Floris But the speed of walking isn't a constant speed is it? I mean, I sometimes walk faster or slower, depending on if I have to go somewhere or not etc. So I'm not really sure if that's the best example...
Unless I'm misunderstanding you
I just used walking as an example to simplify the example.
Obviously light doesn't "slow down" as it gets closed to an object, like how a human would slow down to avoid bumping into someone.
Also, the speed of light is not constant, the speed of light in a vacuum is constant (maybe!!)
http://en.wikipedia.org/wiki/Variable_speed_of_light
Says "concept" right there
There are a few things to understand about light, one is that normally something is either a particle or a wave length, and secondly that 'something' (such as sound) requires a medium to pass through.
Light however, is both a wave and a particle.
And the wave requires no medium.
For the wave:
The velocity is the frequency times the wavelength.
For the particle:
These photons travel at a certain speed, say approx. 3 x 10^8 m/s
It's as constant as can get, and the absolute fastest possible speed in science.
All though they're hoping to confirm neutrons going faster, but nothing has been proven.
It goes "faster" through vacuum because it has no medium to pass through.
It goes "slower" through say water, because it has a medium to pass through.
The speed of light is the same in both instances. It just takes more time to travel through a medium as it's bumping into particles.
[here]-----------------------c------------------[there] == takes less long
[here]---c---p--c---------p-------c----p--c--[there] == takes longer
The "c" (constant), or, the speed of light is the same speed of light.
[edit] I actually found a video explaining it better than what I can do