Question

Imagine that a motorist is travelling so fast that the yellow light (585 nm wavelength) from a traffic signal appears green (555 nm wavelength). Find the velocity of the motorist, assuming the positive direction is defined as moving closer to the light.

Answer 1

Write the equation for Doppler effect:

$$\lambda'=\frac{c+v}{c}\lambda,\\\space\\ v=c\bigg(\frac{\lambda'}{\lambda}-1\bigg)=16.2·10^6\text{ m/s}.$$