Question

Sodium has a work function of 2.46 eV.

(a) Find the cutoff wavelength and cutoff frequency for the photoelectric effect.

wavelength	_________ nm
frequency	_________ Hz

(b) What is the stopping potential if the incident light has a wavelength of 168 nm?
_________________V

Answer 1

(a) As given work function = 2.46 eV = 2.46 x 1.6 x 10^-19 J

Work function = 3.936 x 10^-19 J

Photon must have at least this much energy for knocking out an electron.

Hence, hf = 3.936 x 10^-19

h = planks const

f = frequency

f = 3.936/6.64 x 10^-19+34

f = 5.927 x 10¹⁴ Hz (this is required cut off frequency)

As lambda = c/f = 3.0/5.927 x 10^8-14

lambda = 5.06 x 10^-7 m (which is required cut of wavelength)

(b) energy of incedent photon E = hc/lambda =( 6.64 x 3 / 168 ) x 10^-34+8+9

E = 1.185 x 10^-18 J

As elenrgy of electron will be Ee = (11.85 - 3.936) x 10^-19 = 8.484 x 10^-19 J

As this must be equal to energy of electron in potential,

Ee = eV

V = Ee/e = 8.454 x 10^-19/1.6 x 10^-19 V

V = 5.28375 Volt