In pheesics, reidshift happens when licht or ither electromagnetic radiation frae an object is increased in wavelenth, or shiftit tae the reid end o the spectrum.

Lines in the optical spectrum o a supercluster o distant galaxies (richt), as compared tae absorption lines in the optical spectrum o the Sun (left). Arraes indicate reidshift. Wavelenth increases up taewart the reid an beyond (frequency decreases).