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Author Topic: What happens when light is reflected ?  (Read 12039 times)

Offline chris

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What happens when light is reflected ?
« on: 05/08/2004 13:31:58 »
I was asked the other day what actually 'happens' when light is reflected by a surface. I had to admit that I didn't properly know.

What makes some surfaces reflective, and what is actually happening, at the level of the photon, during a reflection ?

Presumably it's to do with the wavelength of the photon and how it interacts with the chemical structure of the surface concerned ?

Chris

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Offline qpan

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Re: What happens when light is reflected ?
« Reply #1 on: 05/08/2004 15:55:05 »
Well, i'll hazard a guess. Pretty much all surfaces are reflective or partially reflective (otherwise we wouldn't be able to see them). However, usually this reflection is diffuse reflection, where the photons are reflected in random directions due to the rough nature of the surface. (Only random as perceived by the naked eye - light always follows incident angle = reflected angle)

I believe that reflection is caused by light meeting a non-absorbing surface, in which case the wave simply bounces away. Mirrors are just very flat surfaces which do not absorb any (or at least very little) light from the visible spectrum.

Surfaces only absorb light if the wavelength of light corresponds to the length of chemical bonds in the substance or (switching to particle theory of light) the energy per photon corresponds with the energy required to promote an electron into a higher orbital (photoelectric effect in metals).

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Offline gsmollin

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Re: What happens when light is reflected ?
« Reply #2 on: 05/08/2004 18:25:48 »
I can begin to answer that question, on a classical level. It does depend on wavelength, but whatever the wavelength, there must be a discontinuity in medium to get a reflection. For most electromagentic waves, the best total reflectors are conductors. At the conducting interface, the electric wave, E, becomes zero. The magnetic wave, H, doubles in amplitude and reverses phase. The reflected wave has a 180 degree phase reversal from the incident wave.

You can visualize this by tying a rope to a doorknob, and standing back about 10-20 feet. A sharp jerk on the rope will send a traveling wave to the knob, where it will be reflected with a visible phase reversal. Of course, an electromagnetic wave has two components, E, and H, at right angles to each other.
 

Offline chris

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Re: What happens when light is reflected ?
« Reply #3 on: 10/08/2004 09:03:23 »
Thanks guys, for tackling the question, but I am not totally satisfied yet !

What I really want to know is what is happening at the level of the photon when it meets a reflective surface. How is the 'wave' reflected and why is there a phase reversal ? Are some photons quite literally bouncing off the surface ? A red book looks red because it absorbs all wavelengths except red (in the visible spectrum at least) so what is happening to the 'red photons' to result in their reflection as opposed to their absorption ?

Chris

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Offline qpan

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Re: What happens when light is reflected ?
« Reply #4 on: 10/08/2004 13:31:07 »
The other visible photons which "are not red" are all absorbed by the material, usually being converted to thermal energy (by imparting its momentum to the other particles). This is usually due to being absorbed by bonds, collisions or promoting electrons (which propagates a further photon release at a later time). During collision, if the photon's energy corresponds to the atom's resonance energy, it is absorbed and the atom becomes excited. It later emits the photon to return to its unexcited state. This is what happens to "red" photons - they are absobed and then re-emitted at the same energy level. Other photons with different energies are absorbed too, but due to their energy levels they may be radiated out as photons in the infra-red range.

There is no surface which would not absorb any "red" photons at all, but if something appears red, then it will not absorb many "red" photons compared to all the other photons.


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« Last Edit: 10/08/2004 13:32:03 by qpan »
 

Offline tweener

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Re: What happens when light is reflected ?
« Reply #5 on: 11/08/2004 03:22:48 »
I remember covering this in a solid state physics class:  What happens at an atomic/photonic level when photons impinge on a surface (I believe we studied metal).  I remember that the momentum transferred to the metal was two times more if the photon was reflected rather than absorbed.  I also remember that the mechanism was not absorption/re-emission, as that would not support the derivation of Snell's law for a reflective surface.  Unfortunately I'm shooting from the hip and don't remember any details.  Sorry.

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Offline qpan

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Re: What happens when light is reflected ?
« Reply #6 on: 12/08/2004 09:44:50 »
Snell's law only applies to mirrors. When we see colour, this is an example of diffuse reflection which does not have to obey Snell's law. On mirrors, i do believe that the light simply bounces off, which is why incident angle = reflected angle.

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Offline gsmollin

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Re: What happens when light is reflected ?
« Reply #7 on: 12/08/2004 19:27:40 »
You could always do some searches on this. Here are a few hits:

http://www.thespectroscopynet.com/Educational/reflection.htm , not too technical.

http://www.lodestar.com/research/shearedflows/reflection.html , terse, but there may be more there if you look.

http://ntrp.vin.bg.ac.yu/2004_1/Markovic2004_1.htm , more serious.

http://www.newton.dep.anl.gov/askasci/phy00/phy00232.htm , less serious.

Well that's a few, and there are many more.
 

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Re: What happens when light is reflected ?
« Reply #7 on: 12/08/2004 19:27:40 »

 

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