For a combination of two convex lenses of focal lengths $f_1$ and $f_2$ to act as a glass slab, the distance of separation between them is

If a ray of light passes through an equilateral prism such that the angle of incidence and the angle of emergence are both equal to $70 \%$ of the angle of the prism, then the angle of minimum deviation is

Young's double slit experiment is performed with monochromatic light of wavelength $6000 \mathring{A}$. If the intensity of light at a point on the screen, where path difference of $2000 \mathring{A}$ is $I_1$ and the intensity of light at a I point on the screen, where the path difference is $1000 \mathring{A}$ is $I_2$, then $I_1: I_2=$

Two positive point charges are separated by a distance of 4 m in air. If the sum of the two charges is $36 \mu \mathrm{C}$ and the electrostatic force between them is 0.18 N , then the bigger charge is