Question

The apparent wavelength of light from a star moving away from the earth is 0.02% more than actual wavelength. What is the velocity of the star.

• A. $30{kms}^{-1}$
• B. $60{kms}^{-1}$
• C. $90{kms}^{-1}$
• D. None of these

Answer 1

The correct option is B $60{kms}^{-1}$

Answer is B.
The apparent change in the wavelength is given as follows.
$\frac{\Delta \lambda }{\lambda }=\frac{v}{C},HenceV=\frac{\Delta \lambda }{\lambda }C$
$=\frac{0.02}{100}\times 3\times {10}^8{ms}^{-1}=60{kms}^{-1}$.
Hence, the velocity of the star is 60 km/s.