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 wavelength of the second source is
a. 6150A˚
b. 5150A˚
c. 4125A˚
d. 4500A˚