The concept around the Doppler effect is basically how much a wave stretches or bunches due to the speed it is travelling at.

## How the Doppler Effect Applies to Light

- The original wavelength (λ) of the light approaching Earth from a distance sun. We know this because we can recreate the conditions of distance suns on Earth providing the wavelength of light the distance sun emits.
- Therefore, we know the △λ which is the wavelength we receive on Earth – original wavelength.

△λ / λ = v / c

Where v = velocity of the wavelength at which its moving and c being the speed of light which is 3×10^8 m/s. However, this equation only applies when v << c (when the velocity of the wavelength is much much smaller than the speed of light). As a general rule, the maximum velocity of the wavelength for the equation to work is 0.1c.

## Detailed Explanation

△λ / λ = v / c

**Question: Why does this not take account of relativity?****Answer: **Because △t for a stationary star and △t for a moving star were equated and they are **not **the same because t = γΤ.This is light clocks and time dilation which can be looked into more detail in an article on it in under Physics A2. The theory is that time is not a constant in our universe: instead, the speed of light is. Therefore, time slows down as you get closer to the speed of light. However, as the speed v is << (much much less than) c (speed of light), the value of γ will be extremely close to 1. Therefore, the formula is OK to use.

## Summary

- The Doppler effect is when a wavelength is stretched or compressed due to the object of which is producing the wavelength is moving further away or closer to the observer.
- For speeds less than 0.1c, we can use the equation △λ / λ = v / c to work out the proportion of bunching.