Photoelectric effect is a phenomenon of emission of electrons from the surfaces of a metal when the light of suitable frequency falls on it. If light of frequency v falls on a photosenstive surface of work function `phi_(0)`, then the maximum kinetic energy of photoelectron emitted is given by Einstein's photoelectric equation.
`(KE)_(max)=1/2mv_(max)^(2)=hv-phi_(0)`
The value `(KE)_(max)` will increases if the energy of the incident light (hv) increases of work function `(phi_(0))` is decreased. Read above passage and answer the following questions:
(i) Why can visible light not ejected photoelectrons from every surface?
(ii) Light of frequency `7.21xx10^(14)Hz` is incident on a metal surface. Electrons with a maximum speed of `6.0xx10^(5)ms^(-1)` are ejected from the surface. What is the threshold frequency for photo emission of electrons? `h=6.6xx10^(-34)Js, m_(e)=9.1xx10^(-31)kg`.
(iii) What do you learn basically from the above study?

Explanation

(i) This is because, the energy of visible photons is less than work function of most of the metals.
(ii) `1/2mv_(max)^(2)=hv-phi_(0)=hv-hv_(0)`.
or `v_(0)=v-(mv_(max)^(2))/(2h)=7.21xx10^(14)-((9.1xx10^(-31))xx(6xx10^(5))^(2))/(2xx(6.63xx10^(-34)))=4.74xx10^(14)Hz`
(iii) We find that KE of electron ejected can be increased by increasing the incident energy or by decreasing the work function. In day to day life we can prosper by increasing our sincere efforts or by decreasing our requirements.