# Help Calculating focal length for magnifying glass

1. Dec 30, 2011

### Stomper123

So I bought a magnifying glass for a project and I need to find out the focal length of it in order to use it. I'm given the magnification of the converging lens, which is 5, but I don't any methods or how I could find out the focal length given only the magnification. I attempted to set distance object that I actually measured and put it into the magnification formula and then took the distance of the image and put it into the focal length formula. However, when I tested it out it didn't seem quite right. Are there anyways I could do this mathematically? If not how else could I do this?

Last edited: Dec 30, 2011
2. Dec 30, 2011

### Staff: Mentor

Focal length can be calculated knowing the radii of curvature of both lens surfaces. But radius of curvature is a difficult parameter to measure. If you focus the sun to a spot on paper, the distance from the centre of the lens to the burning paper is the focal length. Centre of the lens being a point inside the glass, roughly midway between the two lens surfaces.

In the absence of sun, try reading a newspaper with the lens. The distance from the page to the centre of the lens where you judge print to be sharpest is the focal length.

3. Dec 30, 2011

### Staff: Mentor

Does this second method really work? I've used the sun method, and also focused ceiling lights on the floor and measured the two distances (to use the thin lens equation), but it seems like if you focus the image on your eye, your eye lens enters into things...?

4. Dec 30, 2011

### Stomper123

would a lamp work?

5. Dec 30, 2011

### Staff: Mentor

The point is that you want some bright object that is very far away, so you don't need to measure the distance to that light source. Depending on the accuracy you want for your focul distance number, you could use a streetlight across the street, probably.

Are you familiar with the thin lense formula, which relates the focul distance to the object and image distances?

6. Dec 30, 2011

### Staff: Mentor

7. Dec 30, 2011

### Staff: Mentor

I think I see how this works now. The rays heading to your eye are parallel, so as long as you focus your eyes on infinity, the lens-newspaper distance is the focul length. Interesting (but possible to make a small error if you aren't used to focusing your eyes on infinity...)

8. Dec 30, 2011

### Stomper123

Oh okay
I am familiar with the thin lense equation
Thanks! I'll try this out

9. Dec 30, 2011

### Stomper123

I'm just wondering. How would you focus your eyes on infinity :O?

10. Dec 30, 2011

### Staff: Mentor

As long as you are not nearsighted or farsighted, you just relax your vision. The relaxed state of your eye lens muscles lets the lens go to its thinnest shape, which focuses the eyes on infinity. That's what happens when you look a long way away at something. As you look closer, the eye lense muscles squish the lens some, which makes it thicker in the middle (higher power), to focus the closer image on the retina.

So as a practical matter with this newspaper technique, you could look past the newspaper at something a long ways away, and then try to keep that same relaxed focus as you change your gaze to the newspaper as seen through the magnifying lens.

Try several of the methods we've talked about in this thread, and see what values of focal length you get. Kind of fun experiments...

11. Dec 31, 2011

### Staff: Mentor

If you hold the lens against your eye like a monocle then it probably won't work. But the right way to use a magnifying glass is to hold it away from you at a distance well beyond focal length, so the rays leaving it are parallel. Then it doesn't make any difference whether you move your eyes nearer to, or further back from, the lens, the image remains in focus.

I think that's how it works, anyway.