For a light ray to undergo total internal reflection ( $\mathrm{i}=$ angle of incidence, $\mathrm{i}_{\mathrm{c}}=$ critical angle)

When the same monochromatic ray of light travels through glass slab and through water, the number of waves in glass slab of thickness 5 cm is same as in water column of height 6 cm . If refractive index of glass is 1.56 , then refractive index of water is

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of combination is

In compound microscope, the focal length and the aperture of the objective used is respectively