1. Quantitative Analysis: The metal salt solutions which are coloured for e.g. Cu (II), Fe (III), Ni (II) etc and some organic coloured compounds can be easily analyzed quantitatively by using Beer Lambert’s Law. Thus, from OD we can calculate the concentration of the solution.

2. Identification of functional group (Qualitative analysis): The spectrophotometers are the instruments working on the principle of \({ \lambda }_{ max }\). This \({ \lambda }_{ max }\) is constant for each functional group. If we know λmax for the particular sample, we can decide the type of functional group present in it, as \({ \lambda }_{ max }\) is characteristic property.

3. Distinguish between geometrical isomer: Each geometrical isomer have its own characteristic \({ \lambda }_{ max }\), therefore from \({ \lambda }_{ max }\) we identify geometrical isomers. E.g. Cinnamic acid exists in Cis and Trans form.

4. Chemical Analysis: With the help of spectrophotometer we can find out the rate constant (K) of any chemical reaction. As we know the first order reaction. $$ K = \frac { 2.303 }{ t } \log { \left( \frac { a }{ a-x } \right) } $$ Where a is initial concentration and (a - x) is final concentration.

We will take some ester and keep in the instrument and find out the absorbance then with the help of Beer-Lamberts Law we can find out the concentration.

Now add some reagent the reaction will take place at the time t. Now we will find out the O.D. of the solution in the cuvette which is (a-x) then by using the above eqn. for first order we can find out the K (rate constant).

Similarly we can find out the K for zero order, second order and so on by using its expression.

5. Determination and estimation of tautomers: Keto and Enol form are present in equilibrium which can be determined by spectrophotometer. E.g. Acetoacetic ester exists in two tautomeric forms which show different \({ \lambda }_{ max }\) values.

1. Quantitative Analysis: The metal salt solutions which are coloured for e.g. Cu (II), Fe (III), Ni (II) etc and some organic coloured compounds can be easily analyzed quantitatively by using Beer Lambert’s Law. Thus, from OD we can calculate the concentration of the solution.

2. Identification of functional group (Qualitative analysis): The spectrophotometers are the instruments working on the principle of \({ \lambda }_{ max }\). This \({ \lambda }_{ max }\) is constant for each functional group. If we know λmax for the particular sample, we can decide the type of functional group present in it, as \({ \lambda }_{ max }\) is characteristic property.

3. Distinguish between geometrical isomer: Each geometrical isomer have its own characteristic \({ \lambda }_{ max }\), therefore from \({ \lambda }_{ max }\) we identify geometrical isomers. E.g. Cinnamic acid exists in Cis and Trans form.

4. Chemical Analysis: With the help of spectrophotometer we can find out the rate constant (K) of any chemical reaction. As we know the first order reaction. $$ K = \frac { 2.303 }{ t } \log { \left( \frac { a }{ a-x } \right) } $$ Where a is initial concentration and (a - x) is final concentration.

We will take some ester and keep in the instrument and find out the absorbance then with the help of Beer-Lamberts Law we can find out the concentration.

Now add some reagent the reaction will take place at the time t. Now we will find out the O.D. of the solution in the cuvette which is (a-x) then by using the above eqn. for first order we can find out the K (rate constant).

Similarly we can find out the K for zero order, second order and so on by using its expression.

5. Determination and estimation of tautomers: Keto and Enol form are present in equilibrium which can be determined by spectrophotometer. E.g. Acetoacetic ester exists in two tautomeric forms which show different \({ \lambda }_{ max }\) values.

Spectrophotometers are the instruments which are working at a wavelength where the absorbance is maximum. Such wavelength of the radiations whose absorbance is maximum is called \({ \lambda }_{ max }\).

With the help of spectrophotometers we can have qualitative as well as quantitative analysis of any type of sample it may be solid, liquid or gas. It may be coloured or colourless.

The spectrophotometers are of two types;

(i) Single Beam Spectrophotometers,

(ii) Double Beam Spectrophotometers.

