Question

How redshift is measured?

Answer 1

Doppler Effect.

Explanation:

Redshifts are measured using Spectroscopy. A spectrum of the Object whose Red Shift has to be deteremined is taken and is compared to the reference spectrum like the Spectrum of our Sun and the wavelengths measured in the Laboratories on Earth.

Red Shift is then calculated using the following relationship

z = (`lamda_o` - `lambda_l`)/ `lambda_l`

`lambda_o` is the observed wavelength.
`lambda_l` is the wavelength in the lab.
z is the red shift.

Now if the result is +ve then this would mean that the object whose spectrum was taken has a Red Shift and is moving away from us.

Likewise, if the result is -ve then the object is Blue shifted and is moving toward us.

Let's do this with the help of an example.

a typical hydrogen absorption line in a Lab on Earth appears at 656.3 nm. Now if you take a spectrum of lets say a Star and you found that the Hydrogen alpha absoption line appears at 658 nm then what would this mean?

z = 658 - 656.3/ 656.3

z = 0.00259

This means that the object is Red Shifted as the result is positive.

This is even more easily explained with the help of the following figure.