A clean cadmium photocathode is illuminated in turn with monochromatic light of four…

PHYSICS · Multi-part · Fermi University Physics · Scientific Practice & Experimental Methods · Quantum Mechanics · Origins of quantum theory

Quantum Mechanics: From the Photon to the Hydrogen Atom, Ch Light in lumps: blackbody radiation, the photoelectric effect and Compton scattering

Problem

A clean cadmium photocathode is illuminated in turn with monochromatic light of four frequencies, and the stopping potential

V_s

(the reverse bias at which the photocurrent just falls to zero) is plotted against frequency

\nu

. The four recorded points are:

\nu=1.00,\,1.50,\,2.00,\,2.50

in units of

10^{15}

Hz, with

V_s=0.06,\,2.13,\,4.20,\,6.27

V respectively. The points lie on a straight line. Take the surface as clean, the light as monochromatic, and use

e = 1.602\times10^{-19}

C and

hc = 1240

eV nm. ((a)) From the slope of the line, find Planck's constant

h

in J s. ((b)) From the intercept, find the work function of cadmium in eV. ((c)) Find the threshold wavelength of this cathode, in nm.