The absorption of electromagnetic radiation can be explained by considering transverse waves. Electromagnetic radiation is the transfer of energy by waves travelling at the speed of light (300,000km/s) in a vacuum. These waves are transverse waves and have both and electric and a magnetic component to them. There is a spectrum of electromagnetic radiation from high energy waves (short wave lengths) such as gamma rays and x-rays, through ultraviolet, visible light and infra-red radiation to lower energy waves such as microwaves and radio waves (long wavelengths).
Radiation may be absorbed by many types of materials. It turns out that all objects are able to emit radiation at all wavelengths and similarly absorb all types of radiation. In fact good emitters are also good absorbers of radiation. However, the intensity of the radiation emitted or absorbed will vary enormously with different wavelengths of radiation.
Dark colored objects tend to absorb the thermal energy from radiation more efficiently than light colored or white objects. Light colored materials are more likely to reflect the radiation away from them. The rate of absorption of radiation depends strongly on the temperature difference between the absorbing material and the emitter of the radiation. In fact it varies with the fourth power of T, where T is the temperature difference (T4). The rate of absorption is also dependent on the amount of exposed surface area – the larger the area, the greater the absorption.
As radiation is absorbed, its energy is transferred to the materials constituent atoms and molecules. This energy transfer depends on the wavelength or energy of the incoming radiation. Microwave radiation, for example, will be absorbed in such a way that the bonds between molecules in the material will vibrate and the temperature of the body will rise. Some of the energy, however, may be re-emitted. For example when a poker is placed in a fire, it absorbs infrared radiation and stores part of it as heat and re-emits part as light as it glows. In conclusion, the absorption of electromagnetic radiation would depend on the type of material used and its intensity on the wavelength of radiation.
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