The half life of a radioactive substance is 30 minute. The time taken between 40% decay and 85% decay of the same radioactive substance is

Two monochromatic beams of intensities I and 4 I respectively are superposed to form a steady interference pattern. The maximum and minimum intensities in the pattern are

Three charges each of $$+1 \mu \mathrm{C}$$ are placed at the corners of an equilateral triangle. If the repulsive force between any two charges is $$\mathrm{F}$$, then the net force on either charge will be [$$\cos 60^{\circ}=0.5$$]

Velocity of sound waves in air is '$$\mathrm{V}$$' $$\mathrm{m} / \mathrm{s}$$. For a particular sound wave in air, path difference of 'x' $$\mathrm{cm}$$ is equivalent to phase difference $$n \pi$$. The frequency of this wave is