Topic: Wavelength.
Why red is used as a danger signal?
Well.
Because my teacher said so.
As usual, you are wrong.
The major reason for red to be used as a danger signal is its wavelength.
I know.
Wavelength is the distance between two sea waves.
No.
In the wave theory of light.
The distance between successive crests or troughs is called wavelength.
Longer the wavelength, lesser will be the scattering of light.
Now, among the colors of visible light, red has the longest wavelength.
Hence, it is least scattered by the atmospheric particles.
As a result, whether it is fog or smoke, red light passes comparatively easily through them.
Thus, it can be seen from maximum distance, making red color the right choice to convey any danger.