In which of the following situations is photoelectric effect employed? I. Automatic devices for switching on…

Step 1: Analyze the question We need to pick the devices that work by the photoelectric effect, where light falling on a suitable metal or semiconductor surface releases electrons and produces a current (as in a photocell or light-dependent sensor).

Step 2: The concept In a photocell, light of enough frequency causes electron emission. The resulting current changes with light intensity, so the device can act as a switch that responds to light.

Step 3: Check each situation

• I. Switching on lights at dusk: A photocell (or LDR) senses the fall in light intensity. When it gets dark, the current changes and a relay switches the light on. ✔ Photoelectric effect.
• II. Automatic doors in stores: A light beam falls on a photocell. When a person breaks the beam, the current changes and triggers the door motor. ✔ Photoelectric effect.
• III. Switching on air conditioners: These are usually controlled by temperature sensors (thermostats or thermistors), not light. ✘
• IV. Automatic voltage stabilizers: These work with electromagnetic components such as transformers, relays and electronic circuits that respond to voltage, not light. ✘

Step 3 result: Only I and II use the photoelectric effect.

Step 4: Answer I and II only**

Tip: For "photoelectric effect" questions, look for devices that respond to light: street lights that come on at dusk, automatic doors, burglar alarms with light beams, and light meters. Devices that respond to temperature or voltage are not photoelectric.