The electric charge is stored in a device called

 Which one of the following statements correctly defines power?

In vacuum, the speed of light

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?