Question

A lens of power $+2.0\text{D}$ is placed in contact with another lens of power $-1.0\text{D}$. The combination will behave like

• A. a converging lens of focal length $100\text{cm}$.
• B. a diverging lens of focal length $100\text{cm}$.
• C. a converging lens of focal length $50\text{cm}$.
• D. a diverging lens of focal length
  $50\text{cm}$.

Answer 1

The correct option is A a converging lens of focal length $100\text{cm}$.

Given $P_1=2\text{D},P_2=-1\text{D}$

For lenses in contact, net power,
$P=P_1+P_2$
$\Rightarrow P=(+2.0)+(-1.0)=+1.0\text{D}$

Using the formula, $P=\frac{1}{f\left(inm\right)}$

So, $f=\frac{1}{P}=+1\text{m}=+100\text{cm}$