In which of the following situations is photoelectric effect employed? I. Automatic devices for switching on light at dusk II. Automatic doors in department...

The photoelectric effect is employed in option I only, which is automatic devices for switching on light at dusk. The photoelectric effect is a phenomenon in which electrons are emitted from a material when it absorbs electromagnetic radiation, such as light. This effect is used in devices such as photoelectric cells or photodiodes, which convert light energy into electrical energy. In option I, the use of photoelectric cells is common in automatic devices for switching on lights at dusk, such as streetlights or security lights. These devices use a photoelectric cell to detect the level of ambient light and automatically switch on the lights when it gets dark. Option II, automatic doors in departmental stores, do not use the photoelectric effect. Instead, they use motion sensors or infrared sensors to detect the presence of a person and trigger the door to open. Option III, automatic devices for switching on air conditioners, do not use the photoelectric effect. They use thermostats or temperature sensors to detect the temperature and trigger the air conditioner to turn on or off. Option IV, automatic voltage stabilizers, do not use the photoelectric effect. They use a voltage regulator or stabilizer circuit to maintain a constant voltage output regardless of fluctuations in the input voltage. Therefore, the correct answer is option I, II and IV only.