Question

A biconvex lens a focal of 10 cm.It is cut in half and two piece .The focal length of the final combination is:

• A. 10
• B. 20
• C. 40
• D. not a lens

Answer 1

The correct option is A 10

$\text{According to question we don't know how cutting is made}$.

$\begin{array}{c}\text{If it is cut in this manner then}\\ \text{there will be no change in focal}\\ \text{length i.e. it will remain 10 cm.}\end{array}$

$\begin{array}{c}\text{Because other surface}\\ \text{will behave like}\\ \text{plane mirror and}\\ \text{focal length of}\\ \text{plane mirror is}\\ \text{infinity}\infty \text{.}\end{array}$