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Why Red, Yellow and Green?

By: Janavi Gupta

How many times a day do we drive past traffic lights when they turn green? Or cry in frustration when they turn red just as we approach them? How annoyed does one get when they turn yellow on the very day that one is getting late for an important presentation! However, have we ever wondered why the colours are so? In this article, I aim to explore that very topic.

Follow, Follow, Follow,

Red, green and yellow.

Traffic lights play a very big role in our daily lives. Basic activities such as commuting become a major hassle for people without them. Traffic jams created by the absence of traffic lights are inconvenient for commuters and especially dangerous for pedestrians who wish to cross the road. Much of our lives are dictated by these lights. Sure, we all know that red stands for stop, yellow tell us to slow down and green signals go. However, how many of us know why they are this way?

Light Scattering is defined as the process of absorption and re-emission of light energy by the dust particles and air molecules present in the atmosphere. The wavelength of light (λ) on the other hand is the distance travelled by the light ray in one time period (T) (time taken by the particle to complete one vibration about its mean position) of particle vibration. The air molecules of size smaller than the wavelength of incident light absorb the energy of incident light and then re- emit it without change in its wavelength. The scattering of light is not the same for all wavelengths of incident light. The intensity of scattered light (I) is mathematically found to be inversely proportional to the fourth power of the wavelength of light.

According to the equation:

Velocity of light = frequency of light * wavelength of time,

V = f * λ

Since Time period = 1/frequency,

T = 1/f

F = 1/T

Therefore, V = 1/T * λ

A picture showing the splitting of white light into its constituent colours.

Light is an electromagnetic spectrum made up of a visible and invisible spectrum. The visible spectrum is the one which we can see, and is made up of 8 colours: violet, indigo, blue, green, a double band of yellow, orange and red, just like in the rainbow. These are the constituents that form white light. The wavelength of violet light is the smallest ( approx. 4000 angstrom) and that of red light is the largest ( approx. 8000 angstrom) hence red light is scattered the least and violet is satteder the most. Hence, red light as compared to the other colours constituent of the visible spectrum can penetrate to a longer distance without becoming weak. Thus it can be seen from the farthest distance as compared to light of different colour having the same intensity. Hence, red is used to signal stop so that it may be seen from long distances even in hazy conditions. Similarly, yellow (approx. 5780 angstrom) with a greater wavelength than green (approx. 5000 angstrom) is used to signal to slow down and green, which is scattered the most among the three is used to signal to go. So, next time you get stuck behind a red stoplight, take a moment to realize why you stopped in time...


Selina Publishing Co. Concise Physics Class 10 ICSE


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