# Photon Pressure

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#### ihwip

##### Guest
I don't know the answer to what should be a rather simple question:

Photons produce a tiny amount of force on objects when absorbed, reflected or radiated. Does this pressure vary by wavelength? Does an ultraviolet photon hit harder than an infrared photon?

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#### Saiph

##### Guest
Indeed it does. A photon's momentum (which is transfered to absorbing objects, like any particle's momentum) is related to it's energy: E=pc where E is energy, p is momentum and C is...well, what you think it is (speed of light).

It can be rewritten as E=hv/c or E=h/L where v is freqency, H planck's constant, and L is wavelength (to lazy to figure out lambda)

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#### SteveCNC

##### Guest
which exerted more force against the surface , absorb the photon or reflect it ?

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#### csmyth3025

##### Guest
SteveCNC":19qk1wgz said:
which exerted more force against the surface , absorb the photon or reflect it ?

My guess would be that an absorbed photon would impart its momentun to the object that absorbs it.

If a photon is reflected, its "forward" momentum plus its "backward" relected momentum would represent the total change in momentum of the photon (a vector quantity). By this reasoning I'm also guessing that a relected photon would inpart a greater "forward" momentum to the object that reflected it.

Chris

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#### Saiph

##### Guest
csmyth is correct, the reflected photon imparts more momentum than a straight absorption, and it's all about conservation of momentum.

Scale it up a bit and think about it. You are on a skateboard, and someone hurls a bowling ball at you, which you catch. Naturally you'll start rolling along. Now you have to throw it back...which sends you rolling in the same direction even faster.

Same deal with photons.

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#### drwayne

##### Guest
This was actually part of a question from my oral preliminary examination for my Ph.D. For
an inelastic collision (complete absorption), the illuminated object acquires the photon's
momentum (call it P). For elastic collision (i.e. perfect reflection), it acquires 2 * P, because
the photon has reversed its direction. Yes, it can all be handled under good, old fashioned
conservation of momentum. (Except for a photon, P != mv)

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