# [Q] Do you understand what is numerical aperture and f number?

1. Apr 7, 2009

### good_phy

Hi, I studied concept of numerical aperture and f number

I learned numerical aperture(N.A) is measure of how much light can be entered into the

optical system, its mathematical expression is N.A = $$nsin\theta_m$$ where n is

index of refraction of immersing medium surrounding lens and $$\theta_m$$ is

maximum angle between optical axis and rim of lens.

I got a question. What is the point from which two line(one is from the point to rim of lens,

another is from the point along optical axis to lens) make maximum angle?

I think maximum angle is varying as the point moves along axis, So it is not clear for me.

And why can more light enter the optical system if n increases, causes N.A increase.