# Phys photon questionWatch

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#1
8. A photon collides with an electron which is loosely bound to an atom and, as a result, changes direction. The collision is elastic bacause there are no other forms of energy into which the kinetic energy may be changed. Before the colliosion the electron may be considered to be stationary. Beacuse the photon behaves like a particle we can say that momentum and KE are conserved.

a) why must the electron have some kinetic energy afta the collision?

b) Wat can u sey about the energy of the scattered photon, compared with the energy of the incident photon?

c) How does the wavelength of the scattered photon compare with the wavelength of the incident photon?
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14 years ago
#2
(a) as there is a collision some momentum must be transferred. the electron initially has no momentum and to have a momentum is must have a velocity (p=mv), and with a velocity and a mass comes kinetic energy.

(b) energy is lower as the momentum of the photon has fallen

(c) longer wavelength as the photon has lost energy.

I think. cant remember the equations exactly.
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#3
(Original post by mik1w)
(a) as there is a collision some momentum must be transferred. the electron initially has no momentum and to have a momentum is must have a velocity (p=mv), and with a velocity and a mass comes kinetic energy.

(b) energy is lower as the momentum of the photon has fallen

(c) longer wavelength as the photon has lost energy.

I think. cant remember the equations exactly.
Aren't photon collisions all or nothing tho, i.e. they give out all their energy or none of it?
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14 years ago
#4
(Original post by mc_watson87)
Aren't photon collisions all or nothing tho, i.e. they give out all their energy or none of it?
Its a different photon I believe
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14 years ago
#5
(Original post by mc_watson87)
Aren't photon collisions all or nothing tho, i.e. they give out all their energy or none of it?
I can see where that comes from but "all or nothing" isn't right I don't think.

when an electron absorbs energy from a photon and jumps up a higher energy level, you can think of the electron in terms of a stationary wave round the atom moving to one higher harmonic - you can only have the electron in integer multiples of the first harmonic. eg. 1st, 2nd, 3rd, and higher harmonics mean the electron has more energy.

as the electron can only occupy certain harmonics, it must only take certain amounts of energy from the photon. so you are right in that there are set energies that can be emitted (which leads to emission spectra), but not all or nothing.

I'm not sure how much this has to do with momentum, because you aren't given quantiative measures of the photon's energy or electron's energy levels, so I don't know if the knowledge of this helps in that specific question.
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14 years ago
#6
(Original post by mc_watson87)
8. A photon collides with an electron which is loosely bound to an atom and, as a result, changes direction. The collision is elastic bacause there are no other forms of energy into which the kinetic energy may be changed. Before the colliosion the electron may be considered to be stationary. Beacuse the photon behaves like a particle we can say that momentum and KE are conserved. ?

a) why must the electron have some kinetic energy afta the collision??
so that they can have the speed to be emitted from the atom
?
b) Wat can u sey about the energy of the scattered photon, compared with the energy of the incident photon??
the energy of the scattered photon is lower as it wasn't able to emit any electorn*has lower than the threshold frequency*so it was scattered
?
c) How does the wavelength of the scattered photon compare with the wavelength of the incident photon?
wave length of the scattered photons is longer as it has a lower throshold frequency so a larger wavelength as frequency is inversely proportional to the wavelength
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