(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 = \(f_o\lambda_o\)
\(\lambda_o = \frac{v}{f_o} = \frac{3 \times10^8}{4.02 \times 10^{14}}\)
= 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