The idea of secondary wavelets for the propagation of a wave was first given by

Which of the following equations represents a circle?

Seven white balls and three black balls are randomly arranged in a row. The probability that no two black balls are placed adjacently is

$A$ choke coil works on the principle of

The refracting angle of a prism is $A$ and the refractive index of the material of the prism is $\cot \frac{A}{2}$. The angle of minimum deviation is

Two blocks of masses $1 ~kg$ and $2 ~kg$ are connected by a metal wire going over a smooth pulley as shown in the figure. The breaking stress of the metal is $2 \times 10^9 ~N/m^2$. What should be the minimum radius of the wire used if it is not to break? Take $g = 10 ~m/s^2$.