# Homework Help: Eyeglasses Problem

1. Oct 6, 2006

### Alt+F4

Lenses Problem

In the figure are shown two converging lenses with x2 = 1 m. The lens on the left (Lens 1) has a focal length of 0.25 m while the one on the right (Lens 2) has a focal length of 0.2 m. An object 3.9 cm high is located x1 = 0.397 m to the left of Lens 1.

http://img153.imageshack.us/img153/123/showmeal3.gif [Broken]

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(a) Consider the image of the object formed by Lens 1. Give its distance as measured from Lens 1 and the image height.
Distance = m

SO i was trying to use the thins line equation which is 1/ o + 1/f = 1/I but i dont know what numbers to plug in

Last edited by a moderator: May 2, 2017
2. Oct 6, 2006

### OlderDan

Drawing the ray diagram for this system of lenses would be very useful. The first lens will form an image. You can use the lens equation to help you locate it. Then think about what the second lense does to the light coming from (or going toward) the image of the first lens.

Last edited: Oct 6, 2006
3. Oct 7, 2006

### Alt+F4

i have no idea, can i get more help

4. Oct 8, 2006

### OlderDan

Do the problem as if the lens on the right were not there to locate the image formed by the first lens. It will be between the two lenses. When you know the position of the image, you can also find the height of the image. Use your thin lens formulas.

As far as the second lens knows, the light coming to it from the image of the first lens is light from some object. So treat the image from the first lens as an object for the second lens and do a new problem to find the image formed by the second lens.

Check the lens equation you originally posted. You have some terms mixed up.

Last edited: Oct 9, 2006
5. Oct 9, 2006

### OlderDan

Here is a diagram that might help you through this. For your part a) you only need the first lens. I assume later parts talk about the second lens. Note that the size of the original image is much larger than scale, which is why the lines are so far off axis. The blue rays are actual paths light could take. The red line through the second lens is added to help locate the final image.

All you need to do is apply the lens equation once to find the image between the lenses, and then a second time to find the final image. The image sizes are determined by the ratio of image distance to object distance.

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Last edited: Oct 9, 2006