UPPSC 2020 Prelims GS Paper-I · Question 91 of 150

Correct — A, a photo cell. A photoelectric smoke detector — the commonest type of fire alarm sensor — houses a light source (an LED) and a light-sensitive photo cell set at an angle to it. In clean air no light reaches the cell; when smoke from a fire enters the chamber it scatters the light onto the photo cell, the cell's output changes and the alarm is triggered. The photo cell (a photodetector such as a photoresistor/photodiode) is therefore the sensing element that 'uses' light to detect a fire.

• (b)Water — Water is the extinguishing agent released by a sprinkler system to put out a fire — it does not detect the fire, so it is not what a fire alarm (the detector) uses to sense one.
• (c)Solar cell — A solar cell (photovoltaic cell) is designed to convert sunlight into electrical power, not to sense the small amount of scattered light in a smoke chamber; it is a power-generation device, not a smoke-detection sensor.
• (d)None of the above — Incorrect, because a photo cell is in fact the sensor used in photoelectric fire/smoke alarms — so option (a) is valid and this cannot be the answer.

A photoelectric (optical) smoke detector works on light scattering. A photo cell is a component whose electrical behaviour changes when light falls on it (photoconductive cells/photoresistors lower their resistance under light; photodiodes/phototransistors produce a current). Pairing a light source with a photo cell lets a device 'see' smoke: smoke particles scatter light onto the cell and change its signal, tripping the alarm.

The trap is the family resemblance of the three light-related terms. 'Solar cell' also responds to light, so it is tempting — but its job is generating power from sunlight, not sensing smoke. Reason from function: a fire alarm must DETECT a fire, and the detecting element that reacts to scattered light is the photo cell, not a power cell (solar) and not the water used to fight the fire.

• Photoelectric smoke detector = LED light source + a photo cell (photodetector)
• Smoke scatters the light onto the photo cell → its output changes → alarm sounds
• Photo cell senses light; solar cell generates power from light — different purposes
• Water is the extinguishing agent (sprinklers), not the detection sensor

• Choosing 'solar cell' because it also responds to light — but it generates power, it does not sense smoke
• Confusing the detector (fire alarm) with the extinguisher (water sprinkler)

UPPSC/UPSC ask which device or component is used in a given everyday application (alarms, street lights, cameras), testing whether you can match a sensor/component (here the photo cell) to its real-world use.

No directly related past PYQ was found.

• practice — not a real PYQ

  A light dependent resistor (LDR) is commonly used in which of the following?

  • (a)Automatic street lights
  • (b)Electric heater
  • (c)Ceiling fan regulator
  • (d)Electric kettle

  Answer(a) Automatic street lights — the LDR's resistance falls in daylight and rises at night, switching the lamp on/off automatically.

• practice — not a real PYQ

  A solar cell converts:

  • (a)Heat energy into electrical energy
  • (b)Light energy into electrical energy
  • (c)Electrical energy into light energy
  • (d)Chemical energy into electrical energy

  Answer(b) Light energy into electrical energy — a photovoltaic cell generates electricity directly from sunlight.