The work function of a photoelectric material is 4.0 e V. (a) What is the threshold wavelength? (b) find the wavelength of light for which the stopping potential is 2.5 V.

Explanation

`phi = 4ev = 4 xx 1.6 xx 10^(-19) J`
(a) Threshold wavelength ` = lambda `
` = hc / phi `
` =6.63 xx 10^(-34) xx 3 xx 10^(8) / 4 xx 1.6 xx 10^(-19) `
` 6.63 xx 3 / 64 xx 10^(-27) / 10^(-9) `
`= 3.1 xx 10^(-7) nm `
(b) Stopping potential is 2.5 V
` E = phi + ve `
`rArr hc / lambda = 4 xx 1.6 xx 10^(-19) + 1.6 xx 10^(-19) xx 2.5 `
rArr lmabda = 6.63 xx 10^(-34) xx 3 xx 10^(8) / 6.5xx 1.6 xx 10^(-19)`
` = 6.63 xx 3 xx 10^(-26) / 1.6 xx 10^(-19) xx 6.5 `
` 1.9125 xx 10^(-7) 190 nm `