Identify the physical quantity that cannot be measured using spherometer :

A convex lens of focal length $$40 \mathrm{~cm}$$ forms an image of an extended source of light on a photoelectric cell. A current I is produced. The lens is replaced by another convex lens having the same diameter but focal length $$20 \mathrm{~cm}$$. The photoelectric current now is :

A vessel of depth '$$d$$' is half filled with oil of refractive index $$n_{1}$$ and the other half is filled with water of refractive index $$n_{2}$$. The apparent depth of this vessel when viewed from above will be-

An ice cube has a bubble inside. When viewed from one side the apparent distance of the bubble is $$12 \mathrm{~cm}$$. When viewed from the opposite side, the apparent distance of the bubble is observed as $$4 \mathrm{~cm}$$. If the side of the ice cube is $$24 \mathrm{~cm}$$, the refractive index of the ice cube is