Traffic lights the earliest time only two colors red, green, later improved, increased a yellow light, red light says stop, yellow light said, green passage. Are using these three kinds of color, on the one hand, is the primary colors, other colors is also raised out on the basis of this, on the other hand, in these three kinds of color as a traffic signal and the structure of people's vision and psychological reaction.

 

Human retina contains rods and three kinds of cone photoreceptors, particularly sensitive to yellow light rods, three kinds of cones, respectively for red, green and blue light is the most sensitive. Due to this kind of visual structure, the most easy to distinguish between red and green. Although it is easy to distinguish yellow and blue, because the eye is sensitive to blue light of photoreceptor cells is less, so the color, or in the red, green is preferred.

 

Thus traffic lights are not arbitrarily set.

 

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