A small metal plate of work function `phi` is kept at a distance r from a singly ionised, fixed ion. A monochromatic light beam is incident on the metal plate and photoelectrons are emitted. Find maximum wavelength of the light beam so that some of that electrons may go round the ion along a circle.

Explanation

The particle will movie in a circle when the stopping potential is equal to the potential due the singly charged on at the point .
` eV_0 = hc / lambda - phi `
` rArr V_0 = (hc / lambda - phi )1 /e `
Ke / 2d = (hc / lambda - phi )1 /e `
`rArr Ke^(2) / 2d = (hc / lambda - phi ) `
hc / lambda = Ke^(2) / 2d + phi = Ke^(2) + 2d phi / 2d `
`lambda = (hc) (2d) / Ke^(2) + 2d phi `
` (2hdc / (1) / (4 pi epsilon_0) (e^(2)) (2d phi))`
` = 8 pi epsilon_0 hcd / e^(2) + 8 pi epsilon_0 d phi`