Explain Doppler effect in light. Distinguish between red shift and blue shift.

The Doppler effect in light :
i) When any light source (like sun, moon, star, atom etc.) is approaching or receding away from the observer then the frequency or wavelength of light appears to be changing to the observer. The apparent change in frequency or wavelength of light is called Doppler effect in light.
ii) Blue Shift : When the distance between the source and the observer is decreasing (i.e. the source is approaching the observer) then the frequency of light appears to be increasing or wavelength appears to be decreasing, therefore, spectrum gets displaced towards blue end, hence it is known as blue shift.
$\text{ a) }\mathrm{\Delta \lambda }={\mathrm{\lambda }}^1-\mathrm{\lambda }=\frac{-{\mathrm{V}}_{\mathrm{S}}}{\mathrm{C}}\mathrm{\lambda } ;\text{ here, }{\mathrm{\lambda }}^1<\mathrm{\lambda }$
$\text{ b) }\mathrm{\Delta n}={\mathrm{n}}^1-\mathrm{n}=-\left(\frac{{\mathrm{V}}_{\mathrm{S}}}{\mathrm{C}}\right)\mathrm{n};\text{ here, }{\mathrm{n}}^1>\mathrm{n}$
iii) Red Shift : When the distance between the light source and the observer is increasing (i.e. the source is receding away from the observer) then the frequency of light appears to be decreasing or the wavelength of light appears to be increasing to the observer. Therefore, the spectral line gets displaced towards red end, hence it is known as the red shift.
$\text{ a) }\mathrm{\Delta \lambda }={\mathrm{\lambda }}^1-\mathrm{\lambda }=\frac{+{\mathrm{V}}_{\mathrm{S}}}{\mathrm{C}}\mathrm{\lambda };\text{ here, }{\mathrm{\lambda }}^1>\mathrm{\lambda }$
$\text{ b) }\mathrm{\Delta n}={\mathrm{n}}^1-\mathrm{n}=-\left(\frac{{\mathrm{V}}_{\mathrm{S}}}{\mathrm{C}}\right)\mathrm{n};\text{ here, }{\mathrm{n}}^1<\mathrm{n}$