Question

when two converging lens of same focal length f are placed in contact then the focal length of the combination is​

Answer 1

Answer:

f/2

Explanation:

We know that the combined power of a combination of a number of lenses is given by simple algebraic addition. If it is the power of a concave lens, it is -ve. If it is the power of a convex lens, the focal length is +ve.

Given focal length = f

So, power of each lens is 1/f.

By simple algebraic addition, we get

Power of combination of lenses

$= \dfrac{1}{f} + \dfrac{1}{f}\\\\\\= \dfrac{1+1}{f}\\\\\\= \bold{\dfrac{2}{f}}$

We know that P = 1/f

So,

focal length of the combination is f/2.

Answer 2

Answer:

Answer:

f/2

Explanation:

We know that the combined power of a combination of a number of lenses is given by simple algebraic addition. If it is the power of a concave lens, it is -ve. If it is the power of a convex lens, the focal length is +ve.

Given focal length = f

So, power of each lens is 1/f.

By simple algebraic addition, we get

Power of combination of lenses

\begin{gathered}= \dfrac{1}{f} + \dfrac{1}{f}\\\\\\= \dfrac{1+1}{f}\\\\\\= \bold{\dfrac{2}{f}}\end{gathered}

=

f

1

+

f

1

=

f

1+1

=

f

2

We know that P = 1/f

So,

focal length of the combination is f/2.