# Crazy thing happens when I play with my laser

1. Oct 13, 2011

### beholderseye

I have a strong green laser, about 200 mw. I discovered this a long time ago and have wondered why it does this ever since...

I shine my laser at a mirror. It bounces back, with a brighter beam. That's not what confuses me. What confuses me is when I try to aim the bounceback beam to perfectly intersect with my current beam, I get this CRAZY pattern on the wall right behind my hands:

Kind of like this, but the waves get closer and closer together the further from the center.

I am so confused and confounded by this. Can someone explain the physics behind this?

2. Oct 13, 2011

### Vorde

I can't speak on the specifics of the situation without a picture of the 'ripples', but what is happening is that the light is interfering with itself which causes an interference pattern, I believe the pattern is circular when a beam is interfering with itself, which would explain this.

3. Oct 13, 2011

### beholderseye

Its weird though, because the pattern, while strongest when the laser hits itself, appears even when the beam is not touching itself but still being reflected off of the mirror.

4. Oct 13, 2011

### beholderseye

Very hard to get a picture of it too.

5. Oct 13, 2011

do it anyways, but the answer is interference.

6. Oct 13, 2011

### Vorde

I don't blame you!

I can imagine why that happens, and to me it definitely sounds like self-interference.

7. Oct 13, 2011

### beholderseye

Managed to get a video of it.

Last edited by a moderator: Sep 25, 2014
8. Oct 14, 2011

### SidBala

I would suspect that an interference pattern is being generated from the two interfaces of the mirror.

Try doing the same experiment with the shiny side of tin foil instead of a mirror. I would expect the pattern to be not present if the above hypothesis is true.

9. Oct 21, 2011

### Claude Bile

It looks like an Airy pattern, which is what you would get when scattering off a small particle.

If the laser is powerful enough, it can form a defect which it then scatters off in the manner that you recorded in your youtube video.

In short, it is likely scattering off a point-like source rather than interference per se.

Claude.