Q. Light of wavelength 180nm ejects photoelectrons from a plate of metal whose work function is 2eV. If a uniform magnetic of 5 × 10^{-5} T be applied parallel to the plate, what would be the radius of the path followed by electrons ejected normally from the plate with maximum energy

(a) 0.148m

(b) 0.2m

(c) 0.25m

(d) 0.3m

**Ans: (a)**

Sol: E = 1240/180 = 6.9eV , φ = 2eV

Since , E = φ + K_{max}

⇒ K_{max} = E −φ = 6.9 −2 = 4.9eV

p^{2}/2m = K_{max} ⇒ p=√(2mK_{max} )

Radius , R = mv/eB

R = p/eB = √(2mK_{max} )/eB

On putting Standard values we get ,

R = 0.148 m