Red light

[Omid]

Why the stop lights at the back of cars are red ?
Some say because red light is visible from far distances.
If that's true, is any relation between this property of red light and its wave length ?

[Nenad]

yes, because red light has a higher wavelength (750nm), this means it has the abillity to diffract more, since waves with larger wavelengths diffract more. This is a problem with the auromobile industry, the light up dials on a dashboard are usually blue, they should be red, since blue is the hardest colour to see. Do an experiment, place blue light on a clock and watch the time pass, and then do it in red light, youll see the difference clearly.

[rayjohn01]

I believe the easiest light to see is yellow ( sunlight ) but we are accustomed to certain colors meaning something as well i.e
yellow -- warning
red --- danger
green -- no problem

[Nenad]

the eye is the most sencitive to yellow and green light, that is true, but red light gives the most detail because of its diffraction abilities.

[Doc Al]

...but red light gives the most detail because of its diffraction abilities.
Red light would give the least detail since it diffracts more than blue light.

[LURCH]

I think that RayJohn has the right idea; the reason brake lights and stoplights and stop signs are red has more to do with psychology than the physics of red light.

[Nenad]

Nope, I know this for a fact, it might be used for psychological reasons, but red light allows you to see in better detail tha blue light. Doc Al puts up a good point, but try it for yourself, youll see the result.

[Janitor]

Some studies have indicated that yellow vehicles are the easiest for drivers to see at a distance or in bad weather, and for that reason some municipalities have yellow fire trucks. From that I might think that yellow lights would be the easiest to see at a distance or through fog.

At any rate, I always assumed that red was chosen for stoplights because of its association with spilled blood, i.e. Danger ahead!