A horizontal cesium plate is moved vertically downward at a constant speed v in a room full of radiation of wavelength 250nm and above. What should be the minimum value of v so that the vertically upward component of velocity is nonpositive for each photoelectron?

Explanation

When `lambda = 250nm `
Energy of Photon
` = hc / lambda = 1240 / 250 = 4.96 eV`
`:. K.E. = hc / lambda - W = 4.96 - 1.9 eV `
` = 3.06 eV `
Velocity to be non positive for each photo electron . The minimum value of velocity of plate should be equal to velocity of photo electron
`:. Velocity of photo electron `
` = (sqrt 2KE / M ) = (sqrt 2 xx 3.06 xx 1.6 xx 10^(-19) / 9.1 xx 10^(-31)) `
` = 1.04 xx 10^(6) m/sec`.