# Solid A2 Physics question - anyone help?

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#1
A question came up on the PSA4 paper for Salter Hornor physics that i'm still confused about and even my teachers seem a bit confused about.

The question was quite complicated and i can't remember all the detail. Basically imagine a CD case. The top side being made of a shiny, reflective metal surface and the bottom side being made of a dull surface.

The question basically asked which side will absorb more energy when a photon impacts with it and therefore in which direction will the resultant force be?

I said that the shiny side would absorb the photon (thinking of the photoelectric effect) and therefore have the most energy transfered to it. The resultant force would therefore be down.

At the time i was unsure about this as i was thinking about elastic and inelastic collisions. Because it said shiny, 'reflective' material i thought that the electron would bounce of this side in an elastic collision and transfer no energy. The dull side would absord energy in an inelastic collision and have energy transfered to it. Therefore the resultant force would be up.

After speaking to my teachers today, they tried to relate momentum equations to it to work out the resultant energy and force.

Anyone do this question or have any idea what the anwser could be. Anyone know what is more likely or have a suggestion?
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#2
Anyone?

Aany idea whether i was right??? - it was worth about 5/6 marks for the two parts combined. I'm quite worried about it.
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#3
Anyone?
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15 years ago
#4
I would say the shiny side reflects the photons so these photons have a greater change in momentum so exert a bigger force on the shiny side than is exerted on the dull side.
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15 years ago
#5
(Original post by need_help)
The question was quite complicated and i can't remember all the detail. Basically imagine a CD case. The top side being made of a shiny, reflective metal surface and the bottom side being made of a dull surface.

The question basically asked which side will absorb more energy when a photon impacts with it and therefore in which direction will the resultant force be?

I said that the shiny side would absorb the photon (thinking of the photoelectric effect) and therefore have the most energy transfered to it. The resultant force would therefore be down.
The shiny side should reflect more photons than the dull side, but if the photons bounce off it elastically 2 x the momentum of the photon will be transferred. The dull side, when a photon is absorbed recieves only the original momentum, half that of the shiny side. So I think the resultant force will be downwards, except not for the same reason.

EDIT: Same as Bezza, beat me to it
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#6
(Original post by Golden Maverick)
The shiny side should reflect more photons than the dull side, but if the photons bounce off it elastically 2 x the momentum of the photon will be transferred. The dull side, when a photon is absorbed recieves only the original momentum, half that of the shiny side. So I think the resultant force will be downwards, except not for the same reason.

EDIT: Same as Bezza, beat me to it

Yeah, that's the logic that my teachers tried to apply to it.

Hmm....so why in the photoelectric effect does a clean metal surface absorb the photons but in this case they would be reflected?
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15 years ago
#7
(Original post by need_help)
Hmm....so why in the photoelectric effect does a clean metal surface absorb the photons but in this case they would be reflected?
Some photons are absorbed. In the photoelectric effect they must interact with an electron to be absorbed and in practice not many do interact.
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