Question

If the shift in the star is towards red end, then
A) the star is approaching the earth
B) the star is receding the earth
C) the apparent frequency is lesser than actual
D) the apparent wavelength is lesser than actual

Answer 1

Hint
This type of shift could be understood with the help of the Doppler effect and is defined as the change in sound frequency due to relative motion of sound source and observer. When the sound source is moving towards the observer the frequency heard increases. By using this concept, we can give the above answer.

Complete step by step answer
The Doppler effect of the light is categorised under the relativistic Doppler effect and is defined as the change in the frequency or wavelength of light, caused by relative motion between source and observer.
When the star emits light the colour of the light depends upon the motion of the observer relative to the star.
Let’s think of our earth as an observer. Now if the star is moving towards the earth, its light emitting is of higher frequencies on the colour spectrum. Such a higher frequency shift is known as blue shift, as the blue, violet and ultraviolet rays shift towards the higher frequency end of the light spectrum. In contrast to this, if the star is moving away from the earth then its light will shift to the lower frequency on the colour spectrum and this shift is known as red shift. The red and blue shifts are known as Doppler shifts.
The doppler shift is primarily used for measuring the speed at which celestial bodies (stars and galaxies) are approaching or receding to us.
When an ambulance is approaching us, the siren is high pitched and then switches to the lower power pitch sound once the ambulance passes to the observer and is travelling away from us.
Hence, from the above discussion we concluded that if the shift in the star is shifted to the red end then the star is receding the earth and the apparent frequency is lesser than the actual frequency.
Thus, option (B) and (C) both are correct.

Note
The Doppler effect is not only source of the red and blue shift discuss above Astronomers know of three possible sources of these shifts These are Doppler shifts 9as discussed above); gravitational redshifts due to the light existing in the gravitational field and cosmological expansion where space itself stretches.