Quantum Physics

• Photelectric Effect

Light hitting a metal surface ejects an electron. According to Classical Theory the energy of an emitted electron depends on intensity. But it didn`t match with experimental data. When frequency exceeds certain values electrons, regardless of intensity of the light , started to build up energy that required to produce photoelectrons.

So Einstein offered a generalization of Planck`s postulate of quantization thermal radiation to EM waves, ‘light consist of quanta called photons’

Einstein hypothesized that light of frequency have an energy value

E = nhv ; n = 1, 2, 3, ...

When a photon absorbed by a metal surface, surface gains a energy which is the energy of photon. The necessary energy to eject an electron from the surface is “w” called work function. Thus, the maximum energy of emitted electrons in terms of frequency becomes.

• Compton Effect

Wavelength shifts were observed as the X-Ray scattered at various angles from a carbo foil in 1923.

This incident couldn`t explained from wave like property of light. However, the scattering was easily explained in particle theory of light. Compton considered the incident beam as a beam of photons of energy.

Incident physically occurs as an elastic collision so that energy and momentum must be conserved.

Conservation of momentum;

Conservation of energy;

Shows that wavelength shifts is not depend on initial energy but scattering angle.

• Particle-Wave Duality

Photon is a relativistic particle with zero rest mass;

E = pc = hv ; lambda = h / p

Its momentum is defined by p = E / c and the energy of photon is E = hv. thus we obtain p = h/ lambda. For a particle with mass travelling relativistic velocities, its total energy;

If it`s not;

When wavelength approaches to zero, the wave like property of system disappears.