What kind of reflections are dangerous with a 2W green laser?

Factual Questions
21

Not necessarily. Your green thing reflects light, but it’s diffuse, rather than specular reflection. You’d get similar reslts from pointing the laser at a diffuse white wall.

By the way, just because a surface looks the sam shade of green doesn’t mean that it contains the same spectrum as your laser. There are an infinite number of spectral combinations that look the same to your eye, in addition to the green corresponding to that color. It’s even conceivable to formulate pain hat oesn’t contain any reflection at the laser wavelength at all, yet looks just like it. An interesting challenge.

22

Or going the other way, you can make a surface that reflects only one specific narrow wavelength, or a handful of them. Since such a surface absorbs almost all of the light that hits it, under broad-spectrum illumination like sunlight it’ll appear black. But you can project on it using a projector that only produces those narrow bands of light. This can be used to make projectors and screens that eliminate almost all glare, and hence can be used even in broad daylight.