Ultraviolet (UV) is an electromagnetic radiation with a wavelength from 100 nm (30 PHz) to 380 nm (750 THz), shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight. It is also produced byelectric arcs and specialized lights such as mercury-vapor lamps, tanning lamps, and black lights. Although lacking the energy to ionize atoms, long-wavelength ultraviolet radiation can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.