(a) Resonance occurs when one vibrating body sets other body into vibration at its own natural frequency as a result of impulse received from the first body. (ii) Simple experiment to illustrate resonance.

The apparatus is arranged as shown. A cylinder is 3/4 filled with water and a tube is dipped into the water so that the length I can be varied. A tuning fork f is set vibrating just above the tube but not touching it. As the fork is set vibrating the length I is gradually increased. A point is reached when a large sound is heard. At this point resonance has occurred because the vibrating tuning fork has set the air in the tube vibrating at its own frequency.~(b)
Parallel light rays are reflected by a concave mirror on a piece of carbon paper. The converging of the rays on the carbon paper will cause it to ignite.

The object Ois kept on the centre of curvature of a concave mirror. An exact but inverted image I is formed at the centre of curvature where the screen should be placed.
