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Q & A: light diffraction

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Diffraction & Interference
Q:
What causes light to bend around small objects like wire or impurities?
- Kirsten (age 17)
Amarillo Texas US
A:
Light is a form of wave. (The wave is made of electric and magnetic fields, but that's not too important for the explanation.) Think of waves you can see- say waves on water. When the wave runs into, for example, a pier, you can see the ripples spread out. You don't get just a sharp edge where there's wave on one side and nothing on the other. When you write down the equations for how the water pushes on nearby water, they tell just how much the wave will spread out. This whole effect sometimes is called diffraction. (Names are worth something because they make it easier to search for more information.)

The same thing happens for light waves. The reason you often don't notice it is that the wavelength is very short. For big objects, that makes the spreading not very important. For little wires or little holes, it's a major effect.

Mike W.

(published on 03/06/2011)

Follow-Up #1: diffraction of light

Q:
My response to a question posted on Physics Forum. Your comments welcome! Nacho: Certainly light interacts with the edges of slits. The interaction actually 'refracts' those photons, electrons, etc. that 'hit' the very thin slit edges. Refraction is induced where photons hit very thin parts of the slit (ideally monoatomic +/-thicknesses). Photonic energy is transferred to outer electrons of the slit material atoms, raising their (atom electron) energy levels. As the electron energies fall-back to their 'normal' state, the excess energy is emanated as secondary photonic energy - probably of a frequency close (or harmonic?) to that of the impinging photon. So the poorly-understood "bending of light" around slit edges is dominantly a refraction process. The process does not work well when slit material thickness exceeds the ability of the impinging photon to penetrate the material completely. This explanation 'works for me' . . . no one has been able to explain this ever since Young first discribed the diffraction phenomenon. I have a brief write-up on this "theory", send me an email and I'll forward it to you. Please nominate me for the Nobel Prize in Physics!! . . . haha! Regards, Bill
- Bill Mansker (age 66)
Albuquerque, NM
A:
Bill- I wish I could say that was close, but it's pretty far off.

1. Diffraction (this isn't "refraction") works with fairly thick slits.
2. The wave penetrates even a good conductor to roughly one "skin depth", in this case many atoms thick, not about one atom.
3. The wave isn't absorbed and re-emitted later as fluorescence, but simply scattered.  There's generally no frequency shift and no loss of coherence with the directly transmitted wave, unlike the picture you gave.
4. The phenomenon isn't "poorly understood". The basic effect was understood when Young presented his results to the Royal Society in 1803. It's been understood in greater depth since the development of Maxwell's equations in the 1860's. You can derive the entire thing from Maxwell's equations, just given the conductivity and geometry of the slit material.

Here's what puzzles me.  On this site we make some mistakes, but we try to stay away from the topics where we're clueless. You represent a very large group of people who choose to answer questions on the Web on topics with which you aren't familiar. Why do you all do it?

Mike W.

(published on 07/20/11)

Follow-up on this answer.