UV radiation is present in sunlight, and is produced by electric 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 causechemical 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.