Light is a form of electromagnetic radiation that is visible to the human eye and has wavelengths between 380 and 750 nm. This radiation is generated by the movement of electrically charged particles such as electrons in atoms or heated molecules. The electromagnetic spectrum ranges from gamma rays and X-rays to radio waves, and the wavelength is the fundamental parameter that distinguishes one part of the spectrum from another. Photons, which are discrete parcels of energy emitted as particles and waves, determine the energy level of radiation based on their wavelength. Frequency, measured in Hz, is the number of cycles per second and is inversely proportional to wavelength. Light is also characterized by its intensity, which is determined by the number of photons emitted. Light has a spectrum, which is a graph of the radiant energy emitted at each wavelength. The spectrum can be broad or have sharp "lines" at certain wavelengths, depending on the characteristics of the light-emitting system. The emitted spectrum is related to the temperature of the source and can be generated by heated filaments, halogen lamps, and electrically induced high-temperature plasmas.  Light is essential for life on Earth and has important physical effects on our skin and eyes. The warmth or coldness of light is perceived through the interaction of light with our bodies, and our bodies' response to periodic changes in light leads to the regulation of activity and rest cycles.