Define the term “threshold frequency”, in the context of photoelectric emission.

OR

Define the term “Intensity” in photon picture of electromagnetic radiation.

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why ?

A. How does one explain the emission of electrons from a photosensitive surface with the help of Einstein’s photoelectric equation?

B. The work function of the following metals is given: Na = 2.75 eV, K = 2.3 eV, Mo = 4.17eV and Ni = 5.15 eV. Which of these metals will not cause photoelectric emission for radiation of wavelength from a laser source placed 1 m away from these metals? What happens if the laser source is brought nearer and placed 50 cm away?(the wavelength is not given in the question, I have taken it as 3300 A^o)

Explain with the help of Einstein’s photo electric equation any two observed features in photoelectric effect which cannot be explained by wave theory.