Why am I seeing variations in the reflected laser light when I look at the reflection from a diffuse surface?

Question

Here is the experiment. I reflected my tripod laser off of a CFB at my home. I noticed a strange effect and turned the light out to see it better. As I looked closely, I could see small variations in the reflected light. They looked like tiny points of light within the reflection. My first feeling is that it was an effect of the light hitting the photo receptor cells on my cornea. The problem with that was that as I move one way or another, or change the angle of view, I am moving along with the reflected dots. Has anyone seen this in a lab somewhere?

charcinders · Accepted Answer

I have no idea what a CFB is but I think I know the answer.

Because laser light is coherent and monochromatic when it hits any kind of diffuse object you get regions of constructive and destructive interference. These look like bright and dark spots to the eye, and create a speckle pattern.

If you point the laser at a surface whose structure is constantly changing you will see movement in the speckle pattern. Examples are a cup of tea with milk (brownian motion) and human skin (movement of blood cells). This movement in the pattern can be used to measure peripheral blood flow through the skin.

demongo · Answer

Sounds like you're seeing laser speckle.

It's a well-known phenomenon, a pattern resulting from mutual interference of out-of-phase wavefronts from the diffuse surface.

Anonymous · Answer

Laser Speckle

I don't usually do this (and I can't even believe I'm about to give a wikipedia link....<sigh>.....but it's appropriate here I think), but you can look the link over and get a little more info about it.

Trending News

Why am I seeing variations in the reflected laser light when I look at the reflection from a diffuse surface?

3 Answers