# Focal length calculations for a double convex lens?

1. Jan 6, 2012

### arcanebop

From what I recall, determining the focal length of a double convex lens just involves an infinite light source and holding the lens close to an image-capturing surface such as a wall and adjusting the distance until the image is at its sharpest...the distance from the lens to the surface is the focal length.

^ Correct me if I'm wrong, I want to double-check ^

What I want to know is, how does one show the calculation/determination of this, as is required by an optics project in school? Do I just draw a ray diagram for the lens, all the necessary light rays converging on a point X distance from the lens? Maybe draw it to scale?

Is that all, or is there some kind of formula that must be shown, such as the Thin Lens Equation (which doesn't make sense to me because one would have to use infinity as DO, or object distance)?

Please clarify these things for me :\$

P.S. I apologize for not using the 3 guidelines to stating a question, I didn't think they applied to my question well...and I searched the forum briefly but didn't find a repeat of my question.

2. Jan 6, 2012