In this post, we will learn how to find the equivalent focal length or power of the two thin lenses placed in contact with each other.

By Applying Lens formula in 1st lens,

-------------------------(1)

And by Applying Lens formula in 2nd lens,

-------------------------(2)

Now adding these two lens equations, we get

-------------------------(3)

If "

**Feq"****is the focal length of the equivalent lens, then**
-------------------------(4)

Therefore by eq (3) & (4) we get

Also, we know that the power of a lens is inversely proportional to its focal length.

Therefore,

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