Page 2 - HA-power of a lens

11. Two lenses one convex and other concave of focal length 0∙5 m and 1∙0 m
respectively recombined, the power of combination will be
(a) –1∙0 D (b) +1∙0 D
(c) 0∙5 D (d) –0∙5 D

12. Two lenses of powers P1 = +2 D and P2 = –2 D respectively, are placed in
contact. the power of combination is
(a) –2 D (b) +2 D
(c) +4 D (d) None of above

13. The power of combination of two lenses separated by distance d
(a) decreases as their separation is increased
(b) increases as their separation is increased
(c) remains same
(d) becomes equal to the distance 'd'

14. Two lenses of powers 6D and –5D are in contact with each other. The focal
length of combination will be
(a) 1 cm (b) 20 cm
(c) 100 cm (d) 100/11 cm

15. Two thin lenses are in contact and the focal length of combination is 80 cm. If
focal length of one of the lenses is 20 cm, then power of the other lens is
(a) +6∙0 D (b) –3∙75 D
(c) + 4∙0 D (d) –1∙5 D

16. A convex lens of power +6 D is placed in contact with a concave lens of power –
4 D. The combination will act as a convex lens of focal length
(a) 2 cm (b) 20 cm
(c) 50 cm (d) 100 cm

Subjective Questions:

1. A lens is formed by combining two thin lenses of powers + 12 D and – 8 D in
contact with each other. What will be the focal length of combination ?

2. An optician prescribes spectacles to a patient with a combination of a convex lens
of focal length 40 cm and a concave lens of 25 cm focal length. What will be the
power of spectacles ?

3. A convex lens of power +4 D and a concave lens of power 3 D are placed in
contact. What is equivalent power of the combination ?

4. A convex lens has focal length of 20 cm. find its power in Dioptres.

5. The combined power of two lenses in contact is + 10 D. When they are separated
by 20 cm, their power becomes 6.25 D. Find the power of individual lenses.