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 Reflected. Radiation is not absorbed or emitted from an object but it reaches the
object and is sent backward. Its represents the reflectivity of an object and

describes the percentage of light that is went back.
 Scattered. Scattered light is deflected in all directions, forward, backward,

sideways. It is also called diffused light.
 Transmitted. Radiation not absorbed, reflected, or scattered by a gas, the radiation

passes through the gas unchanged.

The temperature of an object can tell us something about the emitted radiation.

The rate of thermal energy emitted by a surface depends on its quantity and its

absolute temperature. A black surface absorbs all incident radiation.

The amount of radiation emitted by an object is given by:

(1.16)

4
2
-8
where σ – Stefan-Boltzmann constant: 5.67·10 W/(m K );
T- absolute temperature.

For nonblack surfaces

, (1.17)

where ε – hemispherical emittance or emissivity.

ε is a function of the material, condition of its surface.

The emissivity has a value between zero and 1, and is a measure of how efficiently a

surface emits radiation. It is the ratio of the radiation

emitted by a surface to the radiation emitted by a

perfect emitter at the same temperature.

The amount of radiation emitted by an object to

surroundings is given by:


4
Q     F (T  T 4 ) (W) (1.18)
Fig.12 - Interaction between a rad s surr
surface and incident radiation

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