What’s the equal/opposite reaction of a laser beam hitting something?

Question

So it goes like this; when you shoot a gun, there will be a “kick”, this is because of Newton’s third law. However, what’s the equal, and opposite reaction when a laser ram hits an object? I need an answer as simplified as possible so I can explain this to one of my students who asked about this. Is it heat? Is there a minuscule “kick”?

Photons have momentum, so Newton's laws still hold... – Jon Custer May 17 '18 at 14:34 — Jon Custer, May 17 '18 at 14:34
Related: Rocket propelled by a giant monochromatic laser. – rob May 17 '18 at 18:38 — rob, May 17 '18 at 18:38

NullAchtFuffZehn · Answer 1 · 2018-05-17T16:46:06.323

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Just treat the light as a buch of photons. Each one carries an energy depenent momentum $$ p = \frac{E}{c} = \frac{\hbar \omega}{c} = \hbar k$$ When a photon gets absorbed, it transfers its momentum to the target. If it gets reflected, twice its momentum is transferred.

So actually, as already pointed out in a comment, you can apply classical mechanics here.

edited May 17 '18 at 16:46

answered May 17 '18 at 14:45

NullAchtFuffZehn

104
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So, your saying the opposite reaction would be the deflection of the photons off the wall? – Hobo_warrior May 17 '18 at 14:47
If you hold a flashlight and point it away from you, it will actually push you back due to the photon recoil. – NullAchtFuffZehn May 17 '18 at 14:57
Is this the basis for an ion engine? – Hobo_warrior May 17 '18 at 14:58
@Hobo_warrior An ion engine emits charged particles, not photons. – rob May 17 '18 at 18:40

What’s the equal/opposite reaction of a laser beam hitting something?

1 Answers1