When a surface is irradiated with light of wavelength 4950A˚ , a photocurrent appears which vanishes if a retarding potential greater than 0.6 volt is applied across the phototube. When a second source of light is used, it is found that the critical retarding potential is changed to 1.1 volt.
The work-function of the emitting surface is
a. 2.2 eV
b. 1.5 eV
c. 1.9 eV
d. 1.1 eV