because water molecules present in most materials readily absorb infrared waves (many other molecules, for example, CO , NH , also absorb infrared waves). After absorption, their thermal motion increases, that is, they heat up and heat their surroundings. Infrared lamps are used in physical therapy. Infrared radiation also plays an important role in maintaining the earth’s warmth or average temperature through the greenhouse effect.
Incoming visible light (which passes relatively easily through the atmosphere) is absorbed by the earth’s surface and re- radiated as infrared (longer wavelength) radiations. This radiation is trapped by greenhouse gases such as carbon dioxide and water vapour. Infrared detectors are used in Earth satellites, both for military purposes and to observe growth of crops. Electronic devices (for example semiconductor light emitting diodes) also emit infrared and are widely used in the remote switches of household electronic systems such as TV sets, video recorders and hi-fi systems.
. . Visible rays It is the most familiar form of electromagnetic waves. It is the part of the spectrum that is detected by the human eye.
It runs from about × Hz to about × Hz or a wavelength range of about – nm. Visible light emitted or reflected from objects around us provides us information about the world. Our eyes are sensitive to this range of wavelengths. Different animals are sensitive to different range of wavelengths.
For example, snakes can detect infrared waves, and the ‘visible’ range of many insects extends well into the utraviolet. . . Ultraviolet rays It covers wavelengths ranging from about × – m ( nm) down to × – m ( .
nm). Ultraviolet (UV) radiation is produced by special lamps and very hot bodies. The sun is an important source of ultraviolet light. But fortunately, most of it is absorbed in the ozone layer in the atmosphere at an altitude of about – km.
UV light in large quantities has harmful effects on humans. Exposure to UV radiation induces the production of more melanin, causing tanning of