The apparent shift in the wavelength of light emitted by a source moving toward or away from an observer is known as the Doppler effect.

Here's a breakdown:

* Doppler effect: This phenomenon describes the change in frequency (and therefore wavelength) of a wave as the source of the wave and the observer move relative to each other.

* Light: Light is a form of electromagnetic radiation, which travels as waves.

* Redshift: When the source of light is moving away from the observer, the wavelength of the light appears to be stretched, causing it to shift towards the red end of the visible spectrum.

* Blueshift: When the source of light is moving towards the observer, the wavelength appears to be compressed, causing it to shift towards the blue end of the visible spectrum.

The Doppler effect is a fundamental concept in astronomy, used to determine the motion of stars, galaxies, and other celestial objects. It is also used in everyday applications, such as radar and sonar.