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Radiation (heat transfer by electromagnetic waves)
The continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.

Radiation does not require a medium for energy transfer, which means that radiation can take place in a vacuum.

• An example of radiation is the way thermal energy from the Sun reaches the Earth. (As there is a vacuum between the Sun and the Earth, conduction and convection are not possible ways of thermal energy reaching the Earth from the Sun.) The Sun emits electromagnetic waves and the infrared waves within the electromagnetic waves makes us feel warm.

Thermal energy from infrared waves is called radiant heat and all objects emit some radiant heat. The hotter the object, the greater the amount of radiant heat emitted.


ABSORPTION AND EMISSION OF INFRARED RADIATION


Experiment 6 (:

1. Fill two cans (a bright, shiny can and the other a dull black can) with an equal amount of water.
2. Place a thermometer in each can and place both cans under the sun.
3. Start recording the temperature.

• The temperature of the water in the dull, black can increased faster than that in the bright, shiny can as the dull, black can absorbed radiation at a faster rate compared to the bright, shiny can.
  
• Therefore we can conclude that dull, black surfaces absorb infrared radiation faster compared to shiny, white surfaces.

Experiment 7 (:

1. Fill two cans (a bright, shiny can and the other a dull black can) with an equal amount of boiling water at the same time.
2. Place a thermometer in each can and place both of them in the same area.
3. Start recording the temperature

• The temperature of water for the dull, black can falls at a faster rate than the bright, shiny can as the dull, black can emitted thermal energy at a faster rate compared to the shiny tin.
• Therefore we can conclude that dull, black surfaces are better emitters of infrared radiation than shiny, white surfaces.

FACTORS AFFECTING THE RATE OF INFRARED RADIATION

- Colour and texture
From Experiment 6 and 7, we can see that dull, black surfaces are better absorbers of infrared radiation than shiny, white surfaces and that dull, black surfaces are better emitters of infrared radiation than shiny, white surfaces respectively.


NOTE!
This is why you would be cooler if you wear light or white clothes during hot weather! (:

- Surface temperature
The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.

- Surface area
If we compare two objects of the same mass and material, but with different surface areas, the object with the larger surface area.

APPLICATIONS OF RADIATION

Teapots






Sterilisation of Medical Products
To sterilize is to make things free of microorganisms. Large doses of radiation will kill microorganisms and in recent years the medical industry has introduced radiation to sterilize medical products which do not lend themselves to sterilization by heat or steam.

Food Preservation
In the food irradiation process, a variety of fresh vegetables, seafood and meats are subjected to high energy electron beams to obtain various radiation treatments, depending on dose and complexity of the food. Irradiation can delay sprouting in root crops; kill insects in vegetables and stored grains; kill parasites in fresh meats and seafood; delay ripening and decay in fruits; and kill bacterial spores that cause trichinosis and botulism. All this can be accomplished without making the food radioactive.

Crystal Colour Enhancement
Radiation has been used to produce colour in topaz. Uncolored rough, preform or faceted topaz is subjected to the electron beam to form color centers in the material. The irradiation speeds up the natural process of coloration and results in gems with the much sought after sky blue or super blue color. Topaz that would normally be discarded becomes commercially and aesthetically desirable.

Smoke Alarms
Some smoke alarms use the radiation emitted by a radioactive sample to detect fire. Smoke interferes with the radiation emitted by the atoms. This change in radiation levels sets off the alarm.

Mapping Ocean Currents
Radioactive atoms can be detected easily because of the radiation they emit. This allows scientists to track the location of a small sample of radioactive atoms—for example doctors track how a substance spreads through the body and oceanographers track the movement of water currents. Only very small amounts of radioactive elements are needed for this.

Nuclear Power
The energy released by radioactive decay can boil water, and the steam is then used to generate electricity. Nuclear power involves the use of much larger quantities of radioactive material than other uses of radioactivity.

Chemotherapy
Larger amounts of radiation can be used to destroy cancerous cells as an alternative to chemotherapy.

Infrared Cameras
Hot objects give out energy through radiation in the form of electromagnetic waves. Warm objects like the human body mainly radiate infrared radiation. This is why infrared sensitive cameras can "see" people clearly even in the dark. Therefore, police use infrared cameras to track down criminals in the dark.

Flash animation: Radiation energy transfer from sun to Earth and global warming