Radiation

Radiation is the method of heat transfer that does not require a material medium, it can take place in vacuum.

How Radiation Work
In radiation, the source of heat transmit energy in the form of waves. These waves constitute part of electromagnetic spectrum and are known as infrared radiation. These waves are transformed into heat again when they are absorbed by another body. The hotter an object is, the more energy it radiates.

Example: Sun and Earth
Between the Sun and Earth, there is a vacuum. Conduction and convection cannot take place, which means that radiation is the only one that is taking place. The Sun's energy is traveling to Earth as electromagnetic waves, which is why we have solar energy.

Good and Poor Emitters of Radiation
To emit radiation is to radiate heat.

Objects with black, dull surface radiates more heat.
Objects with shiny surface radiates less heat.

E.g The teapot is usually made of shiny materials so that the tea in it will stay hot for a longer period of time.

Good and Poor Absorbers of Radiation
When a body absorbs radiation, the body's temperature rises.

Objects with dull, black surface absorbs more heat.
Objects with shiny surface absorbs less heat.

*Good Emitter = Good Absorber
Bad Emitter = Bad Absorber

E.g. Solar heating panels are painted black in order to absorb as much radiation from the Sun as possible.

Vacuum Flask

















1. Screw-on Stopper
2. Outer plastic or stainless steel case
3. Outer layer of glass coated with reflective material
4. Vacuum.
5. Inner layer of glass (or stainless steel)
6. Supports and keep inner vacuum chamber in place
7. Additional insulation reduces heat losses and cushions flask against impacts.

The vacuum (7) prevents conduction. The tight stopper(1) prevents air from entering or leaving, which stops convection. The inner layer of reflective material (3) prevents radiation as it is of shiny material, which will reflect radiation instead.
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