Question

What would the difference of spectral lines emitted by hydrogen gas in distant galaxies which is normally found at a wavelength of 21 cm on earth, be observed at 21.1 centimeters?

Answer 1

As we know by Doppler's effect

`(\Delta \lambda)/(\lambda) = (v)/(c)`

so here we know that

`\lambda' = 21.1 cm`

`\lambda = 21 cm`

from above formula we have

`(21.1 - 21)/(21) = (v)/(3 * 10^8)`

`(0.1)/(21) = (v)/(3 * 10^8)`

`v = 3 * 10^8 * (0.1)/(21)`

`v = 1.43 * 10^6 m/s`

so the galaxy is moving away from earth with above speed due to which we got larger wavelength