# Photon reflection problem

1. Aug 20, 2013

### aayushgsa

Hello,
I was just reading optics and I thought that if photon doesn't has mass it can't be accelerated..
If it is so then how it gets reflected off a smooth surface, why don't it just passes from the plane. As any reaction force by the plane couldn't accelerate it , it mustn't rebond as rebounding means accelerating.

2. Aug 20, 2013

### Staff: Mentor

You cannot change a photon's speed, but you can change its direction of motion, and thereby change its momentum.

3. Aug 20, 2013

### jfizzix

Exactly. You can change the photon's energy and momentum, but the speed is the same, since the speed of light is a fundamental constant. What changes is the frequency of light (for energy) or the wavelength of the light (for momentum).

Newton's third law does apply here, and since the momentum of the photon is changed, so is the momentum of the mirror (in the opposite direction). If you had enough light, you could push the mirror with a noticeable force. This is what's behind how solar sails work.

Of course, things get iffy when you start treating photons like massless billiard balls moving at the speed of light, but unless you are really looking at an individual photon and not beams of trillions of them in barely visible laser beams, and the mirror has no features small enough to be even close to the wavelength of light, a classical perspective works often enough.

Hope this helps:)

4. Aug 21, 2013

### lightarrow

Sorry jtbell but I don't think this is an answer for the OP. You have stated that a photon's speed can't vary, but you didn't say anything about its velocity and since velocity is a vector, simply changing the velocity's direction there is an acceleration.
Unless the photon reflected actually it's not the same photon sent to the surface...

5. Aug 22, 2013

### aayushgsa

Yes, that's my point @lightarrow

6. Aug 22, 2013

### lightarrow

It's probably meaningless to attribute individuality to single photons, they are not as little balls; even if the mirror doesn't absorb the photon in the sense that its energy doesn't vary at all, I think the photon is actually destroyed and a new one re-created by the mirror, at least this is what vanesch suggested in this thread: