Question

Two thin lenses of power +3.5D & -2.5D are placed in contact. Find the power and focal length of lens combination?

Answer 1

Answer :

Given -

• Two thin lenses
• One of them is convex lens and the other is concave
• Powers are : +3.5 D and -2.5 D
• Both are placed in contact

To Find -

• Focal length of the combination ?
• Power of the combination ?

Solution -

We know that, P = P1 + P2, in the combination of lenses.

Hence, Power of combination will be : P = 3.5 + 2.5 = 6 D.

Now, We know : P = 1/f(m).

So, Focal length = 1/P.

Hence, f = 1/6 = 0.16 m.

Final Answers :

• Focal length of the combination = 0.16 m.
• Power of the combination = 6 D.