Dual Nature of Radiation and Matter

The Photoelectric Effect

 When light of sufficiently small wavelength is incident on a metal surface, electrons are ejected from the metal. This phenomenon is called as 'photoelectric effect' and the ejected electrons are called as 'photoelectrons'.

Particle Nature of Light

     According to wave theory when light falls on a metal surface, energy is continuously distributed over the surface. All the free electrons receive light energy and when the energy received exceeds that of work function, an electron may escape the surface. If we user a low intensity source, it may take hours before an electron to come out.s

Photocells and their Application

     A photocell converts the change in the intensity of light into a change in the electric current. The diagram shows a photocell circuit. The cathode is made of photosensitive material.

Photo Voltaic Cell

     Photo voltaic cell converts light energy into electrical energy. It acts as a cell. It consists of a metal layer of copper over which a semiconductor layer of cuprous oxide coated with a thing film of silver or gold.

Photoconductive Cell

     The electrical resistance of a semiconductor depends on the intensity of the incident light. A photoconductive cell works on the above principle.

Applications of Photoelectric Cells

     Photocells are used in television camera to reproduce sound recorded on films, in counting devices, in burglar and fire alarms, to measure the temperature of stars, to study the spectrum of heavy bodies, to operate street light, to compare the illuminating powers of two sources, in photometers, for locating minor flaws in metallic sheets, to determine the opacity of solids and liquids, to control the temperature in chemical reactions and to determine the Planck's constant.

Matter Waves

     The suggestion that matter may have wave like properties was first put forwarded in 1924-1925 by Louis De Broglie. He argued that if light, which consists of waves according to classical picture, can sometimes behave like particles, then it should be possible for matter, which consists of particles to exhibit wave-like character under suitable circumstances.

De-Broglie Relationship

     De-Broglie noted that according to the theory of relativity, the role played by momentum 'P' and the energy 'E' of the particle is done by angular frequency 'w' and the propagation vector 'k' of a wave.

De-Broglie Wavelength of an Electron

     Similar to the crystal diffraction patterns produced by X-rays, even the beam of electrons of appropriate momentum could produce crystal diffraction pattern.s

Davison and Germer Experiment

     The first experimental proof of the wave nature of electron was demonstrated in 1927 by two American physicists C.J Davison and L.H Germer.

Elementary Idea of Electron Microscope

     In optical microscope, the ray of light can be bent by a lens system. By placing the object and the lens system in such a way, we control the direction of the ray to get a magnified image of the object.

Summary

     The phenomenon of interference, diffraction and polarization can be explained by the wave theory of light.