Question

In a Fresnel biprism experiment, the two-position of the lens give separation between the slits as 16cm and 9cm, respectively. What is the actual distance of separation?
(A) 12cm
(B) 12.5cm
(C) 13m
(D) 14cm

Answer 1

Hint: In Fresnel biprism, two thin lenses are joined close to each other and interference fringes are found on the screen placed behind the biprism. In this, the light gets split into its constituent colours. It is a kind of variation on the Young's Slits experiment.

Complete step by step answer:
The actual distance of separation canal computed with formula is
\[d=\sqrt{{{d}_{1}}\times {{d}_{2}}}\]
A single wave throat impinges on both prism separation between the slits.
\[{{d}_{1}}=16cm\]
\[{{d}_{2}}=9cm\]
Actual distance separation (d) can be computed with the formula as given
\[d.\sqrt{{{d}_{1}}\times {{d}_{2}}}\]
\[d=\sqrt{16\times 9}\]
\[d=\sqrt{144}\]
D = 12 cm
Thus, the actual distance will be 12cm
Additional information:
In general, the biprism is constructed from a single plate of glass by grinding and polishing, the obtuse angle of the prism and this is done neatly and precisely to avoid gallop errors. The eye-piece can be moved at right angles to the length of the optical bench by giving rotations to the screw. The monochromatic source is always used in such an experiment. Interference fringes are observed

Note: By this formula, we can find any actual distance separation. In this, we have to be very careful that the slit should be vertical and narrow. A convex lens of shorter focal length should be used otherwise a lot of deviation will come in the result of the experiment.