Infrared and visible light

Infrared waves are produced by hot bodies and by molecules, through the vibration of atoms and molecules. The band lies next to the red (low-frequency, long-wavelength) end of the visible spectrum, from about 1 mm to 700 nm.

They are sometimes called heat waves. Water molecules, present in most materials, absorb infrared readily (so do CO₂, NH₃ and many others); the absorbed energy increases thermal motion, so the material and its surroundings warm up.

Infrared, being lower in frequency than light, sets whole atoms and molecules vibrating, not only electrons. This raises the internal energy and hence the temperature.

Uses: infrared lamps in physical therapy; infrared detectors on earth satellites, for military purposes and to watch the growth of crops; and infrared-emitting LEDs in the remote controls of TV sets and music systems. Detectors include thermopiles, bolometers and infrared photographic film.

Greenhouse effect: visible sunlight passes fairly easily through the air and is absorbed by the ground, which re-radiates the energy as longer-wavelength infrared. Greenhouse gases such as carbon dioxide and water vapour trap this infrared and keep the earth warm.

Visible light is the band the human eye detects: about 4 × 10¹⁴ Hz to about 7 × 10¹⁴ Hz, or wavelengths of about 700 nm to 400 nm. It is emitted when electrons in atoms move from a higher energy level to a lower one, and detected by the eye, photocells and photographic film.

Other animals see other ranges: snakes can detect infrared, and the visible range of many insects reaches well into the ultraviolet.

Our eyes are most sensitive near the centre of the sun's wavelength distribution, having evolved to see best the wavelengths the sun gives out most strongly.