Predicting Light Bounce: Mirror vs Wall

[Alkatran]

To start, I don't actually mean "how" (the means by which) but rather how to predict where it will bounce.

Is it always going to bounce off like it does in a mirror (first angle = end angle relative to line perp to mirror). If so, why is it when you shine a flashlight on a wall, you see the "circle of light" on the wall from any position (if it always bounced straight wouldn't the light seem to come fom somewhere else dpending on where you stood?)? Does light bounce off walls in a sphere-like shape?

 

[Janitor]

Don't take me too seriously, I am just yammering... I would say that the rule that angle of incidence equals angle of reflection (or whatever the right terminology is) applies when the surface is polished such that its roughness is small compared to the wavelength of the light striking it. For something as rough as painted wallboard, there would presumably be some sort of distribution curve of reflection angles vs. angle of incidence. The curve might have a peak where the two angles are equal, but not a real sharply-defined peak the way a polished surface does.

Maybe a good model for a rough surface would be a whole bunch of tiny little mirrors butted up to one another but pointing in directions in a chaotic sort of way.

 

[Adrian Baker]

It is called scattering, and yes, it is due to the roughness of the surface.
Non reflective camera lenses prevent any scattering by having an incredibly thin layer on the outside. So, when light hits a lens like this, non is scattered or reflected back - it all travels through.
It is clever how it works...

 

[Alkatran]

So, theoritcly, if you had perfectly smooth walls, and you turned a light on... it would be similar to a mirror in that the light would seem to come from the wall.. hmmm

 

[Chi Meson]

THere's a little more going on. Even if you polished a surface that was not metallic, you would still see the wall that is illuminated by your flashlight. THis is unlike a metallic mirror which would be "invisible" (if the mirror was ideal, and no dust was on its surface). That which goes on in non-metallic reflection is a case of absorption and reemission.

THe molecules in the surface of the "wall" absorb the light that falls on it. Some of the light turns into thermal energy and some of the light is reemitted. THe light that is reemitted does not have to follow the law of reflection, so it will be scattered in all directions.

Depending on the chemical make up of the wall's surface, the absorbed light will be of specific ranges of frequencies, and the reemitted light will be other ranges of frequencies. THis means that no matter how well you polish a red wall, it will still look red.

Metallic reflections operate slightly differently due to the "free electrons" that coat the surface of metallic conductors, so smooth metals do reflect according to the law of reflection.

 

[Alkatran]

Ok, so in a way the light actually does hit, then expand in a sphere, hit, expand... So brightness from an object is proportional to 1/DistanceTravelled^2 and the number of impacts... hmm

And I knew it would change color, I just meant it would seem like the light (with the new color) was coming from the wall.

 

[Janitor]

I like Chi's answer better than my own yammering.