An ink mark is made on a piece of paper on which a glass slab of thickness '$$t$$' is placed. The ink mark appears to be raised up through a distance '$$x$$' when viewed at nearly normal incidence. If the refractive index of the material of glass slab is '$$\mu$$' then the thickness of glass slab is given by

Converging or diverging ability of a lens or mirror is called

One of the necessary condition for total internal reflection to take place is

( $$\mathrm{i}=$$ angle of incidence, $$\mathrm{i}_{\mathrm{c}}=$$ critical angle)

A double convex lens of focal length '$$F$$' is cut into two equal parts along the vertical axis. The focal length of each part will be