Radiation is the transfer of heat in the form of electromagnetic waves. It does not require a medium and can occur in a vacuum. Examples include the heat from the Sun reaching the Earth. Unlike conduction and convection, radiation transfers energy through infrared waves or visible light.
Class 11 Physics Chapter 10 Thermal Properties of Matter covers concepts such as heat transfer methods including conduction convection and radiation. It explains thermal expansion and related coefficients like linear superficial and cubical expansion. The chapter also discusses specific heat and calorimetry and introduces laws like Stefan-Boltzmann law and zeroth law of thermodynamics.
Experimental Observations Relevant to Kirchhoff’s Law of Heat Radiation:
Kirchhoff’s law of heat radiation states that for a body in thermal equilibrium, the ratio of its emissive power to its absorptive power is constant and equal to the emissive power of a perfect blackbody at the same temperature. Several experimental observations confirm this law:
1. Emission and absorption by black bodies:
Black bodies are perfect absorbers and thus also the best emitters of radiation. Experiments on cavity radiators, that approximate black bodies, demonstrate that their radiation spectrum does indeed behave according to theory, for example Planck’s law, as demanded by Kirchhoff’s principle.
2. Properties of surfaces and radiation:
Blackened surfaces absorb more radiation and emit more heat, while polished surfaces, which are poor absorbers, are also poor emitters. Comparative experiments confirm the law’s validity.
3. Absorption lines in stellar spectra:
Kirchhoff’s law accounts for the absorption lines that appear in stellar spectra. These lines arise from gases in the atmosphere of the star absorbing certain wavelengths of light emitted by the hotter interior, which proves that a good absorber is also a good emitter.
4. Cooling and heating experiments:
Objects with high absorptivity, such as black surfaces, cool faster because they radiate heat more efficiently than reflective surfaces.
5. Greenhouse effect research:
The gases like carbon dioxide and water vapor are able to absorb infrared radiation strongly as well as radiate heat effectively. These observations have been established experimentally, which are central to the studies on climate.
The above results clearly illustrate that a body which absorbs and emits radiations have reciprocal characteristics.
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