Relativistic Doppler effect invisible ultraviolet wavelengths, infrared light is blueshifted into the visible range. Precisely what changes in the colors one sees depends on the physiology of the human eye and on the spectral characteristics of the light sources being observed. The Doppler effect (with arbitrary direction) also modifies the perceived source intensity: this can be expressed concisely by the fact that source strength divided by the cube of the frequency is a Lorentz invariant This implies that the total radiant intensity (summing over all frequencies) is multiplied by the fourth power of the Doppler factor for frequency. As a consequence, since Planck's
Ultraviolet communication in butterflies communicate their receptivity to males using their ultraviolet communication system. The ultraviolet reflection is thought to be concentrated to the ventral side of a female’s hind wing. Yoshiaki Obara determined this experimentally by comparing the number of approaches to different parts of butterfly wings. The results showed that the female’s ventral hind wing was the greatest releaser of male sexual behavior. Although ultraviolet reflectance releases the sexual behavior, the strongest reflection of UV light is not the strongest releaser for the behavior. As a result, male white cabbage butterflies must have a preference for a certain level of UV reflectance