(a) Explain
what are meant by electron energy levels in an atom.

How can this concept be used to account for the characteristic emission
line spectrum of an element?

(6)

(b) Calculate the minimum wavelength of electrons that are accelerated
through a potential difference of 12 x 10 ^{3} V.

(4)

(Charge on electron= -1.6x 10^{-19} C; Planck constant=6.6x 10-^{14}
J S; speed of electromagnetic radiation= 3.0 x
10^{8} ms-^{1})

*(AEB,
question B15, paper 3, June 1979)
*