Question

Explain the Doppler Effect in light. Hence explain the red and blue shift. State its two applications.

Answer 1

Hint:The relativistic Doppler Effect primarily applies to sound waves and radio waves and is defined as the change in sound frequency due to relative motion between the sound source and the observer. When a sound source is moving towards the observer the apparent frequency heard increases.

Step by Step Solution:
The Doppler Effect in light is categorised under the relativistic Doppler Effect and is defined as the change in the frequency (and wavelength) of light, caused by the relative motion between the source and the observer.

When a star emits light, the colour of its light observed depends upon the motion of the observer relative to the star. Let’s think of our earth as the observer. Now if a star is moving towards the earth, its light is shifted to higher frequencies on the colour spectrum. Such a higher frequency shift is called a blue shift as the blue, violet and ultraviolet rays lie towards the higher frequency end of the light spectrum. In contrast to this, if the star is moving away from the earth, its light is shifted to lower frequencies on the colour spectrum and such a shift is called a redshift. The red and blue shifts are better known as Doppler Shifts.

The Doppler shift is primarily used to measure the speed at which celestial bodies (stars and galaxies) are approaching or receding from us.
When an ambulance is approaching an observer, the siren is high pitched and then switches to a lower pitch sound once the ambulance passes the observer and is travelling away from it.

The Doppler shift is also used in police radar guns to measure how fast your car is going based on how the radio wave shifts in frequency when it bounces off your car.

Note: The Doppler Effect is not the only source of the red and blue shifts discussed above.
-Astronomers know of three possible sources for these shifts.
-These are Doppler shifts (as discussed above); gravitational redshifts due to the light exiting a gravitational field and cosmological expansion where space itself stretches.