(a)(i) State Einstein’s equation of photoelectric effect
(ii) What conservation principle does the equation represent?
(b) List three applications of photocells
(c) A photo emissive surface has a threshold frequency of 4.02 x 10(^{14})Hz. If the surface is illuminated by light of frequency 5.0 x 10(^{15})Hz, calculate the:
(i) threshold wavelength;
(ii) work function;
(iii) kinetic energy of the emitted photoelectrons. [ c = 3.0 x 10(^{8}) ms(^{-1}), h = 6.63 x 10(^{-34}) Js]
Explanation
(a)(i) E = W + K.E, where
E = energy of incident radiation
W = work function of metal
K.E. = Kinetic energy of ejected electrons
(ii) The equation represents the principle of conservation of energy.
(b) Applications of photocells:
1. for burglar alarm
2. for television cameras
3. automatic doors
4. automatic street lighting
5. automatic digital counters.
(c)(i) V = (f_olambda_o)
(lambda_o = frac{v}{f_o} = frac{3 times10^8}{4.02 times 10^{14}})
= 7.46 x 10(^{-7})m
(ii) W = (hf_o = 6.63 times 10^{-34} times 4.02 times 10^{14})
= 2.67 x 10(^{-19})J
(iii) K.E. = (h (f – f_o) = 6.63 times10^{-34} (5.0 times 10^{15} – 4.02 times 10^{14}))
= 3.05 x 10(^{-15})J