Long wavelength UV-A is a normal component of sunlight, that, by virtue of its relatively longer wavelength, can penetrate the atmosphere. It is the least harmful and most commonly found type of UV light, because it has the least energy and is often called black light, and is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light - thus appearing to glow in the dark. Applications have included inspection of fluorescent surfaces, tanning beds, and treatment of skin diseases.