What is light? Scientists have been trying to answer this question for hundreds of years.
The process goes a little like this: we set up an experiment and make observations about
what light does. Then, using the data from our experiment and all the previous experiments,
we make a model—or models—of what we think light is. This process continues as more experiments
are done and the models are refined. In this video, we'll use everything we know about
what light does to explain the different models of what light is.
This video is part of the Representations video series. Information can be represented
in words, through mathematical symbols, graphically, or in 3-D models. Representations are used
to develop a deeper and more flexible understanding of objects, systems, and processes.
Hello. My name is Paola Rebusco. I am a physics instructor in the Experimental Studies Group
at MIT. Today I'll be talking with you about different models of light.
In order to understand this video, you'll need to be familiar with the electromagnetic
spectrum—particularly, the idea that there are varieties of light that our eyes cannot
see. You should also be familiar with the behavior of waves, especially diffraction
and interference. Finally, you should have some experience solving problems with collisions,
so that you better understand how particles interact with each other.
After watching this video, you should be able to explain the major models of light, and
should also be able to explain the different facets of light in terms the models that best
describe them. This video should also help you to understand how models are used and
verified in science.
Let's start by looking at the things that light does. We cannot expect to describe what
light is without having a good picture of its behavior. We are going to make a list
and refer to it later, so be sure to take notes. First, we know that light moves very
quickly.
When it reaches an object, light can interact in many ways. It might be transmitted through
the object, be reflected off, or be absorbed.
However, beams of light that pass through each other come out unchanged.
Light can refract, changing directions as it moves from one material to another.
It can also diffract as it passes through small openings.
When light passes by a charged particle, it can make that particle oscillate back and
forth.
There are many kinds of light, some of them invisible to us.
It can strip electrons away from atoms, ionizing them almost instantly.
Light can collide with particles.
Finally, light can transfer energy and momentum almost instantaneously, but it can also do
so in a gradual way. Some of the atomic-scale phenomena listed are actually the reason for
the macroscopic phenomena. For instance, light that strips away electrons from atoms is absorbed
in the process.
Take a moment to make sure you have all of this in your notes. These observations can
help us create a model for light. When we talk about "what light is," what we're really
doing is creating a model. A model in physics is a representation -- a mental analogy that
we can use to better understand the physical world. Once we have a model, we can predict
the behavior of an object or phenomenon based on the ways that the model would behave.
It's important to understand that no model is perfect -- all models go through cycles
of testing and refinement. There are typically conditions under which a model can break down,
giving incorrect results. However, because models are just analogies, we often create
a new and improved one to better describe how something works. A single phenomenon can
be modeled in many different ways, with different assumptions or simplifications. Therefore,
we can even use more than one model for the same thing. To give an example, imagine describing
gravity as an invisible rope holding objects together. There are several good things about
this model. Ropes can only pull, not push, and this is true for gravity. Ropes can be
used to swing objects in curved paths, the same way that gravity makes the planets move
in curved paths.
Unfortunately, the model breaks down quickly. The amount of force a rope exerts does not
change with the length of the rope, whereas gravity's force definitely does depend on
distance. Ropes can become tangled, whereas gravity has nothing to tangle.
We can see that this model works for some purposes, but not for others. Let's consider
some of the things we might want to use to build our model for light. We know from our
list that light can pass through other light, can diffract and refract, and that it can
make charged particles oscillate.
You may have heard light described in terms of rays of light, or waves, or particles called
photons. Which one of these models seems to fit these observations? Pause the video to
consider.
If we consider waves, we know that waves can pass through each other, that they diffract
and refract, and that waves can make particles oscillate. These match the properties of light
that we chose very well.
The fact that light specifically makes charged particles oscillate is an indication that
we are likely dealing with an electromagnetic phenomenon. Therefore, we can model light
as an electromagnetic wave. As you probably know, this is a common model for light. We
imagine a situation similar to the animation shown to the right, where electromagnetic
waves propagate in the direction that the light is shining. This animation shows the
electric field vectors as the wave moves to the right. Below is another image that shows
the electric field in red and the magnetic field in blue, perpendicular to each other.
