Turning the red light green

What’s the best thing about having a rocket-powered car? If you go fast enough, the red lights look green!

That's an old physics joke on the nature of light, here is more information on colour and the electromagnetic spectrum.

Light is electromagnetic energy, and it comes in waves. What we see as different colours is actually just different frequencies of these waves. The lower frequencies appear to us as colours down at the low end of the spectrum, such as red, orange, yellow. The higher frequencies appear to us as colours at the high end of the spectrum, such as green, blue, and violet.

Now, one neat thing about colours is that you can effectively change their frequencies by moving toward or away from their source. Think of it this way. Imagine that a line of evenly-spaced joggers is running toward you. If you stand still, they will run past you with a certain frequency, such as one jogger per minute. If you run towards them, you can increase that frequency; say to two joggers per minute or more.

It’s the same with light waves. A certain low frequency coming from a stoplight is perceived as red. If you raced towards the light very fast, you could increase the frequency you perceive until the color appeared to be green.

Now, it has to be admitted that to really do this you’d need to go twenty-thousand miles-per-second, much faster than any car, or even rocket, can at present move.

Also, you should never run a red light, but that’s taking the fun out of the joke.