OR (b) (i) Differentiate between a wavefront and a ray. (ii) State Huygen's principle and verify laws of reflection using suitable diagram. (iii) In Young's double slit experiment, the slits $S_1$ and $S_2$ are $3 \text{ mm}$ apart and the screen is placed $1.0 \text{ m}$ away from the slits. It is observed that the fourth bright fringe is at a distance of $5 \text{ mm}$ from the second dark fringe. Find the wavelength of light used.