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## Homework Statement

An optical system consists of a thin lens in air with f = 100 mm. An aperture stop with a diameter of 20 mm is placed 25 mm to the right of the lens. Calculate the numerical aperture of the system. (Hint: Find the size of the entrance pupil.) What would be the numerical aperture if the aperture stop were placed 25 mm to the left of the lens?

## Homework Equations

NA = h_e/f

h_e is the height of the entrance pupil

## The Attempt at a Solution

I've drawn a picture, but I can't figure out how to find h_e. (The hint was rather useless given h_e appears in the relevant equation!) I know the entrance pupil is the image of the aperture stop in the object space (so you can actually see it?). Help is appreciated.