The phenomenon of photoelectric effect is used in the following EXCEPT

Analyze the question: We need the device that does NOT use the photoelectric effect (light causing electrons to be emitted or causing a change in current/conductivity).

Concept: The photoelectric effect is the emission of electrons from a metal surface when light of suitable frequency falls on it. Photocells (photoelectric cells) use this to convert light signals into electrical signals.

Going through the options:

• A. Automatic doors: A light beam falls on a photocell. When someone interrupts the beam, the current changes and the door opens. This uses the photoelectric effect.
• B. Burglar alarm: An invisible (infrared) beam falls on a photocell. When an intruder breaks the beam, the alarm sounds. This uses the photoelectric effect.
• C. Electric fire alarm: The usual fire alarm works by detecting heat (a thermal expansion/bimetallic strip or thermistor) or smoke (ionization of air by a radioactive source). It does not rely on light ejecting electrons. This is the exception.
• D. Sound production from film tracks: In old cinema films, the sound track varies the amount of light reaching a photocell, which produces a varying current that is turned into sound. This uses the photoelectric effect.
• E. Television cameras: The camera tube/sensor converts light from the scene into electrical signals through photoemission or photoconductivity. This uses the photoelectric effect.

Answer: The fire alarm detects heat or smoke, not light-induced electron emission, so it is the odd one out.

Tip: For "photoelectric effect applications," remember the light-beam devices: doors, burglar alarms, sound-on-film, cameras, and light meters. If a device senses heat or smoke instead, it is not a photoelectric application.