red+shift

A redshift is a shift in the frequency of a photon toward lower energy, or longer wavelength. The redshift is defined as the change in the wavelength of the light divided by the rest wavelength of the light

The **Doppler Redshift** results from the relative motion of the light emitting object and the observer. If the source of light is moving away from you then the wavelength of the light is stretched out, i.e., the light is shifted towards the red. These effects, individually called the blueshift, and the redshift are together known as doppler shifts. The shift in the wavelength is given by a simple formula