Procedure

 

For doing simulation

 

1. Move the slider in 'Calibrate Telescope’ and click ‘START’ button.
2. Click on Combo box to select lamp.
3. Click ‘Switch On Light’.
4. Vernier reading is set to 0⁰ and telescope at 90⁰ by clicking on sliders of both.
5. Click the ‘Place grating button’.
6. Turn the telescope to left. Coincide vertical cross wire with the green line on the pattern.
7. Note the readings of vernier 1 and vernier 2.
8. Telescope is moved to right side of direct image and vertical wire is made to coincide with green line of pattern.
9. Note the readings of vernier 1 and vernier 2.
10. Click on slider under Fine angle to get readings more precisely.
11. Difference between the two readings on the same vernier is taken.
12. Mean value of this difference gives 2Θ, twice the angle of diffraction. From this, the value of Θ is obtained for green line.
13. Assuming the wavelength of green line, 546nm, the no. of lines per mm is calculated using equation, N=SinΘ/mλ where m is the order.

 

For doing the real lab

 

• Setting the grating for normal incidence position.

 

1. Vernier table is fixed after making the preliminary adjustments.
2. Illuminate the slit with mercury vapour lamp and make the slit narrow.
3. Bring the telescope in line with collimator. Coincide the direct image’s slit with vertical cross wire.
4. Note down any one of the vernier reading.
5. Turn the telescope exactly through 90⁰ and then clamp it.
6. Place the grating over the prism table with its ruled surface facing collimator and perpendicular to line joining two leveling screws of prism table.
7. Unclamp the Vernier and rotate until the reflected image coincides with the vertical cross wire.
8. The prism table is now fixed and the readings of Vernier are noted.
9. Vernier table is unclamped and rotated exactly 45⁰ in the proper direction so that surface of grating becomes normal to the collimator.
10. Vernier table is now clamped.

 

• Standardizing  the grating

 

1. Move the telescope to observe the direct image. Diffraction patterns are seen on the either side of the direct image.
2. Turn the telescope to left. Coincide vertical cross wire with the green line on the pattern.
3. Note the readings of vernier 1 and vernier 2.
4. Telescope is moved to right side of direct image and vertical wire is made to coincide with green line of pattern.
5. Note the readings of vernier 1 and vernier 2.
6. Difference between the two readings on the same vernier is taken.
7. Mean value of this difference gives 2Θ, twice the angle of diffraction. From this, the value of Θ is obtained for green line.
8. Assuming the wavelength of green line, 546nm, the no. of lines per mm is calculated using equation, N=SinΘ/mλ where m is the order.

 

• To determine the wavelength of other lines

 

Repeat the same procedure as above for other lines, and calculate its wavelength using the equation, λ=sinΘ/Nm.
 

 

Observations

 

 

 

Results

 

The wavelength of the prominent lines of the mercury spectrum are given in nanometre in the tabular column.

 

Number of grating per metre=................/m