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Question

# 1. A ray of light travelling in air falls on the surface of a glass slab at an angle 45° with the normal. The refractive index of glass is 1.5. Calculate the angle of refraction. 2. A point object is placed at a distance of 18 cm from a convex lens on its principal axis. Its image is formed on the other side of the lens at 27 cm. Calculate the focal length of the lens. [5 MARKS]

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Solution

## Each problem: 2.5 Marks 1. Angle of incidence = 450 Refractive index of glass = n = 1.5 n=Sin iSin r Sin r = Sin 4501.5=0.4728 r=280 2. According to the sign convention, the object is placed on the left side of the lens, therefore object distance is negative i.e. u = - 18 cm. Since the image is formed on the other side, the image-distance is positive i.e., n = + 27 cm. Using lens formula, 1v−1u=1f, we have 1+27−1−18=1f f=10.8 cm

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