Optics

The fundamental transverse mode of a laser beam, , follows a Gaussian intensity distribution:
where is the beam waist. The intensity profile is Gaussian-shaped in both transverse dimensions.

The longitudinal mode spacing is given by:
Substituting m/s and cm = m:

For Young’s Double Slit Experiment, the fringe width is given by:
where:
- = wavelength of light
- = distance between slits and screen
- = separation between slits
Since , the fringe width increases with wavelength.

In Newton’s rings experiment, the diameter of the nth dark ring is given by: Since the diameters of the rings are in ratio 10:7, the ratio of refractive indices is: Thus, the correct answer is:

A broad light source is required in the Biprism experiment to ensure sufficient interference fringes. In Newton’s rings, a monochromatic point source is preferable.