The work function of Cs is 2. 14eV.Find

(a) Threshold frequency for Cs

(b) Wavelength of incident light if the photo current is brought to zero by stopping potential of 0.6 V.

(a) Threshold frequency is defined as the minimum frequency of incident light which can cause photo electric emission i.e. this frequency is just able to eject electrons without giving them additional energy. While, minimum amount of energy which is necessary to start photoelectric emission is called Work Function. If the amount of energy of incident radiation is less than the work function of metal, no photo electrons are emitted.

Where, ɸ = work function and h is Planck’s constant ().

Threshold frequency = 2.14 × 1.6 × 10^-19 / 6.6 ×10^-34

= 5.2 ×10¹⁴ hertz

(b) Given The work function of cesium is 2.14 eV, find the wavelength of the incident light if photo current is brought to zero by stopping potential of 0.60 V.

We know the relation hf = K max + C1

Where, photon energy is hf, k-max is kinetic energy of the emitted electrons and work function is C1.

So K-max = eV

Now h f = eV + C1

So frequency of incident light is f = c / λ

h(c / λ) = eV + C1

λ = h c / e V + C1

Substituting the values, we get

λ = 4537 Angstrom