# Optics - Angular Magnification

## Homework Statement

The eyepiece of a compound microscope has a focal length of 2.50 cm and the objective has a focal length of 1.5 cm. The two lenses are separated by 17 cm. The microscope is used by a person with normal eyes (near point at 25 cm). What is the angular magnification of the microscope?

a) 283 x
b) 145 x
c) 97 x
d) 113 x
e) 242 x

## Homework Equations

Angular Magnification = Near point / f (eye piece) = 25cm / f (eye piece)

Total magnification = 25cm(s1') / (f1f2)

## The Attempt at a Solution

At first, I just thought it was asking for total magnification and got 113x, which is wrong. I know that the angular magnification is 25 / f (eye piece) so 25/2.5? Clearly not right. Perhaps I need to use the object-image formula (1/s + 1/s' = 1/f) to derive more information, but I'm not sure how. Help would be appreciated =)

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Simon Bridge
Homework Helper
Why did you use 25cm for the near-point in the formula?
How may the other two measurements be used?

Do you know how the total magnification is related to the angular magnification?

I used 25cm because that is the normal near point, and it says it in the problem statement. The total magnification = angular magnification * lateral magnification, where lateral magnification = s1'/s1
Since I know that the total magnification is 113.333 (as calculated in my original post), I just have to divide by the lateral magnification to get the angular magnification. I don't know which measurements to use for lateral magnification though...

Simon Bridge
Homework Helper
I used 25cm because that is the normal near point, and it says it in the problem statement.
<checks> Oh so it does - well done ;)
The total magnification = angular magnification * lateral magnification, where lateral magnification = s1'/s1
Since I know that the total magnification is 113.333 (as calculated in my original post), I just have to divide by the lateral magnification to get the angular magnification. I don't know which measurements to use for lateral magnification though...
How would you normally compute the lateral magnification in a system of two lenses?

How would you normally compute the lateral magnification in a system of two lenses?
Using the object-image relation formula (1/s + 1/s' = 1/f), the image from the first lens becomes the object of the second lens. I tried doing that, but I'm not sure which lens acts as the object, and would the first object distance be 17cm?

Simon Bridge