The following are the main components of a Double Beam Spectrophotometers:

1. Source of radiations: It may be U.V-light, visible light or I.R. light. U.V. Light can be obtained by heating the filament which is filled with either H2 gas or deuterium. Visible Radiations can be obtained by incandescent lamp with tungsten filament and I.R. Radiations can be obtained by heating Newtons glower at the temp. of 1500-2000 ℃. Newtons glower is metallic oxide of Yettrium, Erbium and Zirconium. I.R. radiations can also be obtained by heating glow bar which is Silicon carbide SiC (Carborandum) at the temp. of 1300-1700 ℃.

2. Collimating Convex Lens: The function is to collect all the rays coming from the source.

3. Diaphragm: To set the 100% transmittance.

4. Monochromators: To obtain the radiation of one wavelength. The Monochromators may be prism or diffraction gratings.

5. Cuvette or Sample Holder: For U.V. light it should be made up of quartz, for visible light it should be made up of glass and for I.R. light it must be that of rock salts for eg. NaCl.
a) If the sample is gas or volatile liquid having low B.P. must be taken in the closed container.
b) If it is liquid, one drop of the liquid must be placed between two parallel plates for U-V. the plates must be quartz and for visible it must be glass and for I.R. it is rock salt.
c) If it is solutions the solvent used for the preparation of solution must be Cyclohexane, methyl alcohol, ethyl alcohol or any other suitable solvent which is not capable to absorb the radiations incident on it.
d) If it is solids then we take about 1mg. of it and is mixed in 100-200 mg. of KBr - prepare slurry. It is cut into smaller circular discs (just like bindle) called pellets. They are dried and placed in the place of cuvette. The diameter of pellets is 10mm. and thickness is 1mm.

6. The Focusing Convex Lens: Its function is to collect all the transmitted light from cuvette and is focused at a point where photo cathode of the photo-cell is placed.

7. Photo-Cell: The transmitted light is incident on it photo cathode converts these radiations into current. It can be amplified using the amplifier if needed.

8. Read out meter or dial: The current is converted into OD or Absorbance in read out meter and by knowing OD we can find out the concentration of the solution i.e. quantitative analysis is possible by applying Beer Lamberts Law.

Working:

First of all the source of radiations from U.V. light to visible light to I.R. radiations is started. The light from the source is collected by Collimating convex lens and allowed to fall on monochromator. For each radiations incident on monochromator we are getting the radiations of one wave length only i.e. \( \lambda \), the monochromatic light is then separated in two beams with the help of two mirrors. One beam is passed through one cuvette containing blank (solvent) and the other through the other cuvette containing sample solution and both are finally incident on detectors. These may be photo calls or thermocouples. Where the heat is produced which is then converted to current and finally O.D. or Absorbance (A).

The signal generated is platted O D Vs \(\lambda\). From the graph we can find out \({ \lambda }_{ max }\) and we can have the qualitative analysis. From O.D. or Absorbance we can have the quantitative analysis.

Value of the wavelength where the absorbance is maximum is known as \({ \lambda }_{ max }\) which is characteristic property of the compound and gives the qualitative analysis of sample.While knowing the value of absorbance we can find out the concentration of solution using beer lambert’s law. Thus quantitative analysis is possible.

Double beam Spectrophotometer have following advantages:

1. As the two beams of radiations are passed simultaneously from sample solution and Blank Therefore any fluctuation in voltage can be compensated (cancelled).

2. As the two beams are passed from Blank and Sample solution any impurities present in the solvent will not affect the absorbance.

3. Accuracy is more than single beam spectrophotometer.

4. The main disadvantage is the instrument is costly.

Spectrophotometers are the instruments which are working at a wavelength where the absorbance is maximum. Such wavelength of the radiations whose absorbance is maximum is called \({ \lambda }_{ max }\).

With the help of spectrophotometers we can have qualitative as well as quantitative analysis of any type of sample it may be solid, liquid or gas. It may be coloured or colourless.

The spectrophotometers are of two types;

(i) Single Beam Spectrophotometers,

(ii) Double Beam Spectrophotometers.

