A star is moving away from earth and shift in spectral line of wavelength 5700 Ao is 1.90Ao. Velocity of the star is

50 km s–1                     70 km s–1  80 km s–1 100 km s–1

100 km s–1

$\Delta \lambda = \frac{v}{c}\lambda $ $\Rightarrow v = \frac{\Delta \lambda}{\lambda}c = \frac{1.9}{5700} 3\times 10^8 = 10^5m/s = 100km/s$