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

  1. 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
       
  2. Double-slit experiment (Thomas Young)
    • Unable to explain the radiation spectrum produced by black bodies
       
  3. Dalton atomic model 
    • Unable to explain the light spectrum produced by atoms
       
  4. Discovery of electrons
    • Unable to explain the line spectrum of light produced by atoms
 

Quantum theory

  1. Max Planck
    • Introduced the concept of quantum (discrete energy) in 1900
       
  2. 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
       
  3. Niels Bohr
    • Explained the production of line spectrum by hydrogen atoms
       
  4. Louis De Broglie
    • Introduced the hypothesis on the wave nature of particles in 1924
 

Electromagnetic spectrum may be a:

  1. Continuous spectrum
  2. Line spectrum
 
  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

 
     
 
 

 

 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

  1. 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
       
  2. Double-slit experiment (Thomas Young)
    • Unable to explain the radiation spectrum produced by black bodies
       
  3. Dalton atomic model 
    • Unable to explain the light spectrum produced by atoms
       
  4. Discovery of electrons
    • Unable to explain the line spectrum of light produced by atoms
 

Quantum theory

  1. Max Planck
    • Introduced the concept of quantum (discrete energy) in 1900
       
  2. 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
       
  3. Niels Bohr
    • Explained the production of line spectrum by hydrogen atoms
       
  4. Louis De Broglie
    • Introduced the hypothesis on the wave nature of particles in 1924
 

Electromagnetic spectrum may be a:

  1. Continuous spectrum
  2. Line spectrum
 
  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