Answer the following with reference to the photo-electric effect.
a. (i) Define threshold frequency and stopping potential.
b. (ii) Explain why no photo-electrons are emitted when the frequency of incident light is lower than the threshold frequency, no matter how high the light intensity is.
c. (iii) Give a reason why the stopping potential depends on the frequency of light but is independent of its intensity.
d. (iv) A metal surface has a threshold wavelength of 620nm. Calculate its work function in electron-volts.