question_answer 35)

The adjoining figure shows an equiconvex lens (of refractive index 1.50) in contact with a liquidlayer on top of a plane mirror. A small needle with its tip on the principle axis is moved alongthe axis until its inverted image is found at theposition of the needle. The distance of the needlefrom the lens is measured to be 45*0 cm. Theliquid is removed and the experiment is repeated. The new distance is measured to be 30.0 cm. What is the refractive index of liquid ?  

Answer:

In presence of the liquid, the distance ofthe needle from the lens is equal to the focallength f of the combination of the convexlens and the piano concave lens formed bythe liquid below it i.e.f= 45 cm. Also 1-5 In the absence of the liquid, the distance ofthe needle and the lens is equal to the focallength of the convex lens only i.e. If is the focal length of plane concavelens formed by the liquid, thenor Using the Lens maker's formula, we get For the piano convex lens formed by the liquid Usingwe get, i.e. i.e.