Electromagnetic radiation

Electromagnetic radiation is one of the fundamental forms of energy. It is associated with oscillating electric and magnetic fields—hence the name— at right angles to each other and to the direction in which the radiation is traveling. It can be regarded as pure energy traveling either in waves or as a stream of particles, called photons. Although the two descriptions sound contradictory, in fact they are not. All types of electromagnetic radiation travel at the same speed, whether it is pictured as an advancing wave or as a stream of photons. In a vacuum it travels at 2.9979 x 108 meters per second—186,629 miles per second (299,790 km s-1). This is known as the "speed of light," light being a particular waveband of electromagnetic radiation. Electromagnetic radiation travels more slowly through a medium, such as air or water.

All bodies that are hotter than their surroundings emit electromagnetic radiation. The Sun is hotter than the space surrounding it and that

Electromagnetic radiation

is why it radiates heat and light. The amount of energy the body radiates is related to its temperature by physical laws that describe the behavior of blackbodies (see the sidebar).

It is the temperature at the surface of the body, where it makes contact with its surroundings, that determines the amount of energy the body radiates and the intensity of its radiation. Deep in the interior of the Sun the temperature is about 27,000,000°F (15,000,000°C). This temperature does not affect solar radiation, however, because the temperature at the visible surface of the Sun—the photosphere—is about 10,800°F (6,000°C). This is the temperature at which the Sun emits blackbody radiation.

Blackbody radiation

All hot objects radiate heat. You can feel the heat when you stretch out your hands toward the hot coals of a fire or a hot central-heating radiator. This is a universal physical law. Any body that is at a temperature higher than the temperature of its surroundings will radiate heat.

A body that absorbs all of the electromagnetic radiation falling upon it and then radiates the energy it has absorbed at the maximum rate possible is known as a blackbody. This is because any body that absorbed all of the radiation falling upon it would reflect no light at all and would therefore be perfectly black, so a blackbody would be visible only by virtue of its own radiation. The concept is theoretical, because in the real world there can be no such thing as a body that reflects no radiation at all (except a black hole).

All forms of electromagnetic radiation travel at the speed of light. Changing the amount of energy the radiation possesses cannot alter its speed. Instead, it alters its wavelength, which is the distance between one wave crest or trough and the next. As the amount of energy increases, the wave length decreases—the crests and troughs move closer together.

The amount of energy a blackbody radiates depends on its temperature. More precisely, the amount of energy radiated is proportional to the fourth power of the temperature. This is expressed as:

where E is the amount of radiation integrated over all wavelengths in the spectrum, T is the temperature in kelvins, and o (Greek sigma) is the Stefan-Boltzmann constant. The relationship between radiant energy and temperature was discovered in 1879 and developed further in 1884 by the Austrian physicists Josef Stefan (1835-93) and his former student Ludwig Eduard Boltzmann (1844-1906), and it is known as the Stefan—Boltzmann law.

The wavelength of the radiation varies inversely with the temperature—the higher the temperature the shorter is the wavelength. This relationship was discovered in 1896 by the German physicist Wilhelm Wien (1864-1928) and is known as Wien's law. It is expressed as:



Blackbody radiation. The graph relates the energy emitted by radiation at different wavelengths.

Blackbody radiation. The graph relates the energy emitted by radiation at different wavelengths.

where Amax is the wavelength of the maximum emission (A is Greek lambda), T is the temperature in kelvins, and C is Wien's constant. This is 2,897 ,um, so the equation becomes:

The graph shows the relationship between temperature, the wavelength of the radiation emit ted, and the energy (in watts per square meter) of that radiation.

This relationship between temperature and wavelength explains why the color of certain materials changes as their temperature rises. A red fire does not radiate as much heat as a white-hot one.

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  • Danny
    How electromagnetic radiation effect weather?
    1 year ago

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