Shining a laser onto a mirror Theoretically, if I shined a laser at a mirror at an angle of 0 degrees so that the light was perfectly reflected back to the light source, then I should not be able to see the light because it is not reflected to my eyes.
|
|  me
|
|============ laser
|
|
|

However this is evidently not the case, as I am able to see the laser point on the mirror. Why is this the case? My initial assumption is that it is due to diffuse reflection on the imperfect mirror's surface, but I am not sure.
 A: Your question touches on several issues that confound the "perfect" situation:
Scattering
This will allow you to see the laser light at angles other than the reflected (specular) direction.  This can (and does) come from:


*

*Dirt on the mirror

*Imperfections in polishing (surface roughness)


Coherent Addition / Subtraction
If you are arranged "perfectly" such that the reflection is coming back towards the laser, this sets up another resonator.  (Lasers are resonating cavities:  https://en.wikipedia.org/wiki/Optical_cavity)  This causes all sorts of problems!


*

*Mirror is closer than the coherence length of the laser


*

*Constructive / destructive interference will cause the laser oscillating frequency to lock onto the new cavity (https://en.wikipedia.org/wiki/Fabry%E2%80%93P%C3%A9rot_interferometer)

*The frequencies emitted by the laser will then be constrained to whatever resonates in this new "double" cavity


*If the new "double" cavity is farther away than the coherence length of the laser, then the light returning to the laser is not "coherent" with the laser and will not have the same effect on the laser.


*

*The light may interact with the gain medium, causing the laser to be unstable


A: There are 2 things:


*

*you might see a spot on the mirror itself, especially if dirty or poor quality surface.

*you already see a laser beam going toward your eye even when you are a bit out of its axe (and you should better put your eye out of its axe !).
It's because a bit of light is scattered in light, and at at grazing view angle (either direct or through the mirror) you traverse a huge region close to the beam, and not when you look orthogonal to the beam. Quite like for god rays through clouds.

A: Also, if the mirror you use is not surface silvered (or aluminised), there will be internal reflections between the front surface of the mirror, and the silvered rear surface.  This show up any imperfections in the mirror - ie its surfaces not being flat, and any irregularities in its composition.  These will all result in light being scatterred, rather than reflected precisely back to its source.
A: You are correct in thinking that the spot that you see is due to diffused reflection from the poorly polished or dirty surface of the mirror. We need to remember that we see things only because the light that gets reflected off of things reaches our eyes and is detected by our retina. That is the reason why we can't 'see' light because until and unless the light ray that we want to observe is directly aimed at our eyes, we can't detect it.
