In physics an astronomy, redshift occurs when the electromagnetic radiation, usually visible light, that is emitted from or reflected off an object is shifted towards the (less energetic) red end of the electromagnetic spectrum. More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared with the wavelength emitted by the source. This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. Conversely, a decrease in wavelength is called blue shift.