Ultraviolet not necessarily coincide; for example one applied physics publication used a boundary of 190 nm between hard and soft UV regions. Very hot objects emit UV radiation (see black-body radiation). The Sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm. Extremely hot stars emit proportionally more UV radiation than the Sun. Sunlight in space at the top of Earth's atmosphere (see solar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m in vacuum. However, at ground
Ultraviolet communication in butterflies rapid opening and closing of the wings, is performed when a male approaches another butterfly; yet, males copulate with females who perform this behavior, while retreating from males who perform the flutter response. By showing that male and female wings are similar in appearance except for UV reflectance and an approaching male is exposed to ultraviolet reflection of a male's wings during a flutter response, as well as ruling out temporal differences in the flutter behavior as a cause for sex recognition, Ronald Rutowski concluded that ultraviolet light was being used as an indicator of the male sex. However, in