Page 84 - Physics - XII

The refractive index of the glass with respect to the air = Real thickness of slab
Apparent thickness of slab
R − R
µ = 3 1
R − R
3 2
where R is the reading of real depth at the bottom of the slab,
1
R is the reading at cross-mark due to the refraction, and
2
R is the reading at the top of the glass slab.
3
Procedure
1. Place the travelling microscope on a horizontal table. Make the base of microscope horizontal by
adjusting the levelling screws, also adjust the position of eyepiece so that the cross wires are
clearly visible.

2. Find the least count of microscope scale.
3. Make a black ink cross mark on the base of the microscope. This mark will act as point P [Fig. 6.3(a)].
1
Now, make the microscope vertical and move it very gently. Using the screw, focus the eyepiece on
cross-mark and bring the cross-mark in focus so that the cross of the cross wires of eyepiece coincides
with the black cross-mark on the base of the microscope. Note the main scale and vernier scale readings
on vertical scale. This gives reading R .
1
4. Place the given glass slab on the black cross-mark. The cross-mark through the glass slab appears to
be raised. Now, move the microscope gradually and gently upward to bring the cross-mark in focus
on the cross wires of the eyepiece. Note this reading [Fig. 6.3(b)] on the vertical scale of travelling
microscope and record it as R .
2
5. Sprinkle a little lycopodium powder on the glass slab and raise the microscope further upward till the
powder particles come into focus. Note this reading on the vertical scale again [Fig. 6.3(c)] and record
it as R .
3
6. Repeat steps 3 to 5 two times more and take observations.
7. Diff erence of the readings R and R gives the real thickness of the slab whereas R – R gives the
3
2
3
1
apparent thickness of the slab.
8. Note all the observations in the observation table.
Observations
1. Least count of travelling microscope:

50 Vernier Scale Divisions = 49 Main Scale Divisions

1 V.S.D. = 49 M.S.D.
50

Least count = 1 M.S.D. – 1 V.S.D. = 1 M.S.D. – 49 M.S.D. = 1 M.S.D.
50 50
Value of 1 M.S.D. = 1 cm = 0.05 cm
20
Least count = 1 × 005. cm = 0.001 cm
50
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