Question

A rocket is receding from earth with a velocity $ 0.2c $ . It emits a signal of frequency $ 4\times {{10}^{7}}Hz $ . The frequency as observed on earth will be:

• A. $ 4\times {{10}^{6}}Hz $
• B. $ 3.3\times {{10}^{7}}Hz $
• C. $ 3\times {{10}^{6}}\,Hz $
• D. $ 5\times {{10}^{7}}Hz $

Answer 1

Answer: (B)

Apparent frequency $ v=v\left( 1-\frac{\upsilon }{c} \right) $ $ =4\times {{10}^{7}}\left( 1-\frac{0.2c}{c} \right)=3.2\times {{10}^{7}}Hz $