Interference Of Light Waves

"Superposition of two light waves having phase coherence traveling in the same direction results in a phenomena called Interference."
Light from a Source S passes through a pinhole and falls on two further pinholes A and B, light and dark fringes appear on the screen where the resultant pencils of light overlap. The pencils of light interfere and produce "Interference fringes", first observed by Thomas Young in 1801. These and similar fringes were used by Fresnel and Young to establish the wave theory of light.
a

  Fig   Young's Double slit Experiment for Interference

        When two waves are allowed to superpose upon each other and, if the resultant intensity of the interfering waves is zero or less than the intensity of the either individual wave then this type of interference is called "Destructive Interference" and it occurs where crest of one wave falls upon trough of other wave.

       Similarly, if the resultant intensity of the interfering waves is greater than the intensity of an individual wave then this type of interference is known as "Constructive Interference" and this occurs where crest of one wave overlaps the crest of other wave or trough of one wave overlaps the trough of other wave.

       

Essential Conditions for the Interference:

         Interference of light waves is not easy to observe because of the random emission of light from a source. The following conditions must be met, in order to observe the phenomena:

1- The interfering beams must be monochromatic that is, of a single wavelength.

2- The interfering beams must be coherent.

Coherent: waves that are in phase both temporally and spatially. Most practical radiation sources are not coherent over an appreciable length of time since waves trains of limited length are emitted at random intervals. The laser is a source of coherent radiations.

      Consider two or more sources of light waves of the same wavelength. If the sources send out crests or troughs at the same instant, the individual waves maintain a constant phase difference with one another. The monochromatic sources of light which emit waves, having a constant phase difference are called coherent source.

     A common method of producing two coherent light beams is to use a monochromatic source to illuminate a screen containing two small holes, usually in the shape of slits. The light emerging from the two slits is coherent because a single source produces the original beam and two slits serve only to split it into two parts. The points on Huygen's wavefront which send out secondary wavelets are also coherent sources of light.

"In an Interferometer, fringes are produced and used to make accurate measurement of wavelength."