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# Question and Answer

## QuestionPhysicsClass 12

What speed should a galaxy move with respect to us so that the sodium line at  $$589\;\text{nm}$$ is observed at $$589.6\;\text{nm}$$ ?

Given data,
Initial wavelength  $$\lambda _0=589\;\text{nm}$$
Final wavelength  $$\lambda =589.6\;\text{nm}$$
$${\Delta}\lambda =589.6-589=0.6\;\text{nm}$$
We know by Doppler’s effect,
$$\frac{-{\Delta}\lambda }{\lambda _0}=\frac{-v} C$$
Where,  $${\Delta}\lambda =$$ shift in wavelength, $$v=$$ speed of galaxy,  $$C=$$ speed of light
Putting values, we get,
$$v=3.06\times 10^5\left(\frac m s\right)$$
$$v=306\;\mathit{km}/ s$$