Question

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. The 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 `1/f = 1/f_1 + 1/f_2` 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 `1/f = 1/f_1 + 1/f_2 + ... + 1/f_n` 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.

1. 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) -26.7 cm
   (b) 60 cm
   (c) 80 cm
   (d) 30 cm
2. 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
3. The lens generally used in magnifying glasses is ______.
   (a) single concave lens
   (b) single convex lens
   (c) combination of a convex lens of lower power and a concave lens of lower focal length
   (d) Planoconcave lens
4. 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 the 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

Answer 1

1. 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 -26.7 cm.
2. A spherical air bubble is embedded in a piece of glass. For a ray of light passing through the bubble, it behaves like a diverging lens.
3. The lens generally used in magnifying glasses is a single convex lens.
4. The magnification of an image by a convex lens is positive only when the object is placed between F and the 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 26.66 cm.