The work functions `(W_(0))` of K, Na, Li, Mg and Cu are 2.25 , 2.30 ,2.42 ,3.70 and 4.80 eV respectively. How many of these metals do not undergo photoelectric effect when a radiation of wavelength 450 nm is allowed to fall on them ? (1eV = `1.602 xx 10^(-19) J) `

Options

A. 2
B. 1
C. 3
D. 5

Answer

A

Explanation

`lambda= 450 ` nm
E = hv
or E = `h(c )/(lambda) implies 6.6xx10^(-34)Jsxx (3xx10^(8)m//s)/(450xx10^(-9)m)`
`E= 0.044xx10^(-17) or 4.4 xx10^(-19)J`
or ` E = (4.4xx10^(-19))/(1.602xx10^(-19))= 2.74 eV`
`[ because 1eV=1.602xx 10^(-19)]`
Thus K Na and Li which have less value of work functions `(W_(0))` than the energy of striking photon (E) , so can undergo photoelectric effect when radiation of wavelength 450 nm is allowed to fall on them.