Read the following paragraph and answer the questions that follow. Types of Lenses and their combination A convex or converging lens is thicker at the centre than at the edges. It converges a beam of light on refraction through it. It has a real focus. Convex lens is of three types: Double convex lens, Plano convex lens and Concavo-convex lens. Concave lens is thinner at the centre than at the edges. It diverges a beam of light on refraction through it. It has a virtual focus. Concave lenses are of three types: Double concave lens, Plano concave lens and Convexo-concave lens. When two thin lenses of focal lengths f1 and f2 are placed in contact with each other along their common principal axis, then the two lens system is regarded as a single lens of focal length f and If several thin lenses of focal length f1, f2, .... fn are placed in contact, then the effective focal length of the combination is given by and in terms of power, we can write P = P1 + P2 + .... + Pn The value of focal length and power of a lens must be used with proper sign consideration.
i. Two thin lenses are kept coaxially in contact with each other and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm, the focal length of the other would be (a) -26.7cm (b) 60cm (c) 80cm (d) 30cm
ii. A spherical air bubble is embedded in a piece of glass. For a ray of light passing through the bubble, it behaves like a (a) converging lens (b) diverging lens (c) mirror (d) thin plane sheet of glass
iii. Lens generally used in magnifying glass is (a) single concave lens (b) single convex lens (c) combination of convex lens of lower power and concave lens of lower focal length (d) Planoconcave lens
iv. The magnification of an image by a convex lens is positive only when the object is placed (a) at its focus F (b) between F and 2F (c) at 2F (d) between F and optical centre
OR
A convex lens of 20 cm focal length forms a real image which is three times magnified. The distance of the object from the lens is (a) 13.33 cm (b) 14 cm (c) 26.66 cm (d) 25 cm
