Question

Explain the difference between interference and diffraction of light. Write the expression for the width of interference fringes in Young's double slit experiment and explain the effect of separation of slits and wavelength of light used on it.

Hint:

In interference, the separation of slits and wavelength both directly affect the fringe pattern. A larger separation leads to closer fringes, while a longer wavelength spreads them out.

Answer 1

Interference: It is the phenomenon where two or more light waves superpose to form a resultant wave. Constructive interference occurs when the waves are in phase, and destructive interference occurs when they are out of phase.
Diffraction: It refers to the bending of light around obstacles and the spreading of light as it passes through small openings. It is most noticeable when the size of the obstacle or aperture is comparable to the wavelength of the light.
Expression for Width of Interference Fringes:
In Young's double slit experiment, the fringe width \( \beta \) is given by the expression:
\[ \beta = \dfrac{\lambda D}{d} \] Where:
- \( \lambda \) is the wavelength of the light,
- \( D \) is the distance between the slits and the screen,
- \( d \) is the separation between the two slits.
Effect of Separation of Slits and Wavelength of Light:
- If the slit separation \( d \) is increased, the fringe width \( \beta \) decreases.
- If the wavelength \( \lambda \) is increased, the fringe width \( \beta \) increases.