Question:
The 6563 Angstrom Ha line emitted by hydrogen in a star is found to be redshifted by 15 Angstrom. Estimate the speed with which the star is receding from the Earth.
Answer:
Wavelength of H0 line emitted by hydrogen,
λ = 6563Å = 6563 x 10-10 m
Star s red shift (λ1-λ) = 15 Å = 15 x 10-10 m
Speed of light (c) = 3 x 108 m/s
Let the velocity of the star receding away from the Earth be v.
The red shift is related with velocity
λ1-λ = vλ/c
v = c x (λ1-λ)/ λ
= 3 x 108 x 15 x 10-10/ 6563 x 10-10 = 6.87 x 105 m/s
Therefore, the speed with which the star is receding away from the Earth is 6.87 x 105 m/s.
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