When a metal plate is exposed to a monochromatic beam of light of wavelength 400nm, a negative potential of 1.1 V is needed to stop the photocurrent . Find the threshold wavelength for the metal.

Explanation

`lambda = 400 nm = 400 xx 10^(-9) m `
` :' hc / lambda = hc / lambda_0 ev_0`
`rArr 6.63 xx 10^(-34) xx 3 xx 10^(8) / 400 xx 10^(-9) `
` = 6.63 xx 10^(-34) xx 3 xx 10^(8) / lambda_0 + 1.6 xx 10^(-19) xx 1.1`
`rArr 6.63 xx 3 / 4 xx 10^(-34) xx 10^(8) / 10^(-7) `
` = 6.63 xx 3 xx 10^(-26) / lambda_0 + 1.6 xx 1.1 xx 10^(-19)`
` rArr 4.97 xx 10^(-19)`
` 19. 89 xx 10^(-26) / lambda_0 + 1.76 xx 10^(-19) `
` rArr 4.97 = 19.89 xx 10^(-7) / lmabda_0 + 1.76 `
` rArr 19.89 xx 10^(-7) / lambda_0 = 4.97 - 1.76 = 3.21 `
` rArr lambda_0 = 19.89 xx 10^(-7) / 3.21 `
` = 6.196 xx 10^(-7) m = 620nm`