To find the refractive index of a liquid(water) by using convex lens and plane mirror | Class 12 | Physics Practical Copy | Handwritten with observations

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 Aim: To find the refractive index of a liquid(water) by using convex lens and plane mirror.

Important Terms:

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 vaccum (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


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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.

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Principal:

In this method, a real and inverted image coincides with the object placed on the principal focus point of a convex lens. The rays from a pin AB placed on the principal focus F of a convex lens emerges out parallel to its axis. When these rays fall normally on a plane mirror placed horizontally below the convex lens, they retrace their path and form a real and inverted image at the principal focal plane of the lens. The size of image pin is equal to the size of object pin at point A and the tip of the pin gives the position of the second principal focus. Then fg (AF) is the focal length of the convex lens (for a thin lens) where O is the optical centre of the lens.


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Now, if the space between the lens and the plane mirror is filled with a transparent liquid (water) having refractive index aμw, and the above procedure is repeated to find the position of the principal focus in the new situation then the distance between the optical centre O of the lens and point A', OA'(F) would be the focal length of the combination of the two lenses. The combination consists of a glass. convex lens (radius of curvature of both the curved surfaces are same, R and a water plano-concave lens of same radius of curvature, R.
The refractive index of the liquid used (water) can be obtained by using the formula,
aμw = 2 -
fg
F
where,
aμw: Refractive index of water with respect to air
fg: Focal length of the convex lens
F : Focal length of the combination of lens and water

Procedure:

  1. Place the plane mirror on the base of a laboratory stand keeping its reflecting surface upwards.
  2. Clean the surface of the plane mirror and the convex lens and place the convex lens on the plane mirror.
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  3. Fix a sharp-edged bright pin in the clamp and place it horizontally and above the lens. Adjust the position of the pin such that its tip lies vertically above the optical centre of the convex lens.
  4. Shift the clamped pin gradually upward looking at the image and bring it to a height such that the tip of the pin exactly coincides with the tip of its image Ensure that there is no parallax between
    the object pin and its image. Measure the distance OA. For this, observe the distances of the pin

    from the upper and lower surfaces of the lens and take OA = fg as the average of these two distances.
  5. With the help of a dropper, put a few drops of water under the lens so that the space between mirror and lens is filled with water.
  6. Move the object pin upward and remove the parallax between the tip of the object pin and its image formed by the lens mirror system. Measure the distance OA'. Here again, as before, measure the distances of the pin from the two surfaces of the lens and take OA' = F as their average.
  7. Repeat the experiment and record your observations.

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