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**Mirror equation help! please urgent!**

## Homework Statement

The distance from the lens of someone's eye to the retina is 20.0 mm. if the image of a book held 40.0 cm is in front of the eye in sharp focus, what is the effective focal length of the lense.

the back of the book says the answer is 19 mm.

thanks for your help everyone!

## Homework Equations

1/f = 1/ do + 1/ di