# Diffraction - how does it actually work?

1. Oct 29, 2013

### bfusco

diffraction -- how does it actually work?

In diffraction for light of a single slit, i know that a bright spot appears in the center and subsidiary bright spots to the sides of the central one, but I have a hard time explaining how diffraction works.

I need some help coming to a satisfactory explanation for why there are dark fringes although the light through the slit propagates in all directions. It was explained to me that dark fringes occur if that angle satisfies asin$\theta'$=$\delta$=$\lambda$ (based on the image provided), but i dont really know what that means. I understand that there is a phase difference between the light propagating from all points between the slit that correspond to the difference the light has to travel at each point, but beyond that I dont understand, how does the math i provided describe dark fringes and light fringes?

Also if there is a double slit there are also fringes which have some mathematical relation that similar to the one i stated above, but not the same. Im not sure of the relation and I could really use an explanation.

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Last edited: Oct 29, 2013
2. Oct 29, 2013

### Simon Bridge

How do you mean "actually"?
What sort of thing would be a "satisfactory" explanation?

Nobody knows how the light ends up in the pattern it does - all we know are a bunch of rules for predicting the pattern that we call "models".

What your teachers are telling you is a wave model - The linchpin is to realize that the light from every point inside the slit may propagate in all directions. So you can replace the slit will a row of point sources that are circular (in your diagram).

The light from each of the point sources adds up on the screen to give the total brightness of that part of the screen. But, since light is a wave, all the bits add up as a wave ... which means they sometimes cancel out by destructive interference. The center of the dark fringe is where you get maximum destructive interference.

The double slit is exactly like the single slit except that you block off some of the middle, so you don't get contributions from some of the point sources that you did before.

For more details look up "fraunhoffer diffraction" - but if you are having trouble with the path-difference and wavelengths thing then you may have to learn some more math before this will make sense.

You may prefer a particle-based explanation.
http://vega.org.uk/video/subseries/8
... see all the lectures.