Appratus :

Spectrometer, prism, prism clamp, mercury vapour lamp, lens.

Procedure

Procedure for simulator

Preliminary adjustments:

1. Open the simulation and focus the telescope on a distant object using the Focusing of telescope slider.
2. When the telescope is properly focused, the START button becomes active. Click START to begin the experiment.
3. Click SWITCH ON LIGHT to illuminate the spectrometer.
4. Adjust the Slit focus slider until the slit is clearly focused when viewed through the telescope.
5. Adjust the Slit width slider to obtain a clear and sufficiently narrow slit.
6. Use the Telescope slider and the Telescope fine-angle adjustment to bring the slit image into coincidence with the cross-wire.

Least Count of Spectrometer

One main scale division (N) =................minute

Number of divisions on vernier (v) = ................

L.C = N/v = .......................minute

Determination of the Angle of Minimum Deviation

1. Click PLACE PRISM to place the prism on the prism table.
2. Adjust the Vernier table using the Vernier table slider so that light from the collimator falls on one face of the prism and emerges through the other face.
3. Use the Telescope slider to observe the refracted image of the slit.
4. Slowly rotate the prism table using the Vernier table slider and observe the movement of the refracted image.
5. Continue rotating the prism table in the appropriate direction until the image reaches the position where it remains momentarily stationary. This corresponds to the position of minimum deviation.
6. Use the Telescope fine-angle adjustment to accurately bring the slit image into coincidence with the cross-wire.
7. Note the telescope/vernier readings shown in the simulation. The zoomed view of Vernier 1 and Vernier 2 can be observed by placing the mouse pointer over the corresponding vernier scales.
8. Remove the prism by clicking REMOVE PRISM.
9. Align the telescope parallel to the collimator using the Telescope slider and note the direct-ray reading.
10. Calculate the angle of minimum deviation, (D), from the difference between the direct-ray reading and the deviated-ray reading.

Determination of the Resolving Power of the Prism

1. Click PLACE PRISM to place the prism on the prism table.
2. Rotate the prism table using the Vernier table slider so that the light from the collimator passes through the prism and emerges from the other face.
3. Observe the spectrum produced by the prism through the telescope.
4. Use the Telescope slider to bring the telescope successively to the required spectral lines.
5. For each selected spectral line, use the Telescope fine-angle adjustment to obtain accurate coincidence of the line with the cross-wire.
6. Note the corresponding telescope/vernier readings for the selected spectral lines. Use the zoomed Vernier 1 and Vernier 2 views for reading the scales accurately.
7. Remove the prism by clicking REMOVE PRISM and note the direct-ray reading.
8. Determine the angle of minimum deviation for each selected spectral line from the difference between the corresponding deviated-ray and direct-ray readings.
9. Calculate the refractive index for each selected spectral line using the angle of the prism and the corresponding angle of minimum deviation. Use the following equation. $$n = \frac{\sin\left(\frac{A+D}{2}\right)} {\sin\left(\frac{A}{2}\right)}$$

• $n$ = refractive index of the prism material
• $A$ = angle of the prism. To determine the value of $A$, refer to the Angle of Prism Using Spectrometer experiment.
• $D$ = angle of minimum deviation for the selected spectral line​

11. Select two spectral lines, such as blue and yellow, and calculate the average refractive index using their refractive indices.
12. Calculate the resolving power for the selected pair of spectral lines using the equation given in the Theory section.
13. Record the calculated values in the result section.

Procedure for Real Lab:

1. Turn the telescope towards the white wall or screen and looking through eye-piece, adjust its position till the cross wires are clearly seen.
2. Turn the telescope towards window, focus the telescope to a long distant object.
3. Place the telescope parallel to collimator.
4. Place the collimator directed towards sodium vapor lamb. Switch on the lamp.
5. Focus collimator slit using collimator focusing adjustment.
6. Adjust the collimator slit width.
7. Place prism table, note that the surface of the table is just below the level of telescope and collimator.
8. Place spirit level on prism table. Adjust the base leveling screw till the bubble come at the centre of spirit level.
9. Clamp the prism holder.
10. Clamp the prism in which the sharp edge is facing towards the collimator, and base of the prism is at the clamp.

1. Rotate the prism table so that the light from the collimator falling on one of the face of the prism and emerges through the other face.
2. The telescope is turned to view the refracted image of the slit on the other face.
3. The vernier table is slowly turned in such a direction that the image of slit is move directed towards the directed ray; ie.,  in the direction of decreasing angle of deviation.
4. It will be found that at a certain position, the image is stationary for some moment. Vernier table is fixed at the position where the image remains stationary.
5. Note the readings on main scale and vernier scale.
6. Carefully remove the prism from the prism table.
7. Turn the telescope parallel to collimator, and note the direct ray readings.
8. Find the difference between the direct ray readings and deviated readings. This angle is called angle of minimum deviation (D).

To determine the Resolving power of prism

1. Rotate the vernier table so as to fall the light from the collimator to one face of the prism and emerged through another face. (refer the given figure ).
2. The emerged ray has different colors.
3. Turn the telescope to each color, and note the readings for different colors.
4. Remove the prism, hence note direct ray reading.
5. Find the angle of minimum deviation for different color.(Say ,violet, blue, green, yellow).
6. Find the refractive index for these colors. Using equation (1).
7. Resolving power for yellow and blue

$$w=\frac{n_{b}-n_{y}}{n-1}$$

Where $n_{b}$ and $n_{y}$ are the refractive index of blue and yellow, and

$$n=\frac{n_{b}+n_{y}}{2}$$

Refractive index for the line ________ n1 =

Refractive index for the line ________ n2 =

Average refractive index $n=\frac{n1+n2}{2}$

Resolving power for ______ and ______ line $ω =\frac{n2-n1}{n-1}$ =

Result:

Angle of the Prism =……………………………….Degrees

Angle of minimum deviation of the prism =……………………………….Degrees
Refractive index of the material of the prism =……………………………….
Dispersive power of the prism =……………………………….