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Quantum Theory of Light

7.1

Quantum Theory of Light

Electromagnetic spectrum

All objects emits electromagnetic radiation.
Cold objects \(\rightarrow\) waves with low frequency (Eg: radio waves, microwave)
Hot objects \(\rightarrow\) waves with higher frequency (Eg: visible light and ultraviolet radiation)

	Black body
	An idealised body that is able to absorb all electromagnetic radiation that falls on it.

	Thermal radiation
	Electromagnetic radiation which includes visible radiation and radiation that cannot be seen by the human eye such as infrared radiation.

Classical Theory

The particle nature of light (Isaac Newton)
- Unsuccessful in explaining the phenomenon of light refraction due to failure in comparing the speed of light in glass and air
Double-slit experiment (Thomas Young)
- Unable to explain the radiation spectrum produced by black bodies
Dalton atomic model
- Unable to explain the light spectrum produced by atoms
Discovery of electrons
- Unable to explain the line spectrum of light produced by atoms

Quantum theory

Max Planck
- Introduced the concept of quantum (discrete energy) in 1900
Albert Einstein
- Einstein's quantum theory of light was susccessful in explaining the characteristics of the photoelectric effect that could not be explained by classical theory
Niels Bohr
- Explained the production of line spectrum by hydrogen atoms
Louis De Broglie
- Introduced the hypothesis on the wave nature of particles in 1924

Electromagnetic spectrum may be a:

	Continuous spectrum
	The dispersion of white light by a prism consisting of seven visible colours.

	Line spectrum
	Produced by an excited atom is a series of colored lines with unique wavelengths and frequencies.

Photon energy, \(E\)

\(E=hf\),

where \(h\) = Planck constant (\(6.63\times10^{-34} \text{ J s}\)

\(f\) = frequency of light waves

Wavelength, \(\lambda\)

\(\lambda = \dfrac{h}{mv}\),

where \(m\) = mass of particle,

\(v\) = velocity of particle