The half-life of a radioactive substance is $$20 \mathrm{~min}$$. The approximate time interval $$\left(t_2-t_1\right)$$ between the time $$t_2$$, when $$\frac{2}{3}$$ of it has decayed and time $$t_1$$ when $$\frac{1}{3}$$ of it had decayed is
In the figure, mass of a ball is $$\frac{9}{5}$$ times mass of the rod. Length of rod is $$1 \mathrm{~m}$$. The level of ball is same as rod level. Find out time taken by the ball to reach at upper end of rod.
The diode used at a constant potential drop of 0.5 V at all currents and maximum power rating of 100 mW. What resistance must be connected in series diode, so that current in circuit is maximum?
An unpolarised beam of intensity $$2 a^2$$ passes through a thin polaroid. Assuming zero absorption in the polaroid, the intensity of emergent plane polarised light is