We only perceive in our senses a small part of the energies to which we are exposed every day as light or heat. A large part of these electro-magnetic energies remain unnoticed by us. Some of these is for example X rays and ultra-violet (UV) rays.

In order to understand these energies generally a so called wave-length model is used. According to this model all these energies differ from each other simply in their wavelength. While radio waves belong to the long waved rays, ultra-violet and gamma rays are in the short wave family of energies. Just glancing at the region of optical rays the percentage of UV rays is a very small part of the spectrum. Optical rays are those, which can be bent, reflected, lead or absorbed.