Question

15. A concave lens of focal lërigth 30 cm and a convex lens of focal
length 20 cm are placed in combination. Calculate the focal length
of combination.​

Answer 1

Explanation:

Equivalent focal length,

F

1

=

f

1

1

+

f

2

1

=

20

1

+

30

1

∴F=

20+30

20×30

=

50

600

=12 cm

Answer 2

Answer :

Focal length of concave lens = 30cm

Focal length of convex lens = 20cm

Both are placed in contact.

We have to find focal length of combination$.$

_________________________________

◈ When two lenses of focal length f₁ and f₂ are placed in contact with each other then focal length of combination is given by

• 1/f = 1/f₁ + 1/f₂

Focal length of concave lens is taken negative and that for convex lens is taken positive.

➝ 1/f = 1/f₁ + 1/f₂

➝ 1/f = 1/(-30) + 1/20

➝ 1/f = -2+3/60

➝ 1/f = 1/60

➝ f = 60cm

Hence, combination will behave as convex lens.