Question

Problem 1: Photoelectric effect [4 marks] Consider a zinc surface that is 100 cm away from a 100-watt light bulb. Suppose that the light energy radiated by the bulb is only 10% of the input power. Treat each zinc atom as a perfectly absorbing circular disc of diameter 0.1 nm and use the classical wave interpretation of light. (a) [1 mark] Find the intensity of light emitted by the bulb at a distance r away. Assume that the light bulb radiates like a point source. (b) [1 mark] Calculate the absorbed power of each atom on the zinc surface. (c) [1 mark] Determine the time required for each atom to absorb an amount of energy equal to the material's work function of 4.3 e'V (d) [1 mark] Comment on your result in (c) and compare this with the observed time delay in photoelectric experiments.

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Answer 1

energy radiated 10 100 a) intensity at a distance r for a point source 10 4x ir2 b) absorbed power P=IA 10 4 x 416 r2 10 10 16 20 0.625 × 10-20W

energy equal to work function E-4.3 x 1.6 x 1019J time required 4.3 × 1.6 × 10-19 0.625 x 1020 t 110.08s 110.08 60 t-1.835min d) here time required to absorb the energy equal to work function of metal has a finite amount of time to escape from metal surface but according to experiments emission of photoelectrons takes place instantly (no time delay) almost immediately But according to classical theory this takes time so this wave theory failed to explain the photoelectric effect