The following are the main components of a Double Beam Spectrophotometers:

1. Source of radiations: It may be U.V-light, visible light or I.R. light. U.V. Light can be obtained by heating the filament which is filled with either H2 gas or deuterium. Visible Radiations can be obtained by incandescent lamp with tungsten filament and I.R. Radiations can be obtained by heating Newtons glower at the temp. of 1500-2000 ℃. Newtons glower is metallic oxide of Yettrium, Erbium and Zirconium. I.R. radiations can also be obtained by heating glow bar which is Silicon carbide SiC (Carborandum) at the temp. of 1300-1700 ℃.

2. Collimating Convex Lens: The function is to collect all the rays coming from the source.

3. Diaphragm: To set the 100% transmittance.

4. Monochromators: To obtain the radiation of one wavelength. The Monochromators may be prism or diffraction gratings.

5. Cuvette or Sample Holder: For U.V. light it should be made up of quartz, for visible light it should be made up of glass and for I.R. light it must be that of rock salts for eg. NaCl.
a) If the sample is gas or volatile liquid having low B.P. must be taken in the closed container.
b) If it is liquid, one drop of the liquid must be placed between two parallel plates for U-V. the plates must be quartz and for visible it must be glass and for I.R. it is rock salt.
c) If it is solutions the solvent used for the preparation of solution must be Cyclohexane, methyl alcohol, ethyl alcohol or any other suitable solvent which is not capable to absorb the radiations incident on it.
d) If it is solids then we take about 1mg. of it and is mixed in 100-200 mg. of KBr - prepare slurry. It is cut into smaller circular discs (just like bindle) called pellets. They are dried and placed in the place of cuvette. The diameter of pellets is 10mm. and thickness is 1mm.

6. The Focusing Convex Lens: Its function is to collect all the transmitted light from cuvette and is focused at a point where photo cathode of the photo-cell is placed.

7. Photo-Cell: The transmitted light is incident on it photo cathode converts these radiations into current. It can be amplified using the amplifier if needed.

8. Read out meter or dial: The current is converted into OD or Absorbance in read out meter and by knowing OD we can find out the concentration of the solution i.e. quantitative analysis is possible by applying Beer Lamberts Law.

Working:

First of all the source of radiations from U.V. light to visible light to I.R. radiations is started. The light from the source is collected by Collimating convex lens and allowed to fall on monochromator. For each radiations incident on monochromator we are getting the radiations of one wave length only i.e. \( \lambda \), the monochromatic light is then separated in two beams with the help of two mirrors. One beam is passed through one cuvette containing blank (solvent) and the other through the other cuvette containing sample solution and both are finally incident on detectors. These may be photo calls or thermocouples. Where the heat is produced which is then converted to current and finally O.D. or Absorbance (A).

The signal generated is platted O D Vs \(\lambda\). From the graph we can find out \({ \lambda }_{ max }\) and we can have the qualitative analysis. From O.D. or Absorbance we can have the quantitative analysis.

Value of the wavelength where the absorbance is maximum is known as \({ \lambda }_{ max }\) which is characteristic property of the compound and gives the qualitative analysis of sample.While knowing the value of absorbance we can find out the concentration of solution using beer lambert’s law. Thus quantitative analysis is possible.

Double beam Spectrophotometer have following advantages:

1. As the two beams of radiations are passed simultaneously from sample solution and Blank Therefore any fluctuation in voltage can be compensated (cancelled).

2. As the two beams are passed from Blank and Sample solution any impurities present in the solvent will not affect the absorbance.

3. Accuracy is more than single beam spectrophotometer.

4. The main disadvantage is the instrument is costly.

#	Photoelectric Colorimeter	Spectrophotometers.
1	In this case the source of radiation used is incandescent lamp with tungsten filament.	In this case the source of radiation used is hydrogen or deuterium lamp, discharge tube, glow bar or Newtons glower.
2	They are working with visible light.	They are working with U.V. light, IR light and visible light.
3	Colour filters are used.	Monochromators are used.
4	Beer-Lamberts Law is not strictly obeyed.	Beer-Lamberts Law is strictly obeyed.
5	Any coloured solutions can be analyzed.	Coloured as well as colourless samples can be analyzed.
6	With the help of this instrument only quantitative analysis is possible.	Quantitative as well as qualitative analysis are possible.
7	These instruments are not costly.	These instruments are costly.

