To obtain the powerful parallel beams of light from a vehicle's headlight, one must use

Two thin convex lenses of focal lengths 4 cm and 8 cm are separated by a distance of 4 cm in air. The combination will have the focal length

Two metallic wires A and B are of same material and have equal length. If the cross-sectional area of B is double that of A, then which one among the following is the electrical resistance of B?

The torque on a rectangular coil placed in a uniform magnetic field is large when the

The thermal conductivity of copper is 4 items that of brass. Two rods of copper and brass having same length and cross-section are joined end to end. The free end of copper is at 0^o C and the free end of brass is at 100^o C. The temperature of the junction is

If the focal length of the biconvex lens is 25 cms, then the power of the lens will be