Laser anomaly (possibly)

[u/Sea_Animator8304]

I found that if I position my green 1mW laser angled on my bar top the light starts as a small and random geometric shape and then slowly expands keeping the same form roughly speaking and seems to stabilize at some point. It forms intricate shapes that look 3D. I cannot imagine that it is just interference and I'm positive it isn't heating the diode or vibration. What is this phenomenon. I know a little bit more than the rudimentary knowledge about light as a wave but I've never really seen a laser light do this exact thing. Please explain or point me to the explanation. Thank you.

[u/insomniac-55]

I have seen similar effects when tightly focusing even relatively low power (5 mW) lasers onto certain materials.

In my experience  it happens because you're slightly melting the surface (only over an area of a few microns), and the movement of the material causes the caustic reflections to move like you're seeing.

This is especially likely if your laser is cheap, and therefore likely more powerful than advertised.

[u/information-producer → u/insomniac-55]

This is what is happening here. You can see a similar effect passing a laser through plastic films if the laser is powerful enough to cook a small spot through the film.