Question

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 Å)

• A. 4600 Å
• B. 5520 Å
• C. 3680 Å
• D. 3920 Å

Answer 1

Answer: (B)

5520 Å

Answer 2

$\frac{\Delta\lambda}{\lambda} = \frac{v}{c} = \frac{6 \times 10^{7}}{3 \times 10^{8}} = 0.2$

$\Delta\lambda = \lambda' - \lambda = 0.2\lambda \Rightarrow \lambda' = 1.2\lambda = 1.2 \times 4600 = 5520Å$