Question

The H line in Calcium is normally at 396.9 nm. However, in a star's spectrum, it is measured at 398.1nm. How fast is the star moving and is it moving towards the Earth or away from the Earth?
1- 904.3 km/s away from the Earth
2- 904.3 km/s towards the Earth
3- 907.0 km/s towards the Earth
4- 907.0 km/s away from the Earth

Answer 1

As we know by Doppler's effect of light we have

$\frac{\Delta \lambda}{\lambda} = \frac{v}{c}$

here we will have

[tex}\frac{398.1 nm - 396.9 nm}{398.1 nm} = \frac{v}{c}[/tex]

here by solving above we have

$3.01 \times 10^{-3} = \frac{v}{c}$

here we have

$v = 904.3 km/s$

since wavelength is increased so we can say that it is moving away

so correct answer is

1- 904.3 km/s away from the Earth