# A convex and a concave lens seperated by distance d are then put in contact.What is the focal length of combination?

Grade:12

## 2 Answers

Saurabh Koranglekar
askIITians Faculty 10335 Points
4 years ago
Dear student

The answer is decrease

Regards
Vikas TU
14149 Points
4 years ago

you have a combination of concave and convex lens both have a focal length of 10 cm .

From sign convention:-

Convex lens has focal length of + 10cm or 0.1m.

Whereas,

Concave lens has a focal length of -10cm or -0.1m.

If you have a combination of lens, then you can calculate net power or focal length of combination by the formula :-

(I/net focal length) = (1/f1)+(1/f2)+…..

Here after putting values of f1=0.1m and f2= -0.1m ,

Answer will be 0.

Therefore, (1/net focal length) =0

Thus, net focal length= infinite

Thus it will behave as a plane mirror with focal length infinity.

But , the answer will be different if convex lens and concave length both has different refractive index.

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