#	Photoelectric Colorimeter	Spectrophotometers.
1	In this case the source of radiation used is incandescent lamp with tungsten filament.	In this case the source of radiation used is hydrogen or deuterium lamp, discharge tube, glow bar or Newtons glower.
2	They are working with visible light.	They are working with U.V. light, IR light and visible light.
3	Colour filters are used.	Monochromators are used.
4	Beer-Lamberts Law is not strictly obeyed.	Beer-Lamberts Law is strictly obeyed.
5	Any coloured solutions can be analyzed.	Coloured as well as colourless samples can be analyzed.
6	With the help of this instrument only quantitative analysis is possible.	Quantitative as well as qualitative analysis are possible.
7	These instruments are not costly.	These instruments are costly.

Spectrophotometers are the instruments which are working at a wavelength where the absorbance is maximum. Such wavelength of the radiations whose absorbance is maximum is called \({ \lambda }_{ max }\).

With the help of spectrophotometers we can have qualitative as well as quantitative analysis of any type of sample it may be solid, liquid or gas. It may be coloured or colourless.

The spectrophotometers are of two types;

(i) Single Beam Spectrophotometers,

(ii) Double Beam Spectrophotometers.

The following are the main components of a Single Beam Spectrophotometers:

1. Source of radiations: It may be U.V-light, visible light or I.R. light. U.V. Light can be obtained by heating the filament which is filled with either H₂ gas or deuterium. Visible Radiations can be obtained by incandescent lamp with tungsten filament and I.R. Radiations can be obtained by heating Newtons glower at the temp. of 1500-2000 ℃. Newtons glower is metallic oxide of Yettrium, Erbium and Zirconium. I.R. radiations can also be obtained by heating glow bar which is Silicon carbide SiC (Carborandum) at the temp. of 1300-1700 ℃.

2. Collimating Convex Lens: The function is to collect all the rays coming from the source.

3. Diaphragm: To set the 100% transmittance.

4. Monochromators: To obtain the radiation of one wavelength. The Monochromators may be prism or diffraction gratings.

5. Cuvette or Sample Holder: For U.V. light it should be made up of quartz, for visible light it should be made up of glass and for I.R. light it must be that of rock salts for eg. NaCl.
a) If the sample is gas or volatile liquid having low B.P. must be taken in the closed container.
b) If it is liquid, one drop of the liquid must be placed between two parallel plates for U-V. the plates must be quartz and for visible it must be glass and for I.R. it is rock salt.
c) If it is solutions the solvent used for the preparation of solution must be Cyclohexane, methyl alcohol, ethyl alcohol or any other suitable solvent which is not capable to absorb the radiations incident on it.
d) If it is solids then we take about 1mg. of it and is mixed in 100-200 mg. of KBr - prepare slurry. It is cut into smaller circular discs (just like bindle) called pellets. They are dried and placed in the place of cuvette. The diameter of pellets is 10mm. and thickness is 1mm.

6. The Focusing Convex Lens: Its function is to collect all the transmitted light from cuvette and is focused at a point where photo cathode of the photo-cell is placed.

7. Photo-Cell: The transmitted light is incident on it photo cathode converts these radiations into current. It can be amplified using the amplifier if needed.

8. Read out meter or dial: The current is converted into OD or Absorbance in read out meter and by knowing OD we can find out the concentration of the solution i.e. quantitative analysis is possible by applying Beer Lamberts Law.

Working:

First of all the source of radiations from U.V. light to visible light to I.R. radiations is started. The light from the source is collected by Collimating convex lens and allowed to fall on monochromator. For each radiations incident on monochromator we are getting the radiations of one wave length only i.e. \(\lambda\), This monochromatic light is passed over the sample kept in cuvette where some part of radiation is absorbed and remaining is transmitted which passes through focusing lens and falls on Photocell. Photocell converts this radiation to the current and OD or Absorbance.

