Page 1 - HA-power of a lens

Objective Questions:

1. An optician prescribes spectacles to a patient with a combination of a convex lens
of focal length 40 cm and concave lens of 25 cm. The power of spectacles is (a) 6∙0
D (b) 1∙5 D
(c) – 6∙0 D (d) – 1∙5 D

2. A convex lens of power +6 D is placed in contact with a concave lens of power –4
D. What will be the nature and focal length of this combination ?
(a) concave, 25 cm (b) convex, 50 cm
(c) Concave, 20 cm (d) convex, 100cm

3. A lens is formed combining two thin lenses in contact having power +12 D and – 8
D. The focal length of the combination is
(a) 25 cm (b) –25 cm
(c) 5 cm (d) – 5 cm

4. Two thin lenses of focal lengths 60 cm and –20 cm are placed in contact . The
focal length of combination:
(a) 15 cm (b) 30 cm
(c) –15 cm (d) –30 cm

5. A convex lens has a focal length of 20 cm. Its power in dioptres is:
(a) –5 D (b) –10 D
(c) +5 D (d) +10 D

6. A convex lens of power +6 D is placed in contact with a concave lens of power –
4D. What will be the nature and focal length of this combination ?
(a) convex, 100 cm (b) convex, 50 cm
(c) concave, 25 cm (d) concave, 20 cm

7. Convex lens of power 4D and a concave lens of power 3D are placed in contact.
What is the equivalent power of the combination ?
(a) 7 D (b) 4/3 D
(c) 1 D (d) None of the above

8. A convex lens has a focal length of 20 cm. Its power in dioptres is
(a) –5 D (b) – 10 D
(c) +5D (d) +10 D

9. The focal length of convex lens is 50 cm. What is its power ?
(a) +2 D (b) – 2D
(c) –50 D (d) +50 D

10. Two converging lenses of equal focal lengths f are placed in contact. the focal
length of the combination is
(a) f (b) 2f
(c) f/2 (d) 3f