Question

The wavelength of light of a particular wavelength received from a galaxy is measured on earth and is found to be $5 \%$ more that its wavelength. It follows that the galaxy is:

• A. Approaching the earth with a speed $3 \times 10^{7} ms ^{-1}$
• B. Going away from the earth with a speed $1.5 \times 10^{-7} ms ^{-1}$
• C. Approaching the earth with a speed $1.5 \times 10^{7} ms ^{-1}$
• D. Going away from the earth with a speed $1.5 \times 10^{7} ms ^{-1}$

Answer 1

Answer: (D)

As wavelength is more that means galaxy is receding away from earth at speed given as
$v=\frac{\Delta \lambda}{\lambda} \times C=0.05 \times 3 \times 10^{8}$
$=1.5 \times 10^{7}\, m / s .$