Constant Deviation Spectrophotometer

Submitted By:

Saloni Sharma

Assoc. Prof in Physics

Constant Deviation Spectrometer

• An instrument used to study the spectra with unaided eye is called spectroscope or spectrometer.
• A constant deviation spectrometer got its name due to the fact that it uses constant deviation prism or Pellin-Broca prism.
• Determine the wave length of the unknown spectra of the given metals in the arc lamp source.

Theory of Prism

• If light is incident on a prism at any angle “i”, will be refracted and then it will emerge from the prism at an angle “e”
• The angle between the incident direction of the light and the emerging direction of the light is called angle of deviation “δ”
• If you vary incident angle then the angle of deviation will also vary, for decreasing or increasing the incident angle from a particular angle.

Figure 1: Angle of deviation in ordinary prism

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Theory of Prism

When refraction through a prism takes place in such a manner that the angle of incidence is equal to the angle of emergence, the refracted ray will be parallel to the base of the prism

When the above conditions are satisfied for a particular value of i, the deviation suffered by the light ray is minimum and the angle of deviation is known as the angle of minimum deviation δm.

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For any other value of i, the value of δ increases.

Figure 1: Angle of deviation in ordinary prism

Delta becomes minimum for a particular value of incident angle.

Constant Deviation Prism or Pellin Broca Prism

• Constant deviation prism is a special type of prism is called Pellin Broca prism named after the person who discovered it.
• Pellin Broca prism a constant deviation dispersive prism. The shape of the prism is a trapezium P Q T S.
• It is a single piece prism having well defined angles. Angle Q is 90⁰ degree, angle T is 60⁰, angle S is 135⁰ and angle P 75⁰.

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• This prism can be considered to consist of three prisms. Prism P Q R and Q S T are ordinary prisms having angle of prism 30⁰ each.
• Prism P R S is a reflecting prism with angle RPS and angle RSP are 45⁰ each. Angle PRS is 90⁰.

BC is the refracted ray which will be incident normally on the interface of prism PQR and PRS hence passes undeviated and is incident on the face PS, here it will be reflected due to the phenomenon of total internal reflection and undergoes a deviation of 90⁰.

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This reflected ray CD will be incident on the face QT and will be refraction from prism material to the environment at an angle θe.

When light falls on the face PQ at incident angle θi, it will be refracted at an angle of refraction θr.

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• If we change the angle of incidence θi, we will get a particular angle where the refracted ray will be parallel to QR or QS.
• Since in a prism, the refracted ray of light is parallel to the base of the prism, when the incident angle is for minimum deviation.
• As if that condition is followed and it is going and falling on the surface and then from the surface, it is reflected in such a way that it is parallel to these to the base SR and is refracted outside at an angle θe.
• The construction of the prism is such that, whatever the deviation of light, we are getting the minimum deviation condition for each prism.
• Angle between the incident ray and refracted ray in this prism is 90⁰.

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• m∠ PQT = 90⁰ and m∠ BCD = 90⁰ (1)
• we have, m∠ QBC + m∠ QDC =180⁰ (2)
• But, since m∠ QBC = 90⁰ + θr and m∠ QDC = 90⁰ - θi’ , (3)
• Therefore, (90⁰ +θr) + (90⁰ - θi’ ) = 180⁰ . (4)
• Thus θr = θi’ and θe = θi.
• Hence emergent ray is perpendicular to the incident ray.
• When the angle of incidence is equal to the angle of emergence and the angle of deviation is 90°, a ray would be passing through a position of minimum deviation.
• This principle is used in constant deviation spectrometer

• If light of different wavelengths is incident at same angle θi, because of dispersive power of the prism the angle of refraction θr will be different.
• The refracted ray will fall at different position and they will be reflected at different angles.
• We can rotate the prism to select a particular wavelength for which the deviation will be 90⁰.
• To ensure that the deviation will be 90⁰, we set the Collimator of the spectrometer and the telescope of the spectrometer at 90⁰.
• Through the eyepiece of the telescope, we will see the light, which is coming at 90⁰ deviation.
• We keep the deviation constant and for that we can rotate the prism and select light of different wavelength at these 90⁰.

Experimental Set-Up

• The collimator and the telescope are fixed and the axes are perpendicular to each other.
• The prism table can be rotated about the vertical axis using a drum which is attached to the table.
• The head of the drum is calibrated for the wavelength and thus the wavelength can be measured directly.

• When the light is incident on the prism the prism table can be rotated till the angles of incidence and emergence are equal.
• The pointer seen in the field of view of telescope can be used for the measurement.
• After clamping the prism the drumhead is rotated to rotate the prism table and the desired wavelength is measured.
• An ordinary prism and a spectrometer can be used for this task, but the process is time consuming because if the adjustment of minimum deviation is disturbed, resetting is troublesome.
• In case of constant deviation spectrometer, if the prism is disturbed it can easily be reset by using a source of known wavelength.

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Procedure to find wavelength

• Level the constant deviation spectrometer by means of a sprit level and focus the telescope.
• Place the constant deviation prism on the prism table so that its 90˚ vertex faces towards the objective of the telescope.
• The drumhead is calibrated using known wavelength of a calibration source. The source is placed in front of the collimator lens.

• For sodium source the drum is rotated so that it reads 5890 Å, the known value of Na yellow line. The spectrum is sought for by looking through the telescope. It is brought on the pointer crosswire by slightly rotating the prism this way or that way.
• Once the spectral line coincides with the pointer, clamp the prism. Thus the CDS is calibrated to measure any unknown wave length.
• Then replace the calibration source by the arc source using a particular metal arc of interest.

• The D.C. power supply is connected to the arc stand holding the pointed metal arc one over the other. Switch ON the power supply and observe the arc begins to glow.
• The spectrum is observed in the CDS. Adjust the drum head to make the pointer coincide on each of the spectral lines and read the characteristic wavelength of the different lines emitted by the metals directly.
• Compare the values of spectral lines obtained for different metals with the literature values.

Calibration

• The prism table can be rotated manually to select a particular wavelength.
• We can also rotate the prism table with the help of drum. There is a scale with this drum.
• If we set this prism for a particular wavelength and change the angle of the prism table, the second light will appear, which means the second wavelength will have that 90⁰deviation now.
• By rotating the drum,we will get light at constant deviation of different wavelength. The reading will be wavelength of light, which we are seeing through the telescope and that means, it is having the constant deviation of 90⁰.

• To calibrate with the known wavelength we use the sodium light average wavelength 5893 angstrom or 589 nanometer.
• Keeping the scale of the drum at 589 nanometer, without disturbing the prism, we will rotate the prism table with the help of the drum, which will change the angle of the incidence. We set the angle of incidence for the sodium light ie, sodium light is at constant deviation of 90⁰, and the drum is exactly at 589nm. This reading is at wavelength of this sodium light 589 nm.
• After that, we cannot disturb this prism position ok. Manually, we will not disturb this prism position ok.
• Now, replace the sodium light with unknown wavelength, and rotate the drum to focus for different colors. The reading of the drum will give the reading of the wavelength of unknown source.

PRECAUTION

• The arc points or the holders should not be touched as they carry high voltage.
• To start the glow the two pointer arcs should be brought very close, nearly to touching position and after the glow a minimum gap is maintained to avoid excess load on the power supply.
• After taking readings, allow the metal rods to cool down before changing to another pair of rods.