The phenomenon of photoelectric effect is NOT used in

1. Analyze the question We need to find the device that does NOT use the photoelectric effect, which is the emission of electrons from a metal surface when light of sufficient frequency falls on it.

2. Concept Photoelectric devices (photocells) convert light into an electric current. They are used wherever a change in light must be detected or measured.

3. Check each option

• 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. Bimetallic strip thermometers: Two metals with different expansion rates are bonded together. When heated, the strip bends because one metal expands more than the other. This is thermal expansion, and no light-induced electron emission is involved. This does not use the photoelectric effect.
• C. Light meters in cameras: A photocell produces a current proportional to the light intensity. This uses the photoelectric effect.
• D. Burglar alarms: An interrupted light beam (often infrared) changes the photocell current and triggers the alarm. This uses the photoelectric effect.
• E. Sound from film tracks: Light passes through the variable-density sound track onto a photocell. The varying light produces a varying current, which becomes sound. This uses the photoelectric effect.

4. Answer The bimetallic strip thermometer works by differential thermal expansion, not by the photoelectric effect.

5. Exam tip Remember that the photoelectric effect is about light → electric current. If a device involves heat expansion, magnetism, or pressure instead of light detection, it is not a photoelectric application. Bimetallic strips go with thermostats, fire alarms, and flashers, all of which are thermal expansion applications.