Question

A spectral line λ = 5000 Å in the light coming from a distant star is observed as a 5200 Å. What will be recession velocity of the star.

• A. $1.15 \times 10^{7}cm/\sec$
• B. $1.15 \times 10^{7}m/\sec$
• C. $1.15 \times 10^{7}km/\sec$
• D. 1.15 km/sec

Answer 1

Answer: (B)

$1.15 \times 10^{7}m/\sec$

Answer 2

$\Delta\lambda = 5200 - 5000 = 200Å$

Now $\frac{\Delta\lambda}{\lambda'} = \frac{v}{c} \Rightarrow v = \frac{c\Delta\lambda}{\lambda'} = \frac{3 \times 10^{8} \times 200}{5000}$

$= 1.2 \times 10^{7}m/\sec \approx 1.15 \times 10^{7}m/\sec$