A Young's double slit experiment apparatus has slits separated by 0.2 mm and a screen 60 cm away from the slits. The whole apparatus is immersed in a liquid medium of refractive index $11 / 9$ and the slits are illuminated with green light ( $\lambda=550$ nm in vacuum). Find the fringe width of the pattern formed on the screen.

If in a Young's double slit experiment the slit separation is doubled and the distance of the screen from the slits is reduced to half, then the fringe widths become how many times their original value?

Which of the following phenomena produces the colour in soap bubble?

Wavelength of light used in an optical instruments are $\lambda_1=4000 \mathop {\rm{A}}\limits^{\rm{o}}$ and $\lambda_2=5000 \mathop {\rm{A}}\limits^{\rm{o}}$, then the ratio of their respective resolving powers (corresponding to $\lambda_1$ and $\lambda_2$ ) is