(a) In Young's double slit experiment, describe briefly how bright and dark frings are obtained on the screen kept in front of a double slit. Hence obtain the expression for the fringe width.
(b) The ratio of the intensities at minima to the maxima in the Young's double slit experiment is 9 : 25. Find the ratio of the widths of the two slits.

Explanation

### Solution:

**(a)** In Young's double slit experiment, bright and dark fringes are formed due to the interference of light waves coming from two closely spaced slits.

1. **Formation of Bright Fringes**:
- Bright fringes occur at points where the light waves from the two slits arrive in phase, which means their path difference is an integer multiple of the wavelength (\( \Delta x = n\lambda \), where \( n = 0, 1, 2, \ldots \)).
- This results in constructive interference, leading to maximum intensity at these points.

...