What is Radiation?

Physics

definition

Radiation

Radiation is the process of transfer of heat from a hot body to a cold body directly without heating the space between the two bodies. This process of heat transfer does not require any medium. The energy so radiated is called radiated energy.
The sun gives light and also heat. Heat transferred by the sun is via radiation. During cold winter nights people gather around a fire to keep warm. This heat is transferred by radiation.

definition

Properties of thermal radiation

  • Thermal radiation once radiated behaves like an electromagnetic wave.
  • It also obeys the laws of optics on account of its wavelength being comparatively small to obstacles.
  • The radiation emitted by a body is a mixture of waves off different wavelengths. However, only a small range of wavelength has significant contribution to the total radiation. 
  • The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases.

REVISE WITH CONCEPTS

Stefan Boltzmann Law

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Special Distribution of Blackbody Radiation - Wien's Law

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Important Questions

The temperature of equal masses of three different liquids and are and respectively. The temperature when and are mixed is and when and are mixed it is . What should be the temperature when and are mixed?

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A black body at a temperature of radiates heat energy at the rate . At a temperature of , the rate of heat radiated per unit area in will be:

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A body cools from C to C in minutes. Calculate the time it takes to cool from C to C. The temperature of the surroundings is C.

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A spherical black body with a radius of 12cm radiates 450W power at 500K. If the radius were halved and the temperature doubled, the power radiated in watts would be :

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State Kirchhoff's law of radiation and prove it theoretically.

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Certain quantity of water cools from to in the first minutes and to in the next minutes. The temperature of the surroundings is ;

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The plots of intensity versus wavelength for three black bodies at temperatures  and  respectively are shown in fig. Their temperatures are such that:

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A pan filled with hot food cools from to in minutes. When the room temperature is . How long will it cool from to ?

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Two spheres of the same material have radii and and temperature and respectively. The energy radiated per second by the first sphere is :

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A body cools down from to in Minutes when the temperature of surrounding is . The temperature of the body after next minutes will be:

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