To find refractive indices of water and turpentine oil using a plane mirror, a equiconvex lens (made from a glass of known refractive index) and an adjustable object needle. Apparatus:
A convex lens, an optical needle, a plane mirror, a clamp stand, a spherometer, a plumb line, metre scale, water and turpentine oil. Theory:
Let’s add small amount of water on a flat, plane surface and place a convex lens over it. This forms a plano-concave lens of water between the lower surface of convex lens and plane mirror. Let f1 and f2 are the focal lengths of water lens and convex lens respectively, then focal length of the combination is: The focal length of the plano-concave lens is,
From Lens Maker’s formula, =( R1 = R and R2 =
for water lens.
The refractive index of water is ,
(where ‘R’ is the radius of curvature of the concave surfaces of the plano-concave lens). The Radius of curvature of the lens, is …(iii)
I. For finding the focal length of convex lens:
(i) (ii) Measure the rough focal length of the convex lens. Place the plane mirror with the convex lens placed on it above the horizontal base of a clamp stand horizontally as its tip lies vertically above the optical centre of the lens. Adjust the needle at a height a little more than the rough focal length of the convex lens. (iii) Try to remove the parallax between the tip of the object needle and its image tip. (iv) Note the distance of the tip of the needle from the centre of the upper surface of the lens. Let it be x1. (Use plumb line). (v) Remove the convex lens and measure the distance of the tip of the needle from the plane mirror. Let it be x2. (Use plumb line). 2
Repeat and record all the observations.
For finding the focal length of the plano-concave lens:
Pour few drops of water over the plane mirror and place the convex lens over it. Repeat steps (ii) to (iv) as done above. Repeat the procedure with turpentine oil...
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