# Narrow slits causes a diffraction pattern

1. Dec 1, 2008

### Tac-Tics

In the famous two-split experiment in your freshman college class, you learn that shining a light at a wall with two narrow slits causes a diffraction pattern.

Something that's always bothered me is in these ideal setups, we always assume the wall is a continuous barrier to the light. But the barrier is made up of a lattice of atoms. The interaction is never as simple as the idealized experiment. I'm curious as to what's actually going on between the light and the wall.

2. Dec 1, 2008

### mgb_phys

Re: Diffraction

In simple terms, the EM field in the light hitting the wall causes electrons in the atoms of whatever the wall is made of to move. Moving charges create an electromagnetic field at the same frequency - which forms the light emitted back at you.

By the time it gets to the screen, it doesn't matter if it came form a slit or just pointing the laser directly at the wall.

A far more puzzling question is how do you get a speckle pattern when you shine a laser at the wall - where is the laser ligth interfering ?

3. Dec 1, 2008

### Tac-Tics

Re: Diffraction

So the same wave of light hits two atoms, and they hesitate a second and then release the wave back out all in all directions, right? Since the atoms are not in the same position, it creates an interference pattern. That doesn't seem so puzzling. (Though I know single-particle interference might seem a bit more so).

One problem I'm having in understanding this is when does like travel like a ray and when does it get sent out in all directions? It seems that the light has momentum in a particular directions when it strikes an atom, so it would need to be moving the same direction when it is re-emitted. (But that contradicts what I just said about it being released in all different directions).