Effect of water on light....

[matt_london]

If you place a pole in water it appears to point at an angle from the
point where it enters the water. Why does this happen
when the speed of light is always the same?

[Hamish]

The speed of light is constant in a given medium (ususally quoted as speed of light in a vaccuum). It is the change in this speed which results in the opical illusion.

[Loosehead]

yes hamish is correct, the speed of light is not constant, it varies depending on the medium.

[matt_london]

So, would you measure a different speed if you were in the water with

 the light or only if you were in a different medium?

[jake-the-peg]

Measured it in water?

Yes, more importantly the angle that light is bent (refracted) going between media depends on the differing speeds it's called Snells law

Have a look here for diagrams

http://www.ms.uky.edu/~skim/SpeedOfLight/

[3shortplanks]

I didn't think it was anything to do with speed rather its to do with the density of the differing materials bending (refracting) the light into a different direction. Your not trying to suggest that there is a significant enough change of speed from 186,000 miles per second to notice the difference visually?

[gen2]

3shortplanks, It is the difference in speed that causes the refraction.

[3shortplanks]

Well! I'll go to the foot of our stairs!

[ugly_bob]

Light travels slower in water than air. When it hits a water-air barrier at an angle it bends at an angle proportional to the ratios of the medias refractive indexes.
If the light hits perpendicular to the barrier, it will not refract, but still slow down.

[Pottsy]

It is similar to driving into sand from concrete. If you drive perpendicular to the boundary, you will simply slow down, if you approach at an angle, the wheel hitting the sand first will slow down and turn you towards the area of lower velocity (the sand). the same happens with the light wavefront, hence the bending of it (refraction) due to it changing velocity when it enters the water.