The Doppler effect is the apparent change in frequency due to the movement of the source and the observer. This means an object making sound moving towards you will appear to have a higher pitch and an object making sound moving away from you will appear to have a lower pitch. This concept also applies to light. How?

Doppler effect changes the wavelength of the light emitted, depending upon whether source is moving away or coming towards the observer (detector).

Explanation:

Doppler effect in light is actually a relativistic effect but somewhat similar to the one which happens in sound waves.

When the source is moving away from the detector, the wavelength of the light emitted from the source appears to be increased as seen by the detector, as a result the frequency decreases (we know that frequency of light = speed of light/wavelength of light. Here speed of light is constant and frequency of light is inversely proportional to its wavelength)

Due to this decrease in frequency the light emitted from the source appears more red, since red color is on low frequency side in the electromagnetic spectrum.

Similarly for the source moving towards the detector, the wavelength appears to be decreased, thereby resulting in increase in frequency and the source appears blue. The shift in frequency is known as doppler shift.

The shift in frequency when the source is moving away is known as redshift and the one where the source is moving towards detector is known as blueshift