Question

What is the Doppler Effect called a red-shift or a blue-shift when it affects light?

Answer 1

Doppler's effect : When there is relative motion between source of light and observer the change in frequency of light is observed is called Doppler's effect.

Doppler shift = observed frequency - Actual frequency

Red-shift : When the observer and Source of light moves away from each other the Doppler shift is neagative because the spectrum of visible light observed is shifted towards shorter frequency i.e. toward red .that's why it is called Red Shift

Blue-shift : When the observer and source of light moves towards each other the Doppler shift is positive because the spectrum of visible light observed is shifted towards longer frequency i.e.toward Blue.that's why it is called Blue shift.