Physics

Radiant Energy



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Radiation energy, or radiation, refers to any form of energy propagated as rays, waves or a stream of particles. The term usually applies to light or other electromagnetic waves but can also be used to describe sound waves, water waves and the emissions from radioactive substances. Like all forms of energy, radiant energy can be measured in Joules.

Radiant energy, in the form of electromagnetic waves, is able to travel through a vacuum. This energy is propagated at the speed of light through oscillating electric and magnetic fields.  Fields that oscillate at the highest frequency transmit the most energy. This can be measured as energy intensity or Watts per Square Metre.

Electromagnetic waves are formed through changes in the energy levels within atoms and other states of matter. They can be formed at a very wide range of frequencies. The most energetic electromagnetic waves are gamma rays with frequencies of 10 20 Hertz. These originate in the atomic nucleus. X-rays, ultra-violet, visible light, infra-red light and radio waves, listed in order of diminishing frequency, or energy intensity, have their origin in transitions in the energy levels of the electrons that orbit the atomic nucleus.

Our most everyday experience of radiant energy comes from the sun. Bright sunlight provides an irradiance of about 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared light, 445 watts is visible light, and 32 watts is ultraviolet light. Our eyes sense electromagnetic energy in the form of light. Our bodies sense the radiation in the form of heat.

Radiant energy is perhaps best understood in the form of heat. When radiation is absorbed by matter it may increase the vibrational or translational energy of the atoms or molecules that make up that substance. This increase in internal energy will increase the internal energy of the material and will usually cause and increase in temperature. All frequencies of the electromagnetic spectrum produce some heating when their waves are absorbed but the effect is most noticeable in the infrared.

Infrared waves have a frequency in the range of 10 11 to 10 14 Hz. These frequencies correspond to the rotational, vibrational, and translational energies associated with atoms and molecules. Infrared waves can be readily absorbed and emitted and reflected by materials.

Scientists have developed a clever way to associate radiant energy with temperature. Image a material which absorbs all the radiant energy that falls upon it. Scientists call this a black body. The best practical idealisation that we have is of a spherical cavity with a small pinhole entrance – think of a ping-pong ball with it’s internal surface painted black. Stefan Boltzmann’s Law states that the power radiated by a black body overall all frequencies is proportional to the fourth power of temperature. This law enables us to make reliable estimates of the temperatures within furnaces which resemble black bodies.

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