December 5, 2019

Simply, the photoelectric effect is the emission of electrons when light hits a material. According to Einstein’s (in 1905) quantum theory of radiation, light is a particle called quantum and the energy carried by each quantum s called photon. Einstein who got Nobel Prize in this theory explains light as a particle though the light is wave because it shows some phenomena like interference, diffraction, and polarization. There are lot of theories to explain the photoelectric effect. But here I’m just going to talk about its application around us which we are unaware that this phenomenon is being used.

Before going to application let’s know about Photoelectric cells. It is a device that converts light energy to electric energy. This also works on the principle of the Photoelectric effect. They are of two types of photo emissive cell, photovoltaic cell and photo conductive cell. Let’s talk about a common application that’s around us which is an automatic street light. Photoelectric cells are used for switching ON or OFF the street lights automatically. In order to switch on and off the street – lamps automatically , a photoelectric cell is connected to a relay system . Relay system is electromagnetic device for remote or automatic control that is actuated by variation in conditions of an effect for electric circuit that operates, in turn, other devices (such as switches) in the same or a different circuit. In the evening, when the sunlight goes dim the photoelectric cell stops working and the relay circuits turn the switch on and light the lamps. In the morning when the sun raises the relay circuit turns the switch off. This is based on the principle of a photo conductive cell that in some semiconductor like Selenium, resistance decreases with the increase of intensity of radiation falling upon it

The photoelectric cell is also used in a burglar alarm in banks, houses, etc. A beam of ultraviolet light is allowed to fall on the photoelectric cell, this keep the bell circuit open. When a burglar enters the premises, the ultraviolet rays falling on the photoelectric cell are blocked and the relay circuit turns the alarms switch on, as a result, the bell rings. The photoelectric cell is used in fire alarms in the house , photoelectric cells which are photosensitive to flame light are installed at various places. So, whenever, a firing takes place, the light from the flame falls on the photoelectric cells and switch on the alarm. The photoelectric cell is used to count the products in the factory, to count people entering a room. The principle is that when the light is blocked for some time on the photocell the system records the number of breaks in the circuit and thus counts.
Not only that Photoelectric cells are used in television cameras, in the industry to locate flaws in metallic sheets, to measure the transparency of opacity of liquid and solids, but Night vision scopes also use special devices base on the photoelectric effect, at infrared frequencies, etc.

Prepared By: Kabir Rajbanshi, Moonlight Secondary School Kumaripati, Lalitpur.