Problem

In a photocell the plates

P

and

Q

have a separation of 5 cm, which are connected through a galvanometer without any cell. Bichromatic light of wavelengths

4000 \text{ \AA}

and

6000 \text{ \AA}

are incident on plate

Q

whose work-function is 2.39 eV. If a uniform magnetic field

B

exists parallel to the plates, find the minimum value of

B

for which the galvanometer shows zero deflection.