Question

Photocells are used for the
A. Reproduction of pictures from the cinema film
B. Reproduction of sound from the cinema film
C. Automatic switching of street light
D. B and C both

Answer 1

Hint: The photocells or photoelectric cells work based on the principle of the photoelectric effect. The main principle of the photocells is they convert the light energy into electrical energy and the application of the photocells is based on this principle.

Complete step by step solution:
Photocells produce energy by the process of the photoelectric effect. The photoelectric effect is when particles of light called photons strike the surface of a metal and as a result of this, an electron will be emitted from the metal surface.

Photo-cells are used to create sounds in making movies. The audio sound to be played is converted into light waves and imprinted on the movie film along with the video to be projected. When playing the video back, this signal is converted from light waves to electrical signals and then to audio waves. This leads to video films always having audio synced with the audio.

When light falls on a photocell during the daytime, due to the incident light’s strength, a huge current is set up in the circuit. During the evening, when the incident light reduces, this current reduces in strength. This variation in the electrical energy in the circuit can be used as a switch to automatically turn on and off the street lights. Therefore, photocells are used in the reproduction of sound from cinema film and in the automatic switching of street lights.

Hence, Option D is the correct answer.

Note:There are three types of Photoelectric cells, they are photo-emissive cell, photovoltaic cell, and photo-conductive cell. Examples of photoelectric devices are Light-emitting diodes (LED), phototransistors, solar cells, etc.