Draw a ray diagram showing the geometry of formation of image of a point object situated on the principal axis and on the convex side of a spherical surface of radius of curvature R. Taking the rays as incident from a rarer medium of refractive index $n_1$ to a denser medium of refractive index $n_2$ derive the relation. $\large\frac{n_@}{v} -\frac{n_1}{u} = \frac{n_2-n_1}{R}$, Where symbols have their usual meaning ? (ii) Explain briefly how the focal length of a convex lens changes with increase in wave length of incident light . (iii) What happens to the focal length of convex lens when it is immersed in water ? Refractive index of the material of lens is greater than water .

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