To find the refractive index of a glass slab using travelling microscope | Class 12 | Physics Practical Copy | Handwritten with observations

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Aim: To find the refractive index of a glass slab using travelling microscope.

Important Terms:

01
Refraction:
The direction of propagation of an obliquely incident ray of light that enters the other medium changes at the interface of the two media. This phenomenon is refraction of light.

Refractive Index:
It is the ratio of speed of light in vacuum (air) to the speed of light in any medium.
Refractive index of a medium tells us that how much it is denser than air.
It is a unitless and dimensionless quantity.
It is denoted by μ.
μ =
Speed of light in vaccum or air
Speed of light in any medium
 =
c
v


02


Laws of Refraction:
  1. The incident ray, the refracted ray and the normal to the interface at the point of incidence all lie in the same plane.
  2. Snell's Law: The ratio of sin i and sin r is constant.
    3. sin i
    sin r
     = 1μ2
    where, 1μ2 is the refractive index of second medium with respect to first medium.
    1μ2 is independent of ∠i and depends upon wavelength λ of the light.
    03

Principal:

If a glass slab is placed in air on a horizontal surface and its bottom surface is viewed from top, it appears to be elevated due to the phenomenon of refraction. The distance between this apparent depth and the top surface of the slab gives the apparent thickness of the slab. In case of normal observation, it can be shown that the refractive index of glass with respect to the medium, air is,

μ =
Real thickness of the slab
Apparent thickness of the slab
or
The ratio of real depth to the apparent depth of a refracting medium is equal to the refractive index of the medium. Thus,
μ =
Real depth
Apparent depth
 =
R3 - R1
R3 - R2


04

Procedure:

  1. Find the least count of the microscope scale, being used.
  2. Put a mark on a sheet of paper.
  3. Place the paper on the horizontal platform of the microscope. Adjust the microscope in such a way that its lens system is vertically above the mark.
  4. Focus the microscope on the mark and record the reading R1 using Main Scale Reading (MSR) and Vernier Scale Reading (VSR) of its coinciding division.
  5. Next, place the glass slab over the mark on the sheet of paper.
    05

  6. Move the microscope upward until mark on the paper, seen through the slab is sharp and clear. Take the reading R2 with the main scale and coinciding division of the vernier scale.
  7. Spread a little lycopodium powder/chalk dust over the top surface of the glass slab.
  8. Raise the lens system of the microscope and focus the microscope to see some of their particles clearly and record reading R3.
  9. Invert the slab and repeat steps 3 to 8.


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