Value of the wavelength where the absorbance is maximum is known as \({ \lambda }_{ max }\) which is characteristic property of the compound and gives the qualitative analysis of sample. While knowing the value of absorbance we can find out the concentration of solution using beer lambert’s law. Thus quantitative analysis is possible.

Spectrophotometers are the instruments which are working at a wavelength where the absorbance is maximum. Such wavelength of the radiations whose absorbance is maximum is called \({ \lambda }_{ max }\).

With the help of spectrophotometers we can have qualitative as well as quantitative analysis of any type of sample it may be solid, liquid or gas. It may be coloured or colourless.

The spectrophotometers are of two types;

(i) Single Beam Spectrophotometers,

(ii) Double Beam Spectrophotometers.

The following are the main components of a Single Beam Spectrophotometers:

1. Source of radiations: It may be U.V-light, visible light or I.R. light. U.V. Light can be obtained by heating the filament which is filled with either H₂ gas or deuterium. Visible Radiations can be obtained by incandescent lamp with tungsten filament and I.R. Radiations can be obtained by heating Newtons glower at the temp. of 1500-2000 ℃. Newtons glower is metallic oxide of Yettrium, Erbium and Zirconium. I.R. radiations can also be obtained by heating glow bar which is Silicon carbide SiC (Carborandum) at the temp. of 1300-1700 ℃.

2. Collimating Convex Lens: The function is to collect all the rays coming from the source.

3. Diaphragm: To set the 100% transmittance.

4. Monochromators: To obtain the radiation of one wavelength. The Monochromators may be prism or diffraction gratings.

5. Cuvette or Sample Holder: For U.V. light it should be made up of quartz, for visible light it should be made up of glass and for I.R. light it must be that of rock salts for eg. NaCl.
a) If the sample is gas or volatile liquid having low B.P. must be taken in the closed container.
b) If it is liquid, one drop of the liquid must be placed between two parallel plates for U-V. the plates must be quartz and for visible it must be glass and for I.R. it is rock salt.
c) If it is solutions the solvent used for the preparation of solution must be Cyclohexane, methyl alcohol, ethyl alcohol or any other suitable solvent which is not capable to absorb the radiations incident on it.
d) If it is solids then we take about 1mg. of it and is mixed in 100-200 mg. of KBr - prepare slurry. It is cut into smaller circular discs (just like bindle) called pellets. They are dried and placed in the place of cuvette. The diameter of pellets is 10mm. and thickness is 1mm.

6. The Focusing Convex Lens: Its function is to collect all the transmitted light from cuvette and is focused at a point where photo cathode of the photo-cell is placed.

7. Photo-Cell: The transmitted light is incident on it photo cathode converts these radiations into current. It can be amplified using the amplifier if needed.

8. Read out meter or dial: The current is converted into OD or Absorbance in read out meter and by knowing OD we can find out the concentration of the solution i.e. quantitative analysis is possible by applying Beer Lamberts Law.

Working:

First of all the source of radiations from U.V. light to visible light to I.R. radiations is started. The light from the source is collected by Collimating convex lens and allowed to fall on monochromator. For each radiations incident on monochromator we are getting the radiations of one wave length only i.e. \(\lambda\), This monochromatic light is passed over the sample kept in cuvette where some part of radiation is absorbed and remaining is transmitted which passes through focusing lens and falls on Photocell. Photocell converts this radiation to the current and OD or Absorbance.

Value of the wavelength where the absorbance is maximum is known as \({ \lambda }_{ max }\) which is characteristic property of the compound and gives the qualitative analysis of sample. While knowing the value of absorbance we can find out the concentration of solution using beer lambert’s law. Thus quantitative analysis is possible.

Monochromator are the dispersing devices used in spectroscopy, Monochromator produces light of single wavelength.

