Photo-electrons are produced from a metal surface using a radiation of wavelength `6561Å` . These photo-electrons, when made to enter a uniform magnetic field of intensity `3 xx 10^(-4)` T, move along different circular paths with a maximum radius of 10mm, then the work function of the metal is close to

Options

A. `1. 8 e V`
B. `0.8 eV`
C. `1.6 eV`
D. `1.1 eV`

Answer

D

Explanation

`K E _(max) = E - phi`
` = (12 400 )/( lamda ("in A"))- phi `(in eV)
`therefore r = ( sqrt (2 m KE ))/( e B )`
`KE _(max) = (r ^(2) e ^(2) B ^(2))/( 2m)`
`= (r ^(2) e B ^(2))/( 2m ) `(in J)
`therefore phi = (12 400)/(6561) - (r ^(2) e B ^(2))/( 2m)`
`= 1.1 e V`