Interference Young’s Double Slit Experiment and Expression for Fringe Width, Coherent Sources and Sustained Interference of Light
The observation of the interference effect is an indication of the presence of overlapping waves. It was postulated by Thomas Young that light is having the wave-like properties and it is subjected to the superposition principle. His experiment is intended to demonstrate the destructive and constructive interference of the light.
In the arrangement shown above, the light from the monochromatic line source is passing through the lens and it is focused on the single slit. After this, it is falling on a double slit and is producing two wave trains that are interfering with each other. Across the double-slit, the separation should be less than 1mm. The source single slit, the double-slit should be parallel to each other. In the alternative means, the laser can be used, and the fringes can be viewed on the screen a few meters away without any need for micrometer eyepiece.
The process of interference is happening at all times, at every moment and every place. But yet it is not possible to see interference patterns everywhere.
The two sources are coherent with the waves emitting from them and they are having constant phase difference and same frequency. The randomly phased light waves are constantly producing the dark and bright fringes. At one moment there is a dark fringe and at another moment there is a bright fringe. In this way, the effect of interference is canceled out and only brightness is observed. To truly observe the interferences, the coherent sources of light are required. The characteristics of coherent sources are that the waves have a single frequency, and these waves are generated following by constant phase difference. Commonly, lasers are used as a coherent source and the phenomenon of stimulated emission is used for the generation of highly coherent light.
Conditions for Coherent Interference
The presence of coherent sources of light is a must. The amplitudes and intensities should be nearly equal for the production of sufficient contrast between the minima and maxima. The sources should be small enough to be considered as a point source of the light. For the production of the wide fringes, the interference sources should be close enough. It is a must for the sources to be monochromatic. It is a must for the sources to emit the light in the same state of polarization. For the production of wider fringes, the screen and source should be far enough.
