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Thermodynamics

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Broad Source Necessity

Interference fringes obtained in the case of Fresnel’s prism, inclined mirrors and Lloyd’s single mirror were produced by two coherent sources. The source used is narrow. These fringes can be obtained on the screen or can be viewed with an eyepiece. In the case of interference in thin films, the narrow source limits the visibility of the film.

Consider a thin film and a narrow source at light S. The ray 1 produces interference fringes because 3 and 4 reach the eye whereas the ray 2 meets the surface at some different angle and is reflected along 5 and 6. Here, 5 and 6 do not reach the eye. Similarly we can take other rays incident at different angles on the film surface which do not reach the eye. Therefore, the portion A of the film is visible and not the rest.

If an extended source of light is used, the ray 1 after reflection from the upper and the lower surfaces of the film emerges as 3 and 4 which reach the eye. Also ray 2 from some other point of the source after reflection from the upper and the lower surfaces of the film emerges as 5 and 6 which also reach the eye. Therefore, in the case of such a source of light, the rays incident at different angles on the film are accommodated by the eye and the field of view is large. Due to this reason to observe interference phenomenon in thin films, a broad source of light is required. With a broad source of light, rays of light are incident at different angles and the reflected parallel beams reach the eye or the microscope objective. Each such ray of light has its origin at a different point on the source.