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Photocell, construction, working, applications, FAQs

Photocell, construction, working, applications, FAQs

If you have ever gone to a mall or airport, you might have used automatic soap or water dispensers. You might have noticed that without even touching the device it automatically dispenses soap or water with just a gesture of your hand. Have you ever wondered how the device got to know that you have passed a hand under the device? To know the reason you might want to know about a device called photocell which is incorporated inside these devices.

Table of contents

  • Photocell
  • Construction
  • Working
  • Applications of photocells
  • FAQs

Photocell

A photocell (also known as an electric eye) is a technological application of photoelectric effect whose electrical properties are affected by the light falling on it. Photocells find application in many automatic devices.

Construction

A photocell consists of an evacuated glass or quartz bulb. Two electrodes named emitter and collector are placed inside this glass. The emitter is a sensitive metal plate in the semi-cylindrical shape and sometimes in its absence a photosensitive material is attached inside the bulb. The collector is placed in front of this emitter. Both the electrodes are connected to the electrical circuit (emitter to negative and collector to positive) that contains an external battery and a microammeter to detect any flow of current in the circuit shown in the image below. A part of the glass bulb is left clean for light to enter in the glass and fall on the emitter plate. The glass is placed on a non-metallic base which has two pins to support external connections.

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Working

A photocell is just a simple application of the photoelectric effect. Light having a suitable frequency (threshold frequency) is allowed to fall on the emitter plate. The photon in the radiation ejects the most loosely bound electrons from the metal surface. These photoelectrons are attracted towards the collector plate and thus set a photoelectric current in the circuit. This photoelectric current is measured with the help of the microammeter. The change in the intensity of the radiation is converted into the change in photocurrent and this finds application in different control systems or light measuring devices. The photocurrent produced is of the order of micro-amperes. Therefore the glass or bulb is filled with suitable inert gasses like helium, neon etc to get an increased value of current.

Applications of photocell

  • Scientific work: Photocells find applications in many scientific works and experiments which need to measure the intensity of light incident. For a similar purpose, they are also used in light meters of photographic cameras.
  • Automatic door opener and closer: Photocells are used in automatic door openers. Whenever a person disrupts the light rays incident, there occurs variation in the photocurrent which is used to operate the control system responsible for door opening and closing.
  • Smoke detectors or fire alarms: Many smoke detectors and fire alarms use photocells nowadays. In these detectors, smoke interrupts and scatters the light falling on the cell. This triggers the alarm even before the fire breaks out. In fire alarms as soon as the light falls on the photocells, it generates a current in the circuit and triggers an alarm.
  • Burglar alarm: The burglar alarm in banks or museums uses photocells for security purposes. Whenever a thief disrupts the light continuously falling on the cells, an electric bell starts ringing. Same technology is used to detect the traffic law defaulters.
  • Industries: Photocells find applications in the industries where they are used to detect the flaws or holes on the surface of the finished products.
  • Counting devices: The devices used for counting objects or persons in auditoriums or halls use photocells. The number of interruptions are recorded and calibrated to find the number of objects passed through the device.

FAQs

Q. Can a photocell work at all the frequencies of light?
A.
It doesn’t work at all frequencies since there is a limit to the minimum frequency needed for electron emission which is further related to the work function of the metal.

Q. What happens when the potential applied is reversed (i.e. emitter is connected to the positive terminal and collector is connected to the negative terminal)?
A.
Electrons being negatively charged are repelled by the collector plate. So, as the potential increases fewer and fewer photoelectrons reach the collector plate and hence, the photocurrent keeps on decreasing and at some potential even the most energetic photoelectron can not move to the collector plate and hence, the photocurrent becomes zero.

Q. Can we detect microwaves in the photocells?
A.
No. The frequency of the microwave is less than the threshold frequency needed to carry out the photoelectric effect used in the photocells.

Q. Photocurrent produced in the photocells increases with an increase in intensity of incident light. Why?
A.
As the intensity of light increases, the number of photons incident per second increases due to which the photoelectrons emitted per second also increases and hence, the photocurrent increases.

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