Light is part of a spectrum of electromagnetic energy that includes radio waves, microwaves, infrared radiation, visible light, ultraviolet "light", x rays, and gamma rays. Electromagnetic energy travels as waves that vary in wavelength. Radio waves and microwaves lie at the longer end of the spectrum of electromagnetic energy (kilometers and meters to centimeters and milimeters), while x rays and gamma rays have very short wavelengths (billionths or trillionths of a meter). Infrared radiation, what we experience as heat when we hold our hand near a warm object, is somewhat longer wavelengths than visible light. Visible light is simply electromagnetic radiation in a range of wavelengths that our eyes are sensitive to. Visible wavelengths range from 0.0007 milimeters for red light, through orange, yellow, green, and blue, to 0.0004 milimeters for violet light. Ultraviolet is shorter wavelengths than violet.