Question

Suppose a rocket ship is receding from the earth at a speed of 2/10 ^th the velocity of light. A light in the rocket ship appears blue to the passengers on the ship. What colour would it appear to an observer on the earth? 

(a) Blue 

(b) Orange 

(c) Yellow 

(d) Yellow-orange

Answer 1

The correct option is: (d) Yellow-orange

Explanation:

The observer on the earth will detect a wavelength given by

Where   λl = original wavelength 

λ₀ = apparent wavelength 

v = velocity of source

Here, source is receding, therefore +ve sign will be taken

The wavelength of blue light varies from 450 nm to 475 nm. Therefore, the observed light will have wavelength ranging from 562.5 nm to 593.75 nm which will be yellow-orange.