Find the threshold wavelengths for photoelectric effect from a copper surface, a sodium surface and a cesium surface. The work functions of these metals are 4.5 e V, 2.3 eV and 1.9 eV respectively.

Explanation

If ` lambda_0` be the threshold wavelength and `varphi` be the
work function,
` ( lambda_0)= hc/varphi`
`= 1242 eV nm/varphi.`
` For copper, lambda_0=(1242 e V nm/ 4.5 e V) = 276 nm. `
` For sodium, lambda_0= (1242e V nm/ 2.3 e V) = 540nm.`
` For cesium, lambda_0 = (1242 e V nm/ 1.9 e V) = 654 nm.`