- Fresnel's Biprism produces two virtual coherent sources from a single source by refracting light through two prisms placed base-to-base. - These two coherent sources interfere, producing interference fringes in the region where the light beams overlap. - The fringe width (β), which is the distance between two consecutive bright or dark fringes, is given by the formula: β = λD / d. - In the experiment, β, D, and d are measured. - β is measured using a microscope by finding the positions of several fringes. - D is the distance from the slit (source) to the eyepiece (where fringes are observed). - d is the distance between the two virtual coherent sources. This is usually determined by measuring the distance between the images of the slit formed by a convex lens placed between the biprism and the eyepiece (using the displacement method) and the distance D. - To find the wavelength (λ), the formula is rearranged as λ = βd / D. - Therefore, the calculated value of λ depends on the values of β, d, and D.