basics of photoelectric cell

1. PHTO ELECTRIC CELL
PRESENTED BY:-
SHASHI KANT
ROLL:- 16PH62R13
IIT KHARAGPUR

2. PHOTO ELECTRIC CELL
 A device used to convert light energy into electrical energy is called photoelectric
cell.
 Photocell is based on the phenomenon of photo - electric effect .
 Einstein in 1905 proposed that the incident light consisted of individual quanta
called photons that interacted with electrons in metal like discrete particles rather
than continuous waves .
 Albert Einstein was honoured with the nobel prize in physics in 1921 for
successfully explaining the photoelectric effect .
Types of photoelectric cell
 Photo-emissive cell
 photo-voltaic cell
 Photo-conductive cell

3. PHOTO EMISSIVE CELL
 There are two types of photo-emissive cells; vacuum type or gas
filled.
 Consists of two elecrodes:- cathode & anode.
 Cathode :- coated with photosensitive material like sodium,
potassium or cesium .
 Anode:-straight wire made of nickel or platinum
 Evacuated glass or quartz bulb
 Photo-electrons flow from catode
to anode , gas ionize the gas
filled and hence current gets
modified.

4. Drawbacks of gas filled cell
Photo- electric current doesnot vary linearly with intensity of
light.
Application
There is no time lag between the incident light and the flow
of electrons and hence current, therefore such cell is
used in
Television
Photometry
Fire-alarm

5. PHOTO-VOLTAIC CELL
 Based of principle of inner photoelectric cell.
 This is called true cell because it generates e.m.f without the
application of any external of any external potential difference .
 It consists of semiconductor layer formed on the surface of metal
plate by either heat treatment or cathode sputtering.
 A film of semitransparent metal is coated over the semiconductor.

6.  I
 Current is propostional to the intensity of incident light.
ADVANTAGES :-
It requires no external voltage for its operation.
APPLICATION :-
Photographic exposures meters , photometers and illuminatio meters.

7. PHOTO-CONDUCTIVE CELL
 Based on inner photoelectric effect .
 It consists of thin film of semiconductor like selenium or thallium sulphide
placed below a thin film of semitransparent metal .
 The iron base and the transparent metal film is connetced through bettery and
resistance.
 When light falls on the cell its resistance decreases and hence the current starts
flowing in the external circuit.

8.  Inverse square law :-
where E :- electric lummincance , d : distance
I :- luminous intensity of an electric lamp
θ :- deflection shown by the microammeter

9.  characteristics
 Application :-
Making street lights turn on and off automatically according to the level
of daylight.
In security devices such as motion sensing lights and cameras.
In alarm systems .
As light meters in photographic applications.