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EXPT NO: 06. R. I. OF GLASS USING TRAVELLING MICROSCOPE

Aim :  To determine the refractive index of glass using a traveling microscope.

Apparatus : Traveling microscope, glass slab, saw dust (chalk powder), magnifying glass, white paper etc.

Diagram :

Formula:R. I. of denser medium (glass) w.r.t. air   m

 Procedure :

      Determine the least count of the traveling microscope.

      Focus the eyepiece of the microscope on the cross wires (by moving the eyepiece inwards or outwards) so that the cross wires are well focused.

      Draw a cross mark or few straight lines on a plane white paper and place this paper, just below the objective of the microscope.

      Make the microscope vertical and focus it, so as to obtain well focused image of the lines (cross mark) marked on the paper. Note the corresponding microscope reading (X).

      Place the given glass slab on the paper. Without disturbing the focusing screw, raise the microscope gradually to obtain well focused image of the line (cross mark) Note the corresponding microscope reading (Y).

      Sprinkle few particles of saw dust on the top surface of glass slab. Without disturbing the focusing screw, raise the microscope gradually to obtain well focused image of saw dust. Note the corresponding microscope reading (Z).

      7).      Calculate real depth (Z-X) and apparent depth (Z-Y)

      Calculate the refractive index of glass by using the formula given above.

Observations :

1)     Smallest division on main scale,                         P = ………………cm

2)     Total number of divisions on vernier scale,        N = ………………cm

3)      L.C. of vertical scale of traveling microscope  

Table for traveling microscope readings : 

Sr.No.	Reading when microscope focused on	Main Scale reading A cm	Vernier Scale reading B	Total reading C=BxL.C	Total reading A+C cm
01	On cross mark without glass slab
02	On cross wire with glass slab
03	On saw dust on surface of glass slab

1)   Real depth            = (Z - X)                     = ………………..cm

2)   Apparent depth   = (Z - Y)                     =………………..cm

Calculations : 

Result :    

Refractive index of the material of the glass slab m =  …………….

 Precautions :

      Adjust the leveling screws so that the base of the microscope becomes horizontal.

      Microscope tube should be in the lowest possible position, when it is focused on line/cross mark.

      While focusing the microscope, see that there is no parallax between cross wires and the image.

      Do not disturb the focusing screw while taking the readings.

      The quantity of saw dust added should be small and which is below the objective.

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