In a photocell bichromatic light of wavelength `2475 Å` and `6000 Å` are incident on cathode whose work function is `4.8 eV`. If a uniform magnetic field of `3 xx 10^(-5) tesla` exists parallel to the plate, the radius of the path described by the photoelectron will be (mass of electron ` = 9 xx 10^(-31) kg`)

Options

A. `1 cm`
B. ` 5 cm`
C. `10 cm`
D. `25 cm`

Answer

B

Explanation

Energy of photons corresponding to light of wave length `lambda_(1) = 2475 Å is E_(1) = (12375)/(2475) = 5 eV`.
and that corresponding to `lambda_(2) = 6000 Å` is
`E_(2) = (12375)/(6000) = 2.06 eV`
As `E_(2) lt W_(0)` and `E_(1) gt W_(0)`
Photoelectric emission is possible with `lambda_(1)` only . Maximum kinetic energy of emitted photoelectrons `K = E - W_(0) = 5 - 4.8 = 0.2 eV`.
Photoelectrons experiences magnetic force and move along a circular path of radius
`r = (sqrt(2 m k))/(QB) sqrt(( 2 xx 9 xx 10^(-31) xx 0.2 xx 1.6 xx 10^(-19))/(1.6 xx 10^(-19) xx 3 xx 10^(-5)))`
`= 0.05 m = 5 cm`.