Visible light is only a small portion of the sea of radiant energy that bathes the Earth. The rest is invisible and can be measured using electronic devices or photographic films. All forms of radiant energy can be identified by their frequency, wavelength and the energy they carry. Every wavelength of radiation carries a fixed amount of energy. Each behaves as if it were a stream of particles or energy packets called "photons". The shorter the wavelength (higher the frequency), the more energy each photon carries. Ultraviolet photons carry up to 1000 times more energy than visible light photons. X-ray photons carry up to 100 times more energy than ultraviolet photons and gamma rays can carry up to 100 times more energy than x-rays. That is why gamma rays can travel through metal and the x-rays the doctor uses can pass through the soft tissue in your body.