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History: Until The end of 19th century (Newtonian Mechanics). 1905 Einstein special Relativity (C play a fundamental role).It does not

modify distinction b/w matter & Radiation (referred as classical). During the late 19th century experimental evidence accumulated new

concepts different from classical Mechanics. i.e Quantization of physical

quantities energy & Angular momentum, The particle properties of

radiation and the properties of matter. As velocity c plays a central role in

relativity so does in planks constant in quantum mechanics .

19th century – Classical physics

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Concept of Photon

Thomas Youngs’s 1801 double slit experiment provides strong evidence that

visible light is wave like. i.e all radiation in EMS have wave like aspect (X ray

diffraction).

Another property of light is particle nature( Packet of energy E= hf) strongly

supported by Photo Electric effect , compton effect etc.

Thermal Radiation

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Q: How photon was introduced?

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* The concept of photon was introduced into physics by studies of the radiation emitted by heated objects .In 1905 Einstein introduced the concept of photon while performing photo electric Effect.

* Most objects absorb light and emit Electromagnetic radiation when their temperature are high. * The radiation emitted by hot objects are called Thermal radiation.

* Such radiation is distributed continuously in wavelenght , having measurable intensity through out broad range of Electromagnetic spectrum.

The Concept of Photon

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• In 19th century investigators discovered two important qualitative

properties of thermal radiation • As the temperature of the body increased, the intensity of the

emitted radiation rises rapidly.• The higher the temperature the shorter the wavelenght of emitted

radiation.• In order to attempt above two qualitative observations

quantitatively was a difficulty i.e nature of emitted radiation, factors affecting intensity , temperature and body surface roughness etc. no fundamental law of physics seems to be lurking behind such a sensitive experiment.

Thermal radiation

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Thermal Radiation• The solution to this difficulty was proposed in 1859 when

Kirchhoff consider cavity in solid body its wall at uniform temperature with a small hole in its wall .

• The radiation emerging from the hole should not depend on the material or the mode of construction of the cavity but depend only on temperature .

• Now stefan- Boltzmann constructed above ist qualitative statements quantitatively .

• I(T)= T4

• I(T) represents the total radiated power integral over all wavelenghts.

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Experimental Results

• The second statement was expressed by wien’s lmaxT = 2.898 x 10-3 m . K

• lmax is the wavelenght at which the radiation emitted by a cavity at temperature T has its max intensity. As the increases body color shifts to smaller wavelenghts and it glows yellow orange . Wiens law can be used to deduce the surface temperature of stars from their color, blue stars being hotter than red stars.

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\ Rayleigh-Jeans Law

•An early classical attempt to explain blackbody radiation was the Rayleigh-Jeans law•At long wavelengths, the law matched experime At short wavelengths, there was a major disagreement between the Rayleigh-Jeans law and experiment.•This mismatch became known as the ultraviolet catastrophe.

– You would have infinite energy as the wavelength approaches zero ntal results fairly well.

I , 4

2 Bπ c k Tλ Tλ

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Planck’s Theory of Blackbody Radiation

•In 1900 Planck developed a theory of blackbody radiation that leads to an equation for the intensity of the radiation.•This equation is in complete agreement with experimental observations.•He assumed the cavity radiation came from atomic oscillations in the cavity walls.•Planck made two assumptions about the nature of the oscillators in the cavity walls

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Planck’s Assumptions•The energy of an oscillator can have only certain discrete values En.

– En = n h ƒ• n is a positive integer called the quantum number• ƒ is the frequency of oscillation• h is Planck’s constant

– This says the energy is quantized.– Each discrete energy value corresponds to a different quantum state.

• Each quantum state is represented by the quantum number, n

The oscillators emit or absorb energy when making a transition from one quantum state to another.

The entire energy difference between the initial and final states in the transition is emitted or absorbed as a single quantum of radiation.An oscillator emits or absorbs energy only when it changes quantum states.The energy carried by the quantum of radiation is E = h ƒ.

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Question(s)• Why classical physics doesn’t explain the Black

body radiation?• Why we can not see objects in dark?• What causes stefan use T4 not other power?• Example of Black body radiation in universe.• Why wiens law called wiens displacement law?