Why does red mean stop and green go?

There are several theories about how red came to be associated with danger and green with safety, but the most common explanation is that it has to do with the way our eyes work.

The human eye has three types of cone cells that are responsible for color vision: short-wavelength-sensitive (S) cones, medium-wavelength-sensitive (M) cones, and long-wavelength-sensitive (L) cones. The S cones are sensitive to blue light, the M cones are sensitive to green light, and the L cones are sensitive to red light.

When we look at a red object, the L cones in our eyes are stimulated more than the other two types of cones. This causes our brains to interpret the object as being red. Red light also has a shorter wavelength than green or blue light, so it scatters more easily in the atmosphere. This means that red objects are more visible from a distance, which is why they are often used as warning signals.

Green light, on the other hand, has a longer wavelength than red light, so it scatters less easily in the atmosphere. This means that green objects are not as visible from a distance as red objects. However, green light is still bright enough to be seen easily, which is why it is often used for traffic signals and other safety signs.

In some cultures, the colors red and green have different meanings. For example, in some Asian cultures, red is associated with luck and happiness, while green is associated with death and mourning. However, in most Western cultures, red is associated with danger and green is associated with safety.