# I LASER beam not coherent

1. Jan 21, 2017

I have a red 650nm and <5mW LASER pointer, but the beam is not very coherent. Actually, when I point to a wall at an approximately 300 meters away, the red point gets very large (about a meter in diameter). I wonder how a LASER could act like this?

2. Jan 21, 2017

### blue_leaf77

Beam diameter is not a measure of coherence.
That's an expected behavior and well-known for laser beam, the reason is diffraction. See this article to get the idea about beam divergence over distance.
As for numerical values, assuming the typical beam divergence for laser pointers which is 1.2 mrad and the beam waist is very close to the laser's output aperture, the beam diameter at 300 m distance would be approximately 300*1.2 = 360 mm = 0.36 m. Since this diameter is measured from one half radial distance from the center, the visible area must look bigger than this. I wouldn't be surprised if it looks like almost 1 m as you observed.

3. Jan 21, 2017

### Andy Resnick

As blue-leaf77 mentioned, diffraction is not the same thing as coherence. Even so, 'raw' laser beams can indeed have a low spatial coherence- this is manifested by speckle.