41. Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is ?

40. A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the Sun is 1.39 × 109 m and its mean distance from the earth is 1.5 × 1011 m. What is the diameter of the Sun’s image on the paper ?

A parallel plate capacitor has a uniform electric field E in the space between the plates. If the distance between the plates is d and area of each plate is A, the energy stored in the capacitor is ?

The electric potential at a point in free space due to a charge Q coulomb is Q × 10^11 volts. The electric field at that point is ?

A thin conducting ring of radius R is given a charge +Q. The electric field at the centre O of the ring due the charge on the part AKB of the ring is E. The electric field at the centre due to the charge on the part ACDB of the ring is ?