Write down Einstein's photoelectric equation and explain it .With the help of a neat diagram,describe briefly the constructin and working of a photoemissive cell.Mention any four applications of phtotlectric effect.

Explain, with the help of a schematic diagram, the principle and working of a Light Emiting Diode. What criterion is kept in mind while choosing the semiconductor material for such a device ? Write any two advantages of light emiting diode over conventional incandescent lamps.