Question

12(b)(i) A kinetic energy of 1.80×10⁻¹⁶ J is possessed by an electron ejected from the surface of a metal when the surface is illuminated by light. If the work function of the metal is 1.20×10⁻¹⁶ J, calculate the frequency of the incident photon. [c = 3.0×10⁸ ms⁻¹, h = 6.6×10⁻³⁴ Js] (ii) State two characteristics of a cathode ray tube.

Accepted Answer

(i) From Einstein's photoelectric equation, hf = W₀ + K.E. → f = (W₀+K.E.)/h = (1.2×10⁻¹⁶+1.8×10⁻¹⁶)/6.6×10⁻³⁴ = 3.0×10⁻¹⁶/6.6×10⁻³⁴ ≈ 4.55×10¹⁷ Hz.
(ii) A cathode ray tube: operates in a high-vacuum environment to minimize electron scattering; uses magnetic/electrostatic fields to deflect the electron beam; is coated with phosphorus on the inside of the screen; uses the electron beam to create images on the screen.

WAEC Physics 2025 Theory — Question 26

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