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”?
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$\begingroup$ Photons have momentum, so Newton's laws still hold... $\endgroup$– Jon CusterCommented May 17, 2018 at 14:34
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$\begingroup$ Related: Rocket propelled by a giant monochromatic laser. $\endgroup$– rob ♦Commented May 17, 2018 at 18:38
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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.
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$\begingroup$ So, your saying the opposite reaction would be the deflection of the photons off the wall? $\endgroup$ Commented May 17, 2018 at 14:47
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$\begingroup$ If you hold a flashlight and point it away from you, it will actually push you back due to the photon recoil. $\endgroup$ Commented May 17, 2018 at 14:57
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$\begingroup$ Is this the basis for an ion engine? $\endgroup$ Commented May 17, 2018 at 14:58
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$\begingroup$ @Hobo_warrior An ion engine emits charged particles, not photons. $\endgroup$– rob ♦Commented May 17, 2018 at 18:40