Question

The power lens of combination is 4d.if convex lens has power 5d,the focal length of concave lens is
(A)100cm
(B)-100cm
(C)-1cm
(D)-100/9cm

Answer 1

Explanation:

(a) Power of convex lens P1 = + 5 D

Power of concave lens P2 = - 7.5 D

The combined power is the algebraic sum of the individual powers of the lenses.

∴ Power of the combination P = P1 + P2

P = 5 D - 7.5 D = - 2.5 D

Therefore, the power of the combination of the lenses is - 2.5 D.

(b) The power and focal length are related as:

p

=

1

f

p=1f

−

2.5

=

1

f

-2.5=1f

∴ f=

−

1

2.5

=

−

0.4

m

=

−

40

c

m

-12.5=-0.4m=-40cm

Therefore, the focal length of the combination of the lenses is 40 cm. The negative sign shows that the combination of the two lenses causes it to behave like a concave lens.