Question

18. Two lenses of power -1.75D and +2.75D are placed in contact. The focal length of the
combination is
(a) 50cm
(b)100cm
(c) 75cm
(d) 125cm​

Answer 1

Given :-

• P 1 = - 1.75 D

• P 2 = + 2.75 D

To find :-

• The focal length of the combination (f )

Solution :

Equivalent power of the lens is given by the formula ,

⟹ P = P 1 + P 2

⟹ P = - 1.75 + 2.75

⟹ P = 1 m

we know that ,

⟹ P = 1 / f (m)

On rearranging ,

⟹ f = 1 / 1

⟹ f = 1 m

In order to convert m into cm simply multiply by 100

⟹ f = 1 x 100

⟹ f = 100 cm

∴ The  focal length of the  combination is 100 cm