Question

If white light is used in a Young’s double slit experiment.
$\left( A \right)$ bright white fringes are formed at the centre of the screen.
$\left( B \right)$ fringes of different colours are observed clearly only in the first order.
$\left( C \right)$ the first-order violet fringes are closer to the centre of the screen than the first-order red fringes.
$\left( D \right)$ the first-order red fringes are closer to the centre of the screen than the first-order violent fringes.

Answer 1

Hint: We will start this question by explaining the actual Young’s Double Slit Experiment and then we will discuss what would happen if we use white light instead of monochromatic light and then we will get the required answer.

Complete Step-by-Step solution:
In 1801, Thomas Young was the first person to demonstrate experimentally the interference of light. In this experiment, a source of monochromatic light (e.g., a sodium vapour lamp) illuminates a rectangular narrow slit $S$, about 1 mm wide. ${S_1}$ and ${S_2}$ are two parallel narrow slits which are arranged symmetrically and parallel to the slit $S$ at a distance of about 10 cm from it. The separation between ${S_1}$ and ${S_2}$ is 2 mm and the width of each slit is 0.3 mm. An observation screen is placed at a distance of 2 m from the two slits. Alternate bright and dark bands appear on the observation screen. These are called interference fringes. When one of the slits, ${S_1}$ or ${S_2} $ is closed, bright and dark fringes disappear and the intensity of light becomes uniform.

White light consists of waves of innumerable wavelengths starting from violet to red colour. Therefore, if monochromatic light in Young’s interference experiment is replaced by white light, then the waves of each wavelength form their separate interference patterns The resultant effect of all these patterns is obtained on the screen.
Therefore, the central fringe is white. As fringe width, i.e., wavelength increases in order of colours denoted by VIBGYOR. That is the violet (V) fringe appears first and the red the last. After this the fringes of many colours overlap at each point of the screen and so the screen appears uniformly illuminated.
Hence, option A, B and C are the correct answers.

Note – When two light waves of the same frequency and having zero or constant phase difference travelling in the same direction superpose each other, the intensity in the region of superposition gets redistributed, becoming maximum at some points and minimum at others. This phenomenon is called interference of light.