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Refractive Index of Glass slab using Travelling Microscope

Materials Required

  • A travelling microscope 
  • A glass slab 
  • Spirit level 
  • Lycopodium powder / chalk dust 
  • Paper  
  • Adhesive tape 
  • A marker 

Real Lab Procedure

  • Find the least count of the travelling microscope  
  • To get a sufficient amount of light, place the travelling microscope near the window. 
  • Now, place a spirit level on the horizontal surface of the travelling microscope. Adjust the levelling screws if required to make base of the microscope horizontal so that the bubble in spirit level comes exactly in the centre. 
  • Put a point mark on a sheet of paper. Let it be point P. 
  • Place the paper on the horizontal platform of the microscope. Use an adhesive tape to stick it in place.  
  • Adjust the microscope in such a way that its lens system is vertically above the mark. 
  • Rotate the screw in front of the travelling microscope anticlockwise to loosen it. Adjust the vertical position of the microscope and tighten it. 
  • For clear visibility of the cross wire, adjust the position of the eyepiece using the rack and pinion screw. Do not change this screw for the rest of the experiment.  
  • Rotate the fine adjustment screw on top of the microscope anticlockwise and focus the microscope on the mark such that the cross of cross-wires coincides with the mark on the paper.  
  • Note the main scale reading (MSR) just before the zero mark of vernier scale. 
  • Use the magnifying lens attached to microscope and note the vernier scale division that coincides with a division of the main scale as the vernier scale reading VSR. 
  • Now, calculate the total reading of microscope R1.  
  • Next, place the glass slab over the mark on the sheet of paper. The microscope will now be out of focus. Due to refraction the point P appears to be raised when seen through a glass slab. 
  • Loosen the screw for vertical adjustment and gently raise the entire lens system upward until mark on the paper, seen through the slab is sharp and clear. Tighten the screw.  
  • Make the fine adjustment.   
  • Take the reading R2 with the main scale and coinciding division of the vernier scale. 
  • Spread a little lycopodium powder / chalk dust over the top surface of the glass slab. Make sure that the layer of powder is very thin. 
  • Raise the lens system of the microscope and focus the microscope to see some of their particles clearly and record reading R3. 
  • Invert the slab and repeat the steps to make two more observations. 
  • Fill the observation table and calculate the refractive index.  

Simulator Procedure

  • Click on the ‘Place Paper’ button to place the sheet of paper with a point mark on the horizontal platform of the microscope. 
  • Click on the arrow buttons near the screws of travelling microscope to adjust the vertical position of the microscope and focus on the cross mark 
  • Note down the main scale reading (MSR) just before the zero mark of Vernier scale, note the Vernier scale division that coincides with a division of the main scale as the Vernier scale reading VSR. 
  • Now, calculate the total reading of microscope R1. 
  • Select the ‘thickness of glass slab’ 
  • Select the ‘glass slab material’ 
  • Click on ‘Place Glass Slab’ button to insert glass slab on the top of paper. 
  • Click on the arrow buttons to focus the cross mark seen through the glass slab. 
  • Calculate the reading R2 with the main scale and coinciding division of the vernier scale. 
  • Click on the Lycopodium Powder button to put powder on the top of glass slab. 
  • Click on the arrow buttons to focus the powder', 
  • Calculate the reading R3 with the main scale and coinciding division of the vernier scale. 
  • Calculate the refractive index of the glass slab as the ratio of real thickness of the glass slab (R3 – R1) to the apparent thickness of the glass slab (R3 – R2). 
  • Click on the checkbox to show the result. 
  • To redo the experiment, click on the ‘Reset’ button. 
  • NOTE: Click on the ‘HELP’ button for help. 

Observations

Least count of travelling microscope:

20 division on main scale = 1 cm

Length of 1 MSD = 0.05 cm 

No of division on vernier scale = 50 

                                                                                                     = 0.01 mm 

                                                                                                      = 0.001 cm 

Sl No. Reading of microscope when focused on Real Thickness Apparent thickness Refractive index
Mark made on the paper Mark on the paper through the slab Particles on the top of the glass surface
R1 = MSR + VSR x L.C. R2 = MSR + VSR x L.C. R3 = MSR + VSR x L.C. R3 – R1 R3 – R2

n=(R3-R1)/(R3-R2)
  (cm)  (cm)  (cm)  (cm)  (cm)   
1            
2            
3            

Calculations

The refractive index of the material of glass with respect to air,nga is given by 

Mean, n = (n1 + n2 + n3 ) / 3 = ______

Results:

The refractive index of glass of the slab was found to be _____ .

Precautions

  • In microscope parallax should be properly removed. 
  • The microscope should be moved in upper direction only to avoid back-lash error. 
  • The layer of lycopodium powder should be thin. 
  • After focussing the microscope for the first reading, don’t change its focus for subsequent readings. 

Sources of error

  • Improper calibration of scale of microscope. 
  • Microscope may not be in perfectly vertical position. 
  • The lycopodium layer on glass slab may be thick.