$A$ sound wave of wavelength $32 \ cm$ enters the tube at $S$ as shown in the figure. Then the smallest radius $r$ so that a minimum of sound is heard at detector $D$ is ... $cm$.

Two identical sinusoidal waves are moving in the same direction along a stretched string and interfere with each other. The phase difference between them is $120^{\circ}$. The amplitudes of both the waves are the same. If the amplitude of the resultant wave due to interference is $2 \,mm$, the amplitude of each wave is:

State the principle of superposition and explain it.

Two pulses in a stretched string whose centres are initially $8 \ cm$ apart are moving towards each other as shown in the figure. The speed of each pulse is $2 \ cm/s$. After $2 \ seconds$,the total energy of the pulses will be

The similarity between sound and light waves is:

Two waves are propagating along a taut string that coincides with the $x$-axis. The first wave has the wave function $y_1 = A \cos[k(x - vt)]$ and the second has the wave function $y_2 = A \cos[k(x + vt) + \phi]$.