# Diffraction and the Concept

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#### betterfuture

##### Full Member
7+ Year Member
I absolutely have no idea what the point of diffraction is or what it's useful for. The only thing I get out of it is passing light through a hole and getting patterns on a wall.

Could someone explain what it is and what the important equations are? I totally skipped over this chapter thinking it was not important, or as important so now when I am going over this stuff, I am seeing a lot of variations of the equations. How many equations are there for diffraction? Aren't there some for double slit too that are different from single slit? Thanks!!

Below is single slit and I believe the diffraction equation. Help

Light spreads out as it passes through the hole. The setup shows that light can behave like a wave (but remember that light can also behave like a particle). Brighter spots on the screen represent constructive interference and dark spots represent destructive interference.

Know the setups for double-slit and single-slit. The same equation applies to both: m * lamdba = d * sin(theta). In double-slit, bright spots form when the path length difference between waves is an integer wavelength apart (constructive) and dark spots form when the path length difference between waves is a 1/2 wavelength apart (destructive). In single-slit, dark spots occur when the slit width is a wavelength apart because light from the middle of the slit has destructive interference with light from the edge of the slit.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sindoub.html

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Light spreads out as it passes through the hole. The setup shows that light can behave like a wave (but remember that light can also behave like a particle). Brighter spots on the screen represent constructive interference and dark spots represent destructive interference.

Know the setups for double-slit and single-slit. The same equation applies to both: m * lamdba = d * sin(theta). In double-slit, bright spots form when the path length difference between waves is an integer wavelength apart (constructive) and dark spots form when the path length difference between waves is a 1/2 wavelength apart (destructive). In single-slit, dark spots occur when the slit width is a wavelength apart because light from the middle of the slit has destructive interference with light from the edge of the slit.

Is the equation you provided, which I believe is the same one I wrote above, the only equation for diffraction we have to know? And also, what does the sin (theta)=y/D help with figuring out? What angle is that?

@betterfuture Yes- know that equation and the concept of light as a wave. y / D is just an approximation to use for sin(theta). It's the angle noted in my figure above.

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So let me get this straight. If you have light coming from two different spots and they meet up and hit the wall at the same spot, if the path length difference between them is 1 wavelength, they constructively interfere (bright spot)? But if the path length difference is 1/2 wavelength, then they destructively interfere (dark spot)?

And, I'm sorry. You said y/D is an approximation for sin (theta). So y/D gives us an angle or you mean we take the inverse sin? Sorry, little slow here.

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So let me get this straight. If you have light coming from two different spots and they meet up and hit the wall at the same spot, if the path length difference between them is 1 wavelength, they constructively interfere (bright spot)? But if the path length difference is 1/2 wavelength, then they destructively interfere (dark spot)?

Yes For example, if you draw two sin waves and stack them up in phase you'll get constructive interference. But if you stack them out of phase by a 1/2 wavelength they will cancel out.

And, I'm sorry. You said y/D is an approximation for sin (theta). So y/D gives us an angle or you mean we take the inverse sin? Sorry, little slow here.

It's an approximation for sin(theta). So the sin(theta) term in m * lambda = d * sin(theta) gets replaced by y / D -> m * lambda = d * y / D

I personally haven't seen a question based on this setup yet. But it would probably be more conceptual like this: Based on your original figure posted, at point 2, which represents the path length difference between light waves?

a) lambda
b) 2 * lambda
c) lambda / 2
d) 0

Having recently read the reactions of the recent test takers about the heavy calculations on chem/phys and straight up physics/gen chem passages, similar to the ones on the old AAMC tests, I'd make sure I prepare for the worst

I personally haven't seen a question based on this setup yet. But it would probably be more conceptual like this: Based on your original figure posted, at point 2, which represents the path length difference between light waves?

a) lambda
b) 2 * lambda
c) lambda / 2
d) 0

I think none of the above because the answer would be 3/2*lambda, right?

OP, the reason why it's useful is that it's used for X-ray crystallography. The intricate details are too minute and advanced to go over here, but you basically shoot X-rays at a crystallized protein, get a diffraction pattern, and reconstruct the 3D shape of the protein from the diffraction pattern. It's very useful because that's how we know what proteins look like and thus how to design inhibitors.

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I just watched a video about X-ray crystallography a couple days ago and I thought that was related to Snell's law and index of refraction. I believe the atoms will interfere with the pathway of the light rays and deflect it to another direction. The atoms absorb some of the energy or something. And that's all I know. Some picture probably shows up on the screen of where the light rays bumped into the atoms or whatever, I am guessing. Haha!

I just watched a video about X-ray crystallography a couple days ago and I thought that was related to Snell's law and index of refraction. I believe the atoms will interfere with the pathway of the light rays and deflect it to another direction. The atoms absorb some of the energy or something. And that's all I know. Some picture probably shows up on the screen of where the light rays bumped into the atoms or whatever, I am guessing. Haha!

It actually has more to do with Bragg's law and you can actually calculate the DNA angles, etc. using X-ray diffraction - that's how Watson and Crick (or rather Rosalind Franklin) did it back in the day.

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