The power of a reflected beam

Does anyone know the power of a 45mW beam reflected in a window. How many percent are reflected, how many percent are absorbed and how many percent goes through the window.

It depends on the window /ubbthreads/images/graemlins/grin.gif
I would say roughly 50% transmission (and of course 50% reflection). But that's just a guess. If you need to split a beam into two equally powered beams there are beam splitters made with specific transmission/reflection properties... A little gets lost due to absorption but at 45mW you won't see that.

This assumes your beam is not normal to the surface (perpendicular).

I'd like to know abit more about laser reflection too,

Just say my IR filter came off my laser for some reason, does IR reflect in a similar way to visible light? How about UV light? Do IR and UV go through glass?

UVA goes through glass, but typically, UVB and UVC wavelengths are filtered out (absorbed).
This is with normal household window glass.

UVB and UVC, are those the waves which give people a sun tan/skin cancer?

What frequency are they at?

UVA (long wave UV) is generally considered to be the range from 315-400nm.

UVB (medium wave UV) is generally considered to be the range from 280-315nm.

UVC (short wave UV) is generally considered to be the range from 200-280nm.

"Vaccume UV" is generally considered to be wavelengths below 200nm. The lower range is sometimes quoted as 10nm, and sometimes 100nm. It is absorbed by air within a few inches, so it really requires a vaccume to propagate.

UVB, UVC, and vaccume UV are bad for the skin and eyes. UVB causes corneal fogging, suntans, sunburns, and skin cancer; and UVC is generally considered to be germicidal.

PS Longwave UV is what a blacklight puts out and what makes those Grateful Dead posters glow...