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| Heat Radiation - Infra Red Radiation |
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Infra red radiation is part of the electromagnetic spectrum. It is sometimes called heat radiation. The transfer of heat energy by radiation does not need matter particles as the photons of energy can travel across a vacuum. All matter releases radiant energy. It is caused by the 'jumping of electrons between orbitals. Hotter matter releases more (and/or higher frequency) radiant energy than cooler matter. The radiant energy can be looked at as either a stream of particles (photons) or as a wave - the 'things waving' are the electric and magnetic fields the photon generates as it moves, not matter particles.
When an object is heated it begins to give off a form of energy called electromagnetic radiation. At low temperatures it is in the infra-red region of the spectrum. If you continue to heat the object to a really high temperature eventually you can see some of the radiation - visible e.m. radiation. The waves have reached a frequency which our eyes can detect as light. The logs on the bonfire have reached the temperature necessary for this to happen. Radiation is emitted from a warm surface as soon as it "sees" a surface near it which has a lower temperature. The heat "wants" to move to the cooler surface. Heat will move from warm to cold. The warm surface could be more than 300° C llike the surface of the logs in the fire or it could be 40°C like the surface of the warm radiator in your comfortable heated home. Both will emit 'heat rays' - infra red rays. The hotter the object is compared to its surroundings the faster it will radiate the heat (more J/s). The physical qualities of an object determine the ability of the object to absorb or reflect radiation.The surface properties of materials (their emissivity) also affects the amount of heat radiated. Two objects at the same temperature, and the same size do not necessarily radiate the same amount of heat..... the darker, rougher, duller one would emit the most heat per second because as a rule, dull, rough surfaces absorb radiant heat very well, they therefore are easily heated by radiation. Smooth polished surfaces are usually good reflectors that do not hold heat efficiently. Objects that absorb heat well, often emit heat well. Objects that reflect heat well, often emit heat poorly. Infra-red cameras, night vision and thermography - click here How a Crookes radiometer works The radiometer consists of a glass bulb, from which much of the air has been removed to form a partial vacuum. Inside the bulb, on a low friction spindle, are several (usually four) lightweight metal vanes. Each vane is polished on one side, and blackened on the other. In sunlight, or exposed to a source of infra-red radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power. Heat radiation excites the black surface of the vanes more than the white surface. This is because black is a good absorber and a good radiator of heat and white is a poor radiator and absorber of heat.
Heat escapes quickly from the black sides of the vanes as dark surfaces are good radiatiors of heat. So the black sides cool off first. Meanwhile, the white side takes longer to lose heat and cool down. The result is that gases from the white vane push off with more force (
See also the electromagnetic
spectrum.
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When you stand in front of
a bonfire you feel the warmth of the fire on your hands and face. If you
move back several feet away you can still feel warmth. It is the radiant
energy that you feel - infra red rays coming off the fire travelling like
invisible light rays towards you. Your skin then absorbs the rays and
you feel warm. If someone moves in front of you, you suddenly notice the
difference. The persons body is blocking the radiant energy from reaching
you - you are in their shadow.
The Crookes radiometer was invented by the chemist Sir William Crookes as the byproduct of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance. Investigating this effect, he devised the device named after him, still manufactured and sold to this day as a curiosity item.