Obtain a relation between phase difference and path difference.

What is Huygen's principle? Explain the optical phenomenon of refraction using Huygen's principle.

What is total internal reflection. Explain the phenomenon using Huygen's principle.

A beam of light consisting of two wavelengths, $650 \mathrm{nm}$ and $520 \mathrm{nm}$, is used to obtain interference fringes in a Young’s double-slit experiment. Find the distance of the third bright fringe on the screen from the central maximum for wavelength $650 \mathrm{nm}$.

A beam of light consisting of two wavelengths, $650 \mathrm{nm}$ and $520 \mathrm{nm}$, is used to obtain interference fringes in a Young’s double-slit experiment. What is the least distance from the central maximum where the bright fringes due to both the wavelengths coincide?