Question

A lens of power $6D$ is put in contact with a lens of power $-4D$. The combination will behave like a:

• A. Divergent lens of focal length $25cm$
• B. Convergent lens of focal length $50cm$
• C. Divergent lens of focal length $20cm$
• D. Convergent lens of focal length $100cm$

Answer 1

The correct option is B Convergent lens of focal length $50cm$

For a combination of lenses, net power of the combination is given by sum of individual powers i.e.

Pnet = $P_1+P_2$

According to question

Pnet = 6D - 4D = +2D

hence it will be convergent lens of focal length = $\frac{100}{2}cm=50cm$