JEE Main Chemistry: Wave Optics questions with solutions

Q1. In a Young's double-slit experiment, the slits are illuminated by orange light of wavelength lambda = 600 nm. A thin transparent plastic slab of thickness t is placed over one slit. The number of fringe shifts N on the screen is plotted against the refractive index mu of the plastic slab. The graph is a straight line through the origin with slope equal to 80 (i.e., N increases by 80 for every unit increase in mu). Find the thickness t of the slab.

1. 4.8 mm
2. 48 micrometers
3. 2.4 micrometers
4. 24 micrometers

Answer: 48 micrometers

From the fringe-shift formula, the slope of N vs mu is t/lambda. With slope = 80 and lambda = 600 nm, t = 80 * 600 nm = 48000 nm = 48 micrometers.