what is the work fuction of the metal if the light of wavelength 4000Å generates photoelectrons of velocity `6xx10^(5) ms^(-1)` from it ?
(Mass o felectron `=9xx10^(-31)`kg
Velocity of light `=3xx10^(8)ms^(-1)`
Planck's constant `=6.626xx10^(-34)Js`
Charge of electron `=1.6xx10^(-19)jeV^(-1)`

Options

A. `4.0 eV`
B. `2.1 eV`
C. `3.1 eV`
D. `0.9 eV`

Answer

B

Explanation

`hv = w_0 + 1/2 mv^2`
`w_0 = hv - 1/2 mv^2`
`= (6.626 xx 10^(-34) xx 3.0 xx 10^8)/(4000 xx 10^(-10)) - 1/2 xx 9 xx 10^(-31) xx (6 xx 10^5)^(2)`
`= 4.96 xx 10^(-19) - 1.62 xx 10^(-19)`
`= 3.34 xx 10^(-19) J`
or `= (3.34 xx 10^(-19))`
`= 2.1 eV`.