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# Ch.2: Quantum theory : Black body radiation

الكلية كلية التربية للعلوم الصرفة     القسم قسم الفيزياء     المرحلة 3
أستاذ المادة فؤاد عطية مجيد       01/11/2014 15:50:31
Chapter Two: Quantum Theory

2.1 Black body

A blackbody is defined as the body which can absorb all energies that fall on it. It is something like a black hole. No lights or material can get away from it as long as it is trapped. A large cavity with a small hole on its wall can be taken as a blackbody.

Any radiation that enters the hole is absorbed in the interior of the cavity, and the radiation emitted from the hole is called blackbody radiation.

2.2.1 Stefan-Boltzmann Law :

It is found that the radiation energy is proportional to the fourth power of the associated
temperature.

2.2.2 Wien’s displacement law:
The peak of the curve shifts towards longer wavelength as the temperature falls and it satisfies Where b is called the Wien’s constant This law is quite useful for measuring the temperature of a blackbody with a very high temperature.

2.3 Rayleigh-Jeans Law
In 1890, Rayleigh and Jeans obtained a formula using the classical electromagnetic (Maxwell) theory and the classical equipartition theorem of energy in thermodynamics.
The formula is given by;

Rayleigh-Jeans formula was correct for very long wavelength in the far infrared but hopelessly wrong in the visible light and ultraviolet region. Maxwell’s electromagnetic theory and thermodynamics are known as correct theory. The failure in explaining
blackbody radiation puzzled physicists! It was regarded as ultraviolet Catastrophe (disaster).

2.4 Wein’s formula:
Later on in 1896, Wein derived another important formula using thermodynamics.
Unfortunately, this formula is only valid in the region of short wavelengths.

Magic formula
In 1900, after studying the above two formulas carefully, Planck proposed an empirical It is surprising that the experience formula can describe the curve of blackbody radiation exactly for all wavelengths.
• Other unbelievable deductions:
(1) For very large wavelength, the Rayleigh-Jeans formula can be obtained from Planck’s formula;

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