Prisms: The collimated beam is dispersed by the prism into its constituent wavelengths. As the prism is having different thickness, light with different wavelengths gets diffracted with different angles and get separated. The prism is rotated to allow a particular wavelength to fall on a focusing lens and then emerge out of an exit slit.

Diffraction Gratings: The grating Monochromator have some advantages over prism which are as follows:
1. A grating gives much better dispersion of light than prism.
2. As the gratings are usually made up of non-corrosive material like aluminium which is not easily attacked by moisture. But metal salts prism are attacked by atmospheric air easily.
3. Gratings can be used for over a larger range of wavelength as compared to prism.

A diffraction grating is made by ruling a large no. of parallel & equidistant lines on highly polished surface of a metal such as Al. For UV and Visible lights these lines must be 15000 - 30000 lines per inch. But for I.R. it should be 1500-2500 per inch.

The light which strikes the ruling is scattered and dissipated (wasted). But the light which strikes the unruled portions or grooves is reflected regularly. The reflected waves produce interference pattern which results in the dispersion of light into its component wavelength. The above type of the gratings is known as Reflection Grating.

The other type of grating is known as Transmission Gratings which are obtained by ruling large no. of parallel and equidistant lines on glass. When the light falls on transmission grating each groove acts as an individual source of light. The transmitted rays from these grooves produce interference pattern which results in the dispersion of the transmitted light into its components wavelength.

The conditions for constructive interference are given by $$ n\lambda = d\left( \sin { i } + \sin { r } \right) $$ where \(n = + ve\) integer,
\(\lambda\) = wave length of light incident,
\(d\) = distance between two grooves. (It is known as grating constant)
\(i\) = angle of incidence
\(r\) = angle of refraction

Monochromator are the dispersing devices used in spectroscopy, Monochromator produces light of single wavelength.

Prisms: The collimated beam is dispersed by the prism into its constituent wavelengths. As the prism is having different thickness, light with different wavelengths gets diffracted with different angles and get separated. The prism is rotated to allow a particular wavelength to fall on a focusing lens and then emerge out of an exit slit.

Diffraction Gratings: The grating Monochromator have some advantages over prism which are as follows:
1. A grating gives much better dispersion of light than prism.
2. As the gratings are usually made up of non-corrosive material like aluminium which is not easily attacked by moisture. But metal salts prism are attacked by atmospheric air easily.
3. Gratings can be used for over a larger range of wavelength as compared to prism.

A diffraction grating is made by ruling a large no. of parallel & equidistant lines on highly polished surface of a metal such as Al. For UV and Visible lights these lines must be 15000 - 30000 lines per inch. But for I.R. it should be 1500-2500 per inch.

The light which strikes the ruling is scattered and dissipated (wasted). But the light which strikes the unruled portions or grooves is reflected regularly. The reflected waves produce interference pattern which results in the dispersion of light into its component wavelength. The above type of the gratings is known as Reflection Grating.

The other type of grating is known as Transmission Gratings which are obtained by ruling large no. of parallel and equidistant lines on glass. When the light falls on transmission grating each groove acts as an individual source of light. The transmitted rays from these grooves produce interference pattern which results in the dispersion of the transmitted light into its components wavelength.

The conditions for constructive interference are given by $$ n\lambda = d\left( \sin { i } + \sin { r } \right) $$ where \(n = + ve\) integer,
\(\lambda\) = wave length of light incident,
\(d\) = distance between two grooves. (It is known as grating constant)
\(i\) = angle of incidence
\(r\) = angle of refraction

Photo cells are the devices which converts the radiant light in to electric current.

Photo Emissive Cells: The simplest form of it consists of an evacuated glass tube fit with photo cathode. The photo cathode is concave shaped. It is made of Cesium (Cs) or it is made of metal and surface is coat with Cesium or in some cases the surface is coated with Potassium oxide and silver oxide this cathode is called as Photo-cathode. At the centre, there is metallic point. This acts as anode known as photo anode, connected in series with battery, galvanometer, resistance, and amplifier.

