Black Body Radiation
Evidence for Particle Nature of EM Radiation | Structure of Atom
1. What is a Black Body?
Example: A hollow sphere coated with platinum black with a small hole acts as a near-perfect black body. Any radiation entering the hole is reflected internally until absorbed.
2. The Phenomenon (Observation)
When solids are heated, they emit radiation over a wide range of wavelengths.
Example: Heating an iron rod.
- At low temp: Dull Red (Long wavelength, Low frequency)
- Higher temp: Bright Red $\rightarrow$ Orange $\rightarrow$ Yellow
- Very high temp: Blue (Short wavelength, High frequency)
3. Failure of Classical Wave Theory
According to classical physics (Wave Theory of Light), energy is emitted continuously. This predicted that at high frequencies (UV region), the intensity should go to infinity. This contradiction is known as the Ultraviolet Catastrophe. Classical theory could not explain the distribution of intensity with wavelength.
4. Planck's Quantum Theory (1900)
Max Planck suggested that atoms and molecules do not emit or absorb energy continuously, but discretely.
- Radiant energy is emitted or absorbed discontinuously in the form of small packets of energy called Quanta. (In case of light, 'Photon').
- The energy of each quantum is directly proportional to the frequency ($\nu$) of radiation.
Where:
- $h$ = Planck's Constant ($6.626 \times 10^{-34} J \cdot s$).
- $\nu$ = Frequency ($Hz$ or $s^{-1}$).
- $c$ = Speed of light ($3 \times 10^8 m/s$).
Total energy emitted/absorbed is always an integer multiple of a quantum: $E = nh\nu$ (where $n=1, 2, 3...$). This is called Quantization of Energy.
Practice Quiz
Test your knowledge on Planck's Theory.
Really helpful,wish all the teachers explained like this.
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