A teacher on a teachers’ listserv asked some fine questions about the nature of light. Here are her questions, and my answers.

1) If light is energy that is emitted by accelerating electric charges – often electrons in atoms – how do teachers explain the fact that light moves through a vacuum?

I’m not sure how teachers explain it, but the accelerating electric charges *emit* light. The light then self propagates. Sort of like how a ballplayers arm throws a ball, but once moving, the ball no longer needs the thrower’s arm to keep moving after the initial toss. Except in the case of light, the ball (photon) keeps itself going, and doesn’t stop moving.

And an additional comment from Paul Doherty:

Take two balloons, hang them from the ceiling of the room with strings so they are about waist high and touching each other. Then, rub them with wool.

They move apart repelling each other.

We say that each balloon creates an electric field that exerts a repulsive force on the other balloon which has the same electric charge. The electric field is a straight line between the centers of the balloons.

If you put the balloons in a vacuum they will still repel. The electric field has no problem going through a vacuum.

Move one balloon and the other will move in response. By moving one balloon you change the electric field direction.

The change in the electric field actually propagates down the electric field line as a wave.

To move the balloon to the side you must accelerate it to the side. And this acceleration makes a kink in the electric field line that propagates along the electric field line and is the electric part of an electromagnetic wave.

I find that kids have an easier time with this model, for some reason they accept the existence of elecric fields in a vacuum better than the existence of electromagnetic waves in a vacuum.

And another teacher weighs in with another way to teach this, with a nice advertisement for the simulations created by our education group here at U. Colorado:

The PhET group at U. Colorado has a neat applet that demonstrates this idea very well.

In the applet, you can grab an electron in an antenna and wiggle it up and down. The screen displays a line of force and the the resulting electric field. The behavior of another electron in an another antenna is displayed on the other side of the screen. It is easy to see how the two electrons interact with each other.

If you have trouble moving the electron in the first antenna smoothly, you can set the applet so that it oscillates the electron. It is really easy to see how the other electron’s motion is effected by the first electron. The applet is in Java, so you will need to have it installed on your computer, but you probably already have it.

As an aside, I can’t say enough about the PhET collection of applets. They are really cool and my students find them very helpful

The teacher’s questions continue:

2) What propagates the light/electromagnetic radiation (photons) from the sun to earth through space?

The simple answer — nothing. That is, nothing outside of the electromagnetic wave propagates it, it propagates itself. It does this by electromagnetic induction. Say you shake electric charges, as you mention above. That creates an electric wave (which is an electric field that changes over distance). But what does a changing electric field make? A changing magnetic field (by electromagnetic induction). And what does a changing magnetic field make? A changing electric field. So, the electric and magnetic fields swap energy between each other, as one grows the other diminishes. It’s like the electric wave throws energy to the magnetic wave, which then throws it back, as the two of them run forward. I picture it like two people running and throwing a ball back and forth, but that is an incomplete analogy. They keep each other going. The energy doesn’t diminish so it keeps going.

It can do this even in a vacuum, since nothing is “shaking” — it’s just an electric and magnetic field feeding each other.

3) Light moving through atoms is easier to grasp then vibrating electric charges self propagating … anyway do photon’s self propogate and do the photons or vibrating electric charges move sort of up & down and forward?

Vibrating electric charges creating the electric wave can move up and down (or some other more complicated movement). That creates the electromagnetic wave, which is just photons. The photons do self propagate (since “photons” is just another way of saying “moving electromagnetic wave”). The photons themselves don’t move up and down. Rather, the magnitude of the electric and magnetic fields increase and decrease as the wave moves along. (How rapidly they increase/decrease gives us the color of light, and of course it always moves at speed c).

Another teacher asked:

4) I don’t understand light, photons, light’s momentum, and the bending
of light. Are photons “real”? Do photons have measurable mass when
they are moving? (You told me once that photons have no rest mass).

And Paul Doherty answered:

Photons are “Real” in the sense that they do carry measurable energy and momentum from one place to another.

Mass in relativity is a tricky concept and photons are relativistic.

Do you want inertial mass? When an atom emits a photon the atom does recoil.

The photon has momentum, the Mercury spacecraft Mariner 10 lost its fuel due to a stuck valve, scientists used the force exerted by solar photons bouncing off and being absorbed by the solar panels to propel the spacecraft and change its orbit, so indeed photons have momentum.

Do you want gravitational mass, the photon does fall under gravity, and photons do exert gravity. When a photon goes straight up against gravity it loses energy and so shifts its wavelength to the red, when it goes straight down in a gravity field it blue shifts.

Relativistic mass, E = mc^2 photons have energy, step out in the sunlight and feel the energy in the photons, so m = E/c^2

But there is a definition in relativity of a type of mass called rest mass, electrons have it 9 x 10 ^-31 kg. It is the mass when the electron is at rest. Photons in a vacuum always move at the speed of light, they are never at rest so what could the rest mass mean? We get around that by defining it as 0. Only objects with zero rest mass travel at the speed of light.

Newton predicted light would fall under gravity, Einstein did too, but Einsteins prediction was just twice Newton’s. During solar eclipses the bending of starlight has been measured and confirms Einstein’s prediction.

And here’s a very nice post from Built on Facts about the fact that light can push stuff around (ie., it has momentum), which is how solar sails work.

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