Doppler Shift: What Is It?

 Now, I am going to assume that 95% of the people who are reading this are not astronomy geeks. I bet that 30% of the people reading this are also not experts in optics. Am I right 😜? So, for the sake of those 95% and 30% people, let me start from the basics. (No offence intended)

What is a spectrum of light?

When light passes through a glass prism, or any similar refracting surface, it splits into its constituent colours (Red, orange, yellow, green, blue, indigo and violet). This is called the spectrum of light. In a spectrum of light, the red colour has the highest wavelength and lowest frequency, and the violet colour has the lowest wavelength and the highest frequency.

The spectrum of light.
Credit: CAREX.

However, a very small part of the spectrum of light is visible to the unaided human eye. There are several forms of light which have a frequency lower than red and, at the other end of the spectrum, a frequency higher than violet. (Refer to the above image)

These forms of light are not visible to the human eye, such as ultraviolet light, infrared light, etc, but can be extremely harmful for our eyes. The main reason you are told not to look at a solar eclipse directly is because the UV(ultraviolet) emissions are high and can damage your eyes.

What is Doppler Shift?

Now, finally getting on to the topic...

Doppler shift is basically a change in the frequency of a wave (sound or light), relative to the observer, who is either moving further away or towards the source of the wave.

This phenomenon was first observed in 1842 by the physicist Christian Doppler, after whom the effect has been named.

Since I am talking in astronomical terms here, I will refer to the change in the frequency of light whenever I mention the Doppler Effect.

What Causes the Doppler Shifts?

If the object is moving towards the source of light, the principle behind Doppler shift is that every time a wave of light is emitted, the source is closer to us (the object). So, it takes lesser amount for the subsequent waves to reach to object than the previous wave.

The exact opposite of this happens when the source of light is moving away from the object. Every subsequent wave of light takes slightly longer than the previous wave to reach the object.

This causes the change of the frequency of the light to higher or lower frequencies.

Blueshifts!

Blueshifts are a form of Doppler Shifts mentioned above, in which the light changes to a lower frequency. Blueshift happens when a far away galaxy or object is moving towards us. Due to the Doppler Effect, such celestial bodies appear blue to us when viewed through a telescope.

Redshifts!

Redshifts are the exact opposites of blueshifts. They occur when an object is moving further away from us. Due to this, the body moving away from us appears red to us when viewed through a telescope.

Image indicating the change in frequency during redshift.
Credit: Wikipedia.

Uses of Doppler Shifts

Doppler shifts are used by astronomers to determine is a celestial object is moving towards us or further away from us.

It can further be used to calculate the distance between the Earth and another object, with the help of some annoying stuff called trigonometry.

That is all for this post. Please follow this blog if you liked it. Over and out.

Clear skies,
Aarav Iyer

References:

(1) Wikipedia

(2) NASA

(3) Space.com

Thumbnail credit-Central Galaxy