(i) Define resonant vibrations.

(ii) Which characteristic of sound, makes it possible to recognize a person by his voice without seeing him ?

(i) If the lens is placed in water instead of air, how does it's focal length change?

(ii) Which lens, thick or thin has greater focal length?

Two waves of the same pitch have amplitudes in the ratio 1:3. What will be the ratio of their:

(i) intensities and

(ii) frequencies?