Question

To focus parallel light rays to the smallest possible spot, should you use a lens with a small f-number or a large f-number? Explain.

Short answer

smaller f-numbered lens

Step-by-step solution

Step1. Given Information 

We have given that , parallel light rays focus to the smallest possible spot.

simply

The minimum Width is given by,${\omega }_{min}\approx \frac{2.44\lambda f}{D}$

Also f-number=,hence$\Rightarrow {\omega }_{min}\approx 2.44\lambda \times (f-number)$

Hence smaller f-number is used.