A charge q is placed at the centre of the line joining two equal charges Q. The system of the three charges will be in equilibrium if q is equal to

Two plane mirrors are inclined at 70^o. A ray incident on one mirror at angle, $$\theta $$ after reflection falls on second mirror and is reflected from there parallel to first mirror. The value of $$\theta $$ is

The reddish appearance of the sun at sunrise and sunset is due to

A source of light is placed at a distance of 50 cm from a photo cell and the stopping potential is found to be V₀. If the distance between the light source and photo cell is made 25 cm, the new stopping potential will be :