Unit – III

Applications of Photoelectric Effect

Construction & Working of Photovoltaic Cell

Photo-electric cell or photo cell is an electronic device which works on the principle of photoelectric effect and converts light energy into
electrical energy.

photoelectric cell

Construction:

1. Photocell consists of evacuated glass tube containing two electrodes emitter (C) and Collector (A)
2. The emitter is shaped in the form of semihollow cylinder. It is always kept at negative potential.

3. The collector is in the form of metal rod and fixed at the axis of semi-cylindrical emitter. The collector is always kept at positive potential.
4. The glass tube is fitted on non-metallic base and pins are provided at the base for external connection.



Working :

1. The emitter is connected to negative terminal and collector is connected to positive terminal of battery.
2. A radiation of frequency more than the threshold frequency of material of emitter is made incident on emitter. Photo-emission takes place . The photo-electrons are attracted towards the collector which is positive w.r.t. emitter, Thus current flows in the circuit.
3. If intensity of incident radiation is increased the photoelectric current increases.

Applications of photoelectric cell:

1. Photo-electric cell is used in reproduction of sound which is recorded on a movie film.
2. Photo-electric cell is used in exposure-meter. Exposure meter is used along with a camera to know the correct time of exposure for having a good photograph.
3. Photo-electric cell is used in lux-meter. This device is used to determine the intensity of light.
4. Photo-electric cell is used in burglar-alarm. This device is kept near a safe to be protected from a thief.

Photoelectric cell in sound reproduction from motion picture.

1. Photo-electric cell is used in reproduction of sound which is recorded on a movie film.
2. In a movie film sound is recorded on the film of actions in the form of thin transparent strip. This thin transparent strip is called sound track.
3. The transparency of sound track depends on the variation of frequency of sound recorded. Using photocell sound is reproduced from the sound track.
4. When the film is run in projector the light of projector this sound track and falls on
photocell. Due to variation in sound track the
variation of intensity of sound takes place and thus the photo-electric current varies. The current is amplified and is fed to speakers.



Photoelectric cell in burglar alarm.

1. Photo-electric cell is used in burglar-alarm. This device is kept near a safe to be protected from a thief. Burglar alarm is a device which is used for locating intruder, thief near precious, valuable things like safe.
2. The device consists of photocell and infrared source of light. The light from infra-red source is made continuously incident on the photocell making photoelectric effect continuous. Thus the photoelectric current in the cell flows continuously.
3. When the path of infra-red light is obstructed by the thief, the light falling on photocell is cutoff and photo-electric current in the cell stops and relay circuit is activated and siren starts hooting.

Photoelectric cell in exposure metre

1. Photo-electric cell is used in exposure-meter. Exposure meter is used along with a camera to know the correct time of exposure of film for having a good photograph.
2. To have good photograph if intensity of light is more the exposure of film should be less. If the intensity of light is less the exposure of film should be more. Exposure meter is device attached to the camera which decides the exposure of film.
3. Exposure meter consists of photo-electric cell with sensitive milliammeter and battery connected in series to it. The photoelectric current produced in the cell is directly proportional to the intensity of light.
4. If deflection in the milliammeter is small the photoelectric current is small. It indicates that the intensity of light is small. Thus the exposure time should be more. If deflection in the milliammeter is large the photoelectric current is large. It indicates that the intensity of light is more. Thus the exposure time should be less.



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