the

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mechanism for an existing phenomena and predicting
new phenomena that may be discovered. Rather

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than ask you to come up with entirely new
predictions of what light might be able to

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do, we will instead look at some known phenomena
that we have not yet discussed.

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Each of the four phenomena listed here can
be described using either the wave model or

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the particle model of light. Your professor
will lead the class in a debate as to which

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model is most could best describe each phenomenon.
Pause the video here. Let's look at #3 on

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that list: that an accelerating electric charge
can generate light. Would the wave model or

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the particle model predict this result?

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The fact that an electric charge is involved
definitely points toward an electromagnetic

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wave description of light. We know that moving
electric charges create both magnetic and

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electric fields. If our charge is accelerating,
then the fields that it creates must be changing.

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As we learned earlier in this course, changing
electromagnetic fields obey a wave equation.

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It seems like this particular phenomenon is
best described using a wave model. Let's review.

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The core idea for today's video was the concept
of a model. We saw that light can be modeled

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as an electromagnetic wave, or as a particle.
Models like these are used to describe existing

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phenomena and to improve our understanding.
They can also be used to predict new phenomena.

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I hope you have enjoyed this introduction
to light and wave-particle duality. Good luck

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in your further studies of physics.