Radiant energy which is invisible to human eyes but can be felt as heat is infrared radiation (IR), or infrared light. Sun and fire are the most apparent source of radiating infrared radiation, but all the objects of the universe emit a level of infrared radiation. When an atom absorbs and releases energy, the phenomenon of electromagnetic radiation occurs of which infrared radiation is a part. Gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves, are the form of electromagnetic radiation from highest to lowest frequency. All these radiations together make up the electromagnetic spectrum. Infrared waves occur at a frequency just below those of visible red light and above those of microwaves, within the electromagnetic spectrum. According to NASA, the range of IR frequency is about 300 gigahertz (GHz) to about 400 terahertz (THz), and its wavelength is estimated to range between 1000 micrometer (μm) and 760 nanometres. Hence, it is longwave radiation.