Question

In a Fresnel biprism experiment, the two position of lens give separation between the slits as $16cm$ and $9cm$, respectively. What is the actual distance of separation?

• A. $12cm$
• B. $12.5cm$
• C. $13cm$
• D. $14cm$

Answer 1

The correct option is A $12cm$

Fresnel used a biprism to obtain two coherent sources for producing interference fringes in the laboratory. It consists of two acute angled prisms with their bases in contact. In actual practice, the biprism is constructed as a single prism of obtuse angle of about ${179}^{\circ }$ and the remaining two acute angles are ${30}^{\prime }$ each. In biprism, the virtual images act as two coherent sources and from these two virtual images rays superimposed and interference fringes are formed in overlapping region AB on a screen placed at O.
In order to measure the distance $2d$ between the virtual sources $S_1$ and $S_2$. Glaze brook gave a method, known as magnification method due to Glaze book. If $d_1$ and $d_2$ are distance between real images of $S_1$ and $S_2$, then
$2d=\sqrt{d_1{d}_2}$
Given, $d_1=16cm,d_2=9cm$
$\therefore 2d=\sqrt{16\times 9}=12cm$