Question

What is the effect on the Fresnel biprism experiment when the white light is used?
A. Fringe is affected
B. The diffraction pattern is spread more
C. Central fringes are white, and all others are colored
D. None of these

Answer 1

Hint: When waves face obstacles or any openings, they tend to bend around the edge of the obstacle or the opening if the wavelengths of the wave are comparable to the order of dimension of the obstacle. This bending of waves around the edge of an obstacle is called diffraction.

Complete step by step answer:
According to Huygens’s wave theory of light, there are secondary wavefronts associated with each progressive wave. According to Fresnel, the effect of a wavefront at an external point will depend on the source of light, the distance between the screen and the aperture through which light passes, and the size of the aperture. Fresnel’s assumptions were as follows
A wavefront can be further divided into a large number of small strips called zones.
Depending on the distance of the point from the zone, the effect at that point due to that particular zone will vary.
The result at the point will also depend on the obliquity of the point in consideration of the zone we are referring to.
When we consider a white light, there will not be any path difference at the central point. For all the other points from the central point, the other wavelengths contained in the white light will satisfy the conditions for maxima. Therefore, they will appear colored.
The answer is Option (C): central fringes are white, and all others are colored

Note: Diffraction results from the interaction of light coming from the different parts of the same wavelength. The fringes formed due to diffraction will not be of the same width. The minimum intensity regions will appear perfectly dark. The maximum intensity is for central maxima, and different maxima have different intensities.