Introduction to Dual Nature



Particle and wave nature of light

Dual nature of light:
  • Sometimes it behaves like a particle (called a photon), which explains how light travels in straight lines
  • Sometimes it behaves like a wave, which explains how light bends (or diffracts) around an object 
Light also shows interference properties, for example the formation of dark and bright fringes in Young's Double Slit experiment illustrates this fact. Light also exhibits polarization. Natural light coming from the sky is partially polarised but it can be made fully polarised by passing through a polaroid.


Problems on Millikan's drop experiment

In Millikan's oil drop experiment a fine mist of oil droplets was sprayed into a chamber above the plates. The oil was of a type usually used in vacuum apparatus and was chosen because it had an extremely low vapour pressure. Ordinary oil would evaporate under the heat of the light source causing the mass of the oil drop to change over the course of the experiment. Some oil drops became electrically charged through friction with the nozzle as they were sprayed. Alternatively, charging could be brought about by including an ionising radiation source (such as an X-ray tube). The droplets entered the space between the plates and, because they were charged, could be made to rise and fall by changing the voltage across the plates. It was found that the charge on an oil droplet was always an integral multiple of an elementary charge, . Thus, it was established that electric charge is quantized and from the values of charge (e) and specific charge (e/m, the mass (m) of the electron could be determined.

Example: In a Millikans oil drop experiment, an oil drop of mass , carrying a charge remains stationary between two plates separated by a distance of 5 mm. Given ; find the voltage that must be applied between the plates.

Let the voltage applied by ,
According to equilibrium eqn.,