(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 10⁸ ms-¹)

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