Infrared is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with waves that are just longer than those of red light, the longest waves in the visible spectrum, so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750 nm to 1 mm. IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the solar spectrum. Longer IR wavelengths (30–100 μm) are sometimes included as part of the terahertz radiation band. Almost all black-body radiation from objects near room temperature is in the IR band. As a form of electromagnetic radiation, IR carries energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon.

In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. Types of EMR include radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays, all of which are part of the electromagnetic spectrum.