# Homework Help: Fog and light

1. Feb 11, 2008

### An Architect

light and particles in water

I'm an architecture student with a very basic knowledge of phyics! My project involves shining a beam of light through water that contains particles (in this case salt). What I want to know is why the brightness of the illuminated particles changes when you look at it from different angles i.e. the beam almost disappears when you stand perpendicular to it and the intensity increases as you walk through a 90 degree angle to stand parallel to the beams of light (caught in the headlights!). Obviously, when you stand facing the beam of light its at its brightest, but why is is it that you don't see the reflection of light off the salt when standing perpendicular to the beam direction?

Last edited: Feb 11, 2008
2. Feb 11, 2008

uh ? Its optics!

When you stand in front of a headlight, the intensity is more because the light rays are coming from the headlight straight to your eyes. In the other case, they aren't.

3. Feb 11, 2008

### mgb_phys

2 simple rules
Light travels in straight lines
You can only see light which ends up in your eye!

So you can only see light which either goes directly form the source to you or is reflected from another object.
In the case of the fog tank - you can see the brightest light looking straight through the tank toward the light source.
As you go to the side you see light which has been only slightly deflected by hitting a few particles, then at 90deg you onlysee light which happens to have bounced off particles at exactly 90deg. Finally as you go round the back it brightens again because you see light which has bounced back from any particle in the fog.

It's all a question of probabilities and the number of available paths - it's a bit like the path of a ball in a pinball machine.

4. Feb 11, 2008

### An Architect

equivalent set up but with smoke

ok, I understand.

I've set up a similar experiment projecting a beam of light through smoke (rather than particles and water) but in this case you can see the beam of light refected off the smoke particles from whichever angle you view it. How come there's a difference?

5. Feb 11, 2008

### mgb_phys

The scattering depends on the size of particles.
For paticles roughly the size of a wavelenght of light raleigh scattering (http://en.wikipedia.org/wiki/Rayleigh_scattering) is th emost important, this is about the size of dust in the air. Smoke particles are a little bigger so you don't get the colour effects - they scatter all wavelengths similairly.

Re-reading your original post, you sholuldn't get much scattering from salt in water - it dissolves so there shouldn't be any particles to scatter from?