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How fast should you drive towards a red traffic light to see it appear as green? traffic light


To calculate the speed for which the traffic light appears to become green, it is necessary to apply Einstein's relativity theory. One can show that
L/L_0 = (1 - v/c)^(1/2) / (1 + v/c)^(1/2)
In this formula, L_0 is the wavelength of the red traffic light and L the observed wavelength of somebody who drives with speed v towards the traffic light. The speed of light c = 300,000 km/s.

If you solve this equation with L = 500 nm and L_0 = 700 nm, you will find that v/c = 0.32. Hence if you are driving with about 300 million km/h (0,3 times the speed of light), you will see a green traffic light.