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, as

*z* = (Observed wavelength - Rest wavelength)/(Rest wavelength)

Note that postive values of *z* correspond to increased
wavelengths (redshifts).

Different types of redshifts have different causes.

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

(Observed wavelength - Rest wavelength)/(Rest wavelength) = (v/c)

so long as the velocity *v* is much less than the speed of light.
A relativistic doppler formula is required when velocity is comparable
to the speed of light.

The **Cosmological Redshift** is a redshift caused by the expansion
of space. The wavelength of light increases as it traverses the
expanding universe between its point of emission and its point of
detection by the same amount that space has expanded during the
crossing time.

The **Gravitational Redshift** is a shift in the frequency of a
photon to lower energy as it climbs out of a gravitational field.