However, there is a problem. Light cannot be just a wave. Let's look at the reasons
why.
In addition to the aspects that are wave-like, light has other properties. Light can also
collide with particles of matter. Waves, on the other hand, move through or around particles.
Light can transfer momentum and energy almost instantaneously through these collisions;
waves must transfer energy and momentum gradually. Finally, light can ionize atoms almost instantly,
but electromagnetic waves would need more time to do this because of that gradual energy
transfer.
It is clear that light cannot be just a wave. Waves model some aspects of light well, but
not others. How else might we model light? Consider the aspects that we just discussed,
and try to come up with an alternative model. Pause the video here to discuss.
The other major model for light is the particle model. Particles can collide with each other,
and in the process, they transfer momentum and energy almost instantly. Through these
collisions, they can quickly knock electrons loose from their atoms.
It is because of this that a beam of light can also be modeled as a large number of tiny
particles, called photons. If we return to our list, we can see that some of the items,
shown in red, are described well by the wave model. Others, shown in blue, are the items
we just discussed for the particle model. We cannot deny that light does all of these
things. Experiments show us that it behaves like a particle in some ways, and like a wave
in other ways. This leads to a phenomenon called "wave-particle duality." In some situations
light seems to behave in a manner predicted by the wave model. At other times it behaves
in a particle-like way. We cannot rely just on the wave model of light, or just on the
particle model. Neither of our models works in every possible situation. Scientists and
engineers typically set up situations to take advantage of one model or the other, using
the particle-like or wave-like behaviors of light to their advantage.
We should also mention here that it's not just light that has the properties of both
particles and waves. All things -- electrons, protons, any form of matter -- can be made
to behave like particles or like waves. This was one of the driving forces behind the development
of quantum physics in the early 19th century. One of the most striking experiments in this
area is the double-slit experiment, in which light is directed towards a pair of thin slits
and a pattern appears on a detecting screen.
The screen is capable of detecting individual photons one at a time, which can be explained
by the particle model.
However, the pattern is created through the interference and diffraction of light, which
can only be explained through the wave model. This experiment gives the same results with
matter as with light. If we use an electron source rather than a laser, we can create
a beam of electrons forming the same pattern. This shows that matter can also be represented
as a particle and as a wave. In the modern view of quantum physics, objects are often
represented as "probability wave packets." These are localized collections of waves that
describe where those objects may be found. The wavepacket approach is more accurate than
the wave model or the particle model, but is also difficult to fully understand and
to make calculations with. The separate wave and particle models are both still in common
use today. Models are useful for many purposes. Two of the most common are describing the
mechanism for an existing phenomena and predicting new phenomena that may be discovered. Rather
than ask you to come up with entirely new predictions of what light might be able to
do, we will instead look at some known phenomena that we have not yet discussed.
Each of the four phenomena listed here can be described using either the wave model or
the particle model of light. Your professor will lead the class in a debate as to which
model is most could best describe each phenomenon. Pause the video here. Let's look at #3 on
that list: that an accelerating electric charge can generate light. Would the wave model or
the particle model predict this result?
The fact that an electric charge is involved definitely points toward an electromagnetic
wave description of light. We know that moving electric charges create both magnetic and
electric fields. If our charge is accelerating, then the fields that it creates must be changing.
As we learned earlier in this course, changing electromagnetic fields obey a wave equation.
It seems like this particular phenomenon is best described using a wave model. Let's review.
The core idea for today's video was the concept of a model. We saw that light can be modeled
as an electromagnetic wave, or as a particle. Models like these are used to describe existing
phenomena and to improve our understanding. They can also be used to predict new phenomena.
I hope you have enjoyed this introduction to light and wave-particle duality. Good luck
in your further studies of physics.