In Young's double slit experiment, two wavelengths $\lambda_1=780 \mathrm{~nm}$ and $\lambda_2=520 \mathrm{~nm}$ are used to obtain interference fringes. If the $n^{\mathrm{th}}$ bright band due to $\lambda_1$ coincides with $(n+1)^{\text {th }}$ bright band due to $\lambda_2$, then the value of $n$ is

In Young's double slit experiment, slits are separated by 2 mm and the screen is placed at a distance of 1.2 m from the slits. Light consisting of two wavelengths $$6500\mathop A\limits^o $$ and $$5200\mathop A\limits^o $$ are used to obtain interference fringes. Then, the separation between the fourth bright fringes of two different patterns produced by the two wavelengths is

According to Huygens' principle, during refraction of light from air to a denser medium

In a system of two crossed polarisers, it is found that the intensity of light from the second polariser is half from that of first polariser. The angle between their pass axes is