Electrons are emitted with zero velocity from a metal surface when it is exposed to radiation of wavelength ` 6800` Å . Calculate threshold frequency `( v_0)` and work function `(W_0)` of the metal.

Explanation

Energy of one photon (E) `=hv=hc//lambda`
`h=6.626xx10^(-34)J s , c=3xx10^(8) ms^(-1) , lambda= 0.57xx10^(-6) m` ltbr. `E=((6.626xx10^(-34)Js)xx(3xx10^(8)ms^(-1)))/((0.57xx10^(-6)m))=3.48xx10^(-19)J`
Rate of emission of quanta per second `=("Power")/("Energy")`
Power (P)=25 watt=`25 Js^(-1) , E = 3. 48 xx 10^(-19)J`
`=((25"watt"))/((3.48 xx 10^(-19)J))=((25 J s^(-1)))/((3.48 xx 10^(-19)J))=7.18 xx 10^(19) s^(-1)`.