The document discusses the history and development of theories of blackbody radiation and the concept of the photon. It describes how (1) classical physics could not fully explain experimental observations of blackbody radiation, (2) Planck resolved this issue by proposing that the radiation emitted by cavity walls was quantized into discrete energy packets called quanta (later known as photons), and (3) his theory accurately described blackbody radiation distribution and resolved prior inconsistencies like the ultraviolet catastrophe.

1. 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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2. 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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4. * 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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5. • 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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6. 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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7. 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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8. 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 T
λ T
λ
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10. 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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11. 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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12. 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?
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