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Question
What are the two methods for obtaining coherent sources in the laboratory?
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Solution
In the laboratory, coherent sources can be obtained by using (1) Lloyd's mirror and (2) Fresnel's biprism.
- Lloyd's mirror:
This is an extensively used device. The light from a source is made to fall at a grazing angle on a plane mirror as shown in figure.
Some of the light falls directly on the screen as shown by the blue lines in the figure and some light falls after reflection, as shown by the red lines. The reflected light appears to come from a virtual source and so we get two sources. They are derived from a single source and hence are coherent. They interfere and an interference pattern is obtained as shown in the figure. Note that even though we have shown the direct and reflected rays by blue and red lines, the light is monochromatic having a single wavelength. - Fresnel's biprism:
A biprism is a prism with a vertex angle of nearly 180°. It can be considered to be made up of two prisms with very small refracting angle ranging from 30′ to 1°, joined at their bases. In experimental arrangement, the refracting edge of the biprism is kept parallel to the length of the slit. Monochromatic light from a source is made to pass through a narrow slit S as shown in Figure and fall on the biprism.
The two halves of the biprism form virtual images S1 and S2. These are coherent sources having obtained from a single secondary source S. The two waves coming from S1 and S2 interfere and form interference fringes like that in Young’s double-slit experiment in the shaded region shown in the figure.
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