# Two lenses. Find the image.

## Homework Statement

Two lenses with the focal length of 50 mm are placed tightly. An object is placed 130 mm away from the lenses. Where is the image generated by the lenses?

## Homework Equations

$$\frac{1}{a}+\frac{1}{b}=\frac{1}{f}$$
$$L = f_{ocular} + f_{objective}$$

## The Attempt at a Solution

[/B]
Frankly, I tried starting out by simply using the first equation. I got b to around 81 mm to the right.

I don't know if it's correct. The question never specifies which type of lens (convex or concave). Doesn't specify which kind of optical setup (meaning I don't know if to use the second equation). I thought about dividing the problem with respect to each lens but I don't know the kind of lenses used so I have no clue.

kuruman
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## Homework Statement

Two lenses with the focal length of 50 mm are placed tightly. An object is placed 130 mm away from the lenses. Where is the image generated by the lenses?

## Homework Equations

$$\frac{1}{a}+\frac{1}{b}=\frac{1}{f}$$
$$L = f_{ocular} + f_{objective}$$

## The Attempt at a Solution

[/B]
Frankly, I tried starting out by simply using the first equation. I got b to around 81 mm to the right.

I don't know if it's correct. The question never specifies which type of lens (convex or concave). Doesn't specify which kind of optical setup (meaning I don't know if to use the second equation). I thought about dividing the problem with respect to each lens but I don't know the kind of lenses used so I have no clue.
You have solved half the problem and found the image formed by the first lens. This image serves as an "object" for the second lens so you need to apply the equation once more. Be careful with your signs. The kind of lenses these are should be obvious from their focal lengths, which are positive. See link below for how to treat multiple lens systems.

Two thin lenses in contact are equivalent to a single thin lens whose focal length is given by 1/f = 1/f1 + 1/f2. (Serway). Then 1/i = 1/f - 1/o as usual.