Q. A rocket is moving away from the earth at a speed of $6 \times 10^{7}m/s$. The rocket has blue light in it. What will be the wavelength of light recorded by an observer on the earth (wavelength of blue light $= 4600\ \mathring{\mathrm{A}}$)
A) $4600\ \mathring{\mathrm{A}}$
B) $5520\ \mathring{\mathrm{A}}$
C) $3680\ \mathring{\mathrm{A}}$
D) $3920\ \mathring{\mathrm{A}}$
Explanation
(b) $\frac{\Delta\lambda}{\lambda} = \frac{v}{c} = \frac{6 \times 10^{7}}{3 \times 10^{8}} = 0.2$
$\Delta\lambda = \lambda^{'} - \lambda = 0.2\lambda \Rightarrow \lambda^{'} = 1.2\lambda = 1.2 \times 4600 = 5520\mathring{\mathrm{A}}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.