Register to reply

Photon reflection problem

by aayushgsa
Tags: photon, reflection
Share this thread:
Aug20-13, 12:17 AM
P: 20
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.
Phys.Org News Partner Physics news on
Researchers demonstrate ultra low-field nuclear magnetic resonance using Earth's magnetic field
Bubbling down: Discovery suggests surprising uses for common bubbles
New non-metallic metamaterial enables team to 'compress' and contain light
Aug20-13, 12:32 AM
jtbell's Avatar
P: 11,738
You cannot change a photon's speed, but you can change its direction of motion, and thereby change its momentum.
Aug20-13, 01:16 PM
P: 221
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:)

Aug21-13, 07:05 AM
P: 1,521
Photon reflection problem

Quote Quote by jtbell View Post
You cannot change a photon's speed, but you can change its direction of motion, and thereby change its momentum.
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...
Aug22-13, 07:30 AM
P: 20
Yes, that's my point @lightarrow
Aug22-13, 01:55 PM
P: 1,521
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:

Register to reply

Related Discussions
Whats the difference between reflection and scattering of photon? (QFT) Quantum Physics 0
Feynman's QED and photon reflection Quantum Physics 5
Photon reflection from a metal surface Atomic, Solid State, Comp. Physics 1
Change of momentum of photon upon reflection Advanced Physics Homework 1
Can both a photon and it's reflection hit a particle? General Physics 5