AS physics question
Physics and electronics discussion, revision, exam and homework help.
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AS physics questionTwo things i'm not sure about when i was revising:
Why can we change the wavelength of a matter particle but not of a photon? still unanswered need help on this urgently please.
Why can't an electron absorb several photons to escape from a metal?
Help would be greatly appreciated
Last edited by coolstorybrother; 14-05-2012 at 10:28. -
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Re: AS physics questionNot sure about the first one but for the second one I'm pretty sure it's because the electron will absorb the photon but then instantaneously re emit it. Just way too fast for the electron to absorb multiple ones. But then again, by saying that I am stating that the electron will move up then down energy levels faster than the speed of light...
Anyway yeah that is one of the pieces of evidence that quantum theory is applicable isn't it? If it was purely a wave phenomena then if you left it in the sun for ages it would supposedly discharge.
Sorry if I'm wrong
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Re: AS physics questioni thought photon emission was only possible when an electron de-excites? Or when a charged particle loses energy by stopping speeding up or slowing down, realeasing packets of em waves in all directions - photons? Well, if we refer to the photoelectric effect, wave theory does incorrectly predict that emission should take place at any frequency and that at lower frequencies it should take longer than at higher?(Original post by moogoomonkey)
Not sure about the first one but for the second one I'm pretty sure it's because the electron will absorb the photon but then instantaneously re emit it. Just way too fast for the electron to absorb multiple ones. But then again, by saying that I am stating that the electron will move up then down energy levels faster than the speed of light...
Anyway yeah that is one of the pieces of evidence that quantum theory is applicable isn't it? If it was purely a wave phenomena then if you left it in the sun for ages it would supposedly discharge.
Sorry if I'm wrong
hmm im confused
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Re: AS physics questionwhat equations do you know about wavelength? What equations could this tie into? Why would a photon be a special case?(Original post by coolstorybrother)
Two things i'm not sure about when i was revising:
Why can we change the wavelength of a matter particle but not of a photon?
What do you know about electrons; in particular their energy when it is not yet high enough to escape the atom/ metal?Why can't an electron absorb several photons to escape from a metal?
Help would be greatly appreciated
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Re: AS physics questionhmmmm is it because they can only absorb photons which are equal to the exact difference between their energy levels? so they could technically absorb "several" just it has to be the right several, and provided they would not gain enough energy to cause ionisation maybe? Or would they HAVE to absorb a photon that is equal to the distance between the ground state and the ionisation energy?(Original post by Llewellyn)
what equations do you know about wavelength? What equations could this tie into? Why would a photon be a special case?
well theres the standard c=f(wavelength) and also e=hf however cant you also have the de Broglie wavelength as well that would be h/mv... im not really sure why the photon would be a special case though...
What do you know about electrons; in particular their energy when it is not yet high enough to escape the atom/ metal? -
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Re: AS physics questionhmmmm is it because they can only absorb photons which are equal to the exact difference between their energy levels? so they could technically absorb "several" just it has to be the right several, and provided they would not gain enough energy to cause ionisation maybe? Or would they HAVE to absorb a photon that is equal to the distance between the ground state and the ionisation energy?(Original post by Llewellyn)
what equations do you know about wavelength? What equations could this tie into? Why would a photon be a special case?
well theres the standard c=f(wavelength) and also e=hf however cant you also have the de Broglie wavelength as well that would be h/mv... im not really sure why the photon would be a special case though...
What do you know about electrons; in particular their energy when it is not yet high enough to escape the atom/ metal? -
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Re: AS physics questionThis is the exam definition which you should definitely learn.(Original post by Pangol)
The particle model of light has as one of its assumptions the fact that it deals with a one-to-one interaction between electrons and photons.
I was wondering if anyone has revision notes for electricity. The theory questions keep tripping me up (like where you have to link an increase in resistance to a decrease in current) -
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Re: AS physics questionaah hence why only one photon can be absorbed at a time?(Original post by Pangol)
The particle model of light has as one of its assumptions the fact that it deals with a one-to-one interaction between electrons and photons.
yea will do(Original post by SuperFantasticB)
This is the exam definition which you should definitely learn.
I was wondering if anyone has revision notes for electricity. The theory questions keep tripping me up (like where you have to link an increase in resistance to a decrease in current) umm surely that electricity stuff would be in the textbook?
why cant we change the wavelength of a photon
Is this something to do with the dual wave partiality of light - as the charged particle emits wave packets of em waves (photons) - it can not have a definite wavelength as a wavelength varies over the distance of the packet?Last edited by coolstorybrother; 13-05-2012 at 21:28. -
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Re: AS physics questionWhat determins the wavelength of a matter particle? Its mass and velocity, as given in the deBroglie equation. So what would we have to do to change this wavelength?
What determines the wavelength of a photon? This is set in stone when the photon is generated. For example, the wavelength of radio photons is determined by the frequency of the oscillating electrons in a radio transmitter. What can we do to change them once generated? Assuming they don't cross from one medium into another, not a lot! -
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Re: AS physics questioni think i get it - the only factor we can change to alter the wavelength in a matter particle is its velocity. However we can not change the velocity of a photon as when it is created it cannot be changed? Thus we can not change the wavelength of a photon? Just to clarify, do we use c=f times lambda when it is a wave, and the de Broglie equation when it is a particle?(Original post by Pangol)
What determins the wavelength of a matter particle? Its mass and velocity, as given in the deBroglie equation. So what would we have to do to change this wavelength?
alter its velocity.
What determines the wavelength of a photon? This is set in stone when the photon is generated. For example, the wavelength of radio photons is determined by the frequency of the oscillating electrons in a radio transmitter. What can we do to change them once generated? Assuming they don't cross from one medium into another, not a lot!
Also, when a photon changes from one medium to another, is there literally nothing we can do? -
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Re: AS physics questionYou are right to say that the velocity of a matter particle is a contribritory factor to its wavelength, and is the only one that we have any control over.
The speed of a photon is not within our control. I'm sure you know what it is in a vacuum. It is different in other media, but there is no way to alter it within a medium.
Speed = wavelength x frequency is a vaild expression for all waves. So yes, for a photon, the wavelength os the photon is given by c/f, where c is the speed of light (in whatever medium you are considering).
And yes, the deBroglie equation is the way to determine the wavelength of matter.
