(a) A burglar alarm uses a photoemissive cell illuminated by a lamp. Explain how an intruder triggers it. (b) A small solar calculator cell of area 10 cm² receives sunlight of 800 W/m² and works at 15% efficiency. What electrical power does it deliver? Solution: (a) The lamp's beam continuously ejects electrons from the photocell cathode, maintaining a steady current. When the intruder blocks the beam, emission stops, the current collapses, and the control circuit — wired to trigger on current loss — sounds the alarm. (b) Incident power = 800 W/m² × 10×10⁻⁴ m² = 0.8 W. Electrical power = 0.15 × 0.8 = 0.12 W = 120 mW — plenty for a calculator chip that needs only milliwatts.