When the transmitted light from sample solution is incident on photo cathode, as the surface is alkali metal, then quickly electrons are ejected out and these electrons are attracted towards anode. This flow of electrons will produce current. If the current value is small then it is to be amplified by amplifier. The current produced is directly proportional to the number of electrons ejected from cathode and number of electrons ejected is directly proportional to transmitted light incident on it. It means current produced is directly proportional to transmitted light incident on photo cathode.

Photomultiplier Tube (PMT): It is also known as electron multiplier tube. It consists of cylindrical tube which is evacuated. The tube is having one cathode called photo cathode and number of the electrodes called Dynodes which are arranged as shown in the diagram.

A dynode is electrode with the coating of Cs and Sb or Cs and Silver Oxide. All the dynodes are kept at increasing value of potential. When radiations are incident on photo cathode, it ejects the electrons. These electrons are incident on dynodes. The primary electron gets accelerated in electrostatic field between the cathode and first dynode and then impregnate on the dynode.

As many as TEN secondary electrons may be emitted by dynode for each electron which strike on it. Thus, process is repeated at the several dynodes to which increasing potential is applied.

As a result, the electronic current gets amplified and the pulse of current gets increased by million time greater than the initial current emitted by cathode (Photo Cathode).

Photovoltaic cell or Barrier Layer cells: These cells generate their own e.m.f. and require no external source for operation. The photo voltaic cell consists of metal plate usually made of iron on which a thin layer of selenium is deposited. A thin transparent film of silver is spread over the selenium. It acts as the collector electrode.

The radiant energy which is incident on the semi-conductor surface i.e. Selenium-Silver surface release the electrons at the silver selenium interface and these electrons pass to the collector electrode. If the galvanometer is connected in the circuit it shows the deflection indicating the current is produced.

Photo cells are the devices which converts the radiant light in to electric current.

Photo Emissive Cells: The simplest form of it consists of an evacuated glass tube fit with photo cathode. The photo cathode is concave shaped. It is made of Cesium (Cs) or it is made of metal and surface is coat with Cesium or in some cases the surface is coated with Potassium oxide and silver oxide this cathode is called as Photo-cathode. At the centre, there is metallic point. This acts as anode known as photo anode, connected in series with battery, galvanometer, resistance, and amplifier.

When the transmitted light from sample solution is incident on photo cathode, as the surface is alkali metal, then quickly electrons are ejected out and these electrons are attracted towards anode. This flow of electrons will produce current. If the current value is small then it is to be amplified by amplifier. The current produced is directly proportional to the number of electrons ejected from cathode and number of electrons ejected is directly proportional to transmitted light incident on it. It means current produced is directly proportional to transmitted light incident on photo cathode.

Photomultiplier Tube (PMT): It is also known as electron multiplier tube. It consists of cylindrical tube which is evacuated. The tube is having one cathode called photo cathode and number of the electrodes called Dynodes which are arranged as shown in the diagram.

A dynode is electrode with the coating of Cs and Sb or Cs and Silver Oxide. All the dynodes are kept at increasing value of potential. When radiations are incident on photo cathode, it ejects the electrons. These electrons are incident on dynodes. The primary electron gets accelerated in electrostatic field between the cathode and first dynode and then impregnate on the dynode.

As many as TEN secondary electrons may be emitted by dynode for each electron which strike on it. Thus, process is repeated at the several dynodes to which increasing potential is applied.

As a result, the electronic current gets amplified and the pulse of current gets increased by million time greater than the initial current emitted by cathode (Photo Cathode).

Photovoltaic cell or Barrier Layer cells: These cells generate their own e.m.f. and require no external source for operation. The photo voltaic cell consists of metal plate usually made of iron on which a thin layer of selenium is deposited. A thin transparent film of silver is spread over the selenium. It acts as the collector electrode.

The radiant energy which is incident on the semi-conductor surface i.e. Selenium-Silver surface release the electrons at the silver selenium interface and these electrons pass to the collector electrode. If the galvanometer is connected in the circuit it shows the deflection indicating the current is produced.