mjkzz wrote: ↑Fri Jan 21, 2022 5:22 pm
1. If I shine a laser onto a glass at 45 degree angle (same as shining straight to a 45 degree angled glass), and the glass is 1mm thick, with Rik's analysis, do you see multiple dots on projection plane, spaced at 1.414mm (square root of 2) apart? Is this really true for beamsplitters?
The spacing is not exactly that, because the angles change when the light enters the glass. But as to the multiple dots, the answer is simply "yes".
I set up a casual demo just now, consisting of a sheet of window glass, roughly 3 mm thick, a laser pointer with an aluminum foil pinhole, a white target, and a camera with macro lens, shooting a picture of the laser dot with and without the glass in place.
2. If I shine a laser onto a glass at different angle, say, 44.5 degress, do I get the same?
Sure -- what would be special about 45 degrees?
I remember some phenomenon that when light travel in a denser medium (hence I agree with Ray's idea of trying different material), they get trapped and never get out, I do not remember what this is called.
This is called "total internal reflection", and it cannot be made to happen by shining light onto one of the two parallel sides. The reason is that any light entering the glass will be refracted to an angle that is too steep to be totally reflected on the other side. See
https://en.wikipedia.org/wiki/Total_internal_reflection , especially figures 5 and 6 which show the angles.
3. If I shine a laser at some extreme angle at the glass, do I even get a dot coming out of the other side of glass?
Have you tried it? Or for that matter, have you tried looking through a window at a steep angle? Is there any angle at which the window glass becomes opaque?
Or maybe those secondary beams are definitely there, just very weak?
Yes, just so!
We can calculate how weak they are. If the glass surface is untreated, then at 45 degrees the reflection is certainly less than 10%, probably closer to 5%. Even if it's 10%, then the reflection of the reflection will be only about 10% squared, roughly 1% of the light transmitted straight. If it's only 5% at one surface, then the double bounce is more like 0.25% of the light transmitted straight.
As a side note, this is very different from the issue of ghosting with a second-surface mirror, where the ghost from the front surface needs only one bounce and is correspondingly brighter.
Anyway, yeah, the double-bounced ray is quite weak, and that one fact is all that's needed to explain why it's not a problem except in the most demanding situations.
All your other convoluted explanations are not needed and mostly aren't even right, but I have no enthusiasm to try explaining where the flaws are.
One point does merit further comment:
lights reflected by the glass reaching subject are surprisingly parallel and directional
Imagine yourself at the subject's location, looking upward at the glass. What you see looks just like the light source, as if it were positioned above shining down, and possibly clipped by the edges of the reflecting glass. If light striking the subject is surprisingly parallel and directional, then either that's what the light source is putting out to be reflected, or the reflector is small and/or distant so it's clipping the reflection. So, if you want less parallel and directional illumination, you need a larger/closer reflector, and a light source that is correspondingly large and diffuse, so as to fill the reflector as seen by the subject.
In any event, nothing about my analysis assumed omnidirectional light sources. If some of the black paths are not actually occupied, the red paths still act the same.
--Rik