In a photocell bi chromatic light of wave length `2480Å` and `6000Å` are incident on a cathode whose work function is 4.8eV. If a uniform magnetic field of `3xx10^(-5)T` exists parallel to the plate, find the radius of the circular path described by the photoelectron. (mass of electron is `9xx10^(-31)kg)`

Explanation

`E_(1)=(12400)/(lamda_(1))=(12400)/(2480)=5eV`
`E_(2)=(12400)/(lamda_(2))=(12400)/(6000)=2.06eV`
As `E_(2)ltW_(0)` and `E_(1)gtW_(0)`, photo electric emission is possible only with `lamda_(1)`.
Maximum K.E of emitted photo electrons
`K=E_(1)-W_(0)=0.2eV`.
Photo electrons experience magnetic force and move along a circular path of radius
`r=(mv)/(Bq)=(sqrt(2mK))/(Bq)impliesr=5cm`.