Question

briefly describe the procedure for the diffraction grating experiment

Answer 1

The grating experiment was done with the help of a spectrometer. so, first we have to do the spectrometer adjustment

1.spectrometer adjustment:

1.Find out the least count of spectrometer.  

2. By using the levelling screw provided and spirit level, made the base of the spectrometer horigental.

3. Adjust the eyepiece of telescope till the cross wires are clearly focused. To avoid parallax error, adjust the telescope by focusing a distant object.

4. Keep the telescope in line with the axis of the collimator and the source. Adjust the width of the slit to get a narrow line image. Adjust the distance of the slit from the collimating lines till a sharp image of the illuminated slit is obtained.

Then, we have to do grating adjustment

2. Grating adjustment:

1.Bring the telescope in line with the collimator and the source to get image of the slit of vertical cross wire.

2. Take the direct ray reading on the scale ( i.e. main scale reading/MSR, vernier coincidence/VC, vernier scale reading/VSR and total ). Let the total reading be X.

3.Fix the grating on the prism table. now, turn the telescope exactly 90⁰ from the initial position and clamp the telescope in that position. Let the new reading be X+90⁰

4.Rotate the prism table slowly without disturbing the verniers so that the reflected image of the slit is made to coincide exactly with the vertical cross wire. in this position, the angle of incidence of light on the grating surface will be 45⁰

5. Rotate the vernier scale along with the prism table through 45⁰ to adjust the grating for normal incidence position. Lock the verniers in this position.

The next step is measurement of angle of diffraction.

3. Measurement of angle of diffraction:

1.Adjust the telescope for the second order spectrum on the left hand side, clamp it by slow motion screw.Bring the intersection of cross wire exactly in the middle of the second order D₂ line.

2. Note the readings ( MSR,VC,VSR and total) on both the verniers V₁ and V₂ .The diffrence between this two reading should approximately 180⁰.

3. Adjust the telescope for second order D₁ line and fist order on left hand side and note down the readings.

4.Then turn the telescope to catch D_2,D₁ and first order line on right hand side and note down the readings.

5. the difference between the two readings of the same order is double the angle of diffraction (2). Find out mean for different orders.