Description of prism
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- The collimator renders a parallel beam of light through the two coaxial cylindrical tubes. One end of the collimator has a slit through which light enters the tube and falls on lens L situated at the other end.
- Prism table is a circular plate fixed over a vertical stand of adjustable height. The free end of stand consists of a circular scale graduated in degrees from 0o to 360o along with verniers to enable to read the position of the prism.
- Telescope is meant for observing the spectrum and is mounted horizontally on a vertical stand attached to the circular scale. The telescope can be rotated about the prism table.
Adjustments in prism
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The telescope is turned towards a distant object and is focused to see a clear image of object. It is then brought in line with the collimator. A clear image of the slit is obtained by adjusting the screws in the collimator. The prism is kept over the prism table.
Determination of m Material of a Prism
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The prism is placed over the table such that parallel rays from collimator falls on the sides AB and AC. Move the telescope in the position T1 to catch the brightest image of the slit formed by reflection of light at faces AB and AC. The cross wire is made to coincide with image and reading on the circular scale is noted. The telescope is turned to position T2 and the same procedure is repeated. If q is the difference between the two readings through which the telescope is turned then
`/_` = `theta/2`
Determination of angle of prism
`/_` = `theta/2`
Determination of Angle of Minimum Deviation
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To determine the angle of minimum deviation the side AB of the prism is made to face the ray of light. On looking through the face AC and rotating the prism table, the image of slit also turns. For a particular position of the prism, the slit becomes stationary. On further rotating the prism table, image of slit turns in the opposite direction. Fix the prism when the image of the slit is stationary. This is the position of minimum deviation. Coincide the cross wires of the telescope in this position and note the reading. Remove the prism and catch the direct ray and once again note the reading. The difference between the two readings gives the angle of minimum position.
Knowing dm and A, the refractive index (m) of the material of the prism can be calculated using the prism formula
Prism Binoculars
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Ordinary binoculars or opera glasses consist of two Galilean telescopes mounted side by side for observation with both the eyes and operated by one focusing screw.
The prism binoculars are a double field astronomical telescope using two sets of totally reflecting prisms. These prisms are interposed between the objective and the eyepiece, the edges of the prisms being at right angles to each other. The prisms not only erect the image horizontally and vertically, but also effectively fold the optical path ABCDEF, thus increasing the effective length of the telescope. High magnification is possible with a small actual length.
Direct Vision Spectroscope
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In a direct vision spectroscope, a number of thin prisms of crown and flint glasses are alternately placed in a small tube with refracting edges turned opposite to each other. The angles are such that they produce dispersion without any deviation for the mean yellow ray.
Generally three crown glass and two flint glass prisms are connected together with a transparent adhesive called Canada balsam and prisms are mounted in the tube. At one end of the tube a slit (S) of adjustable width and an achromatic lens L are provided. The distance between S and L is equal to the focal length of L. A narrow parallel of light from the lens through the prism assembly.
A spectrum is obtained in which the mean (yellow) colour is in the same direction as the incident beam. The spectrum can be observed through an eyepiece E. This spectroscope is a pocket sized instrument used for quick examination of various parts of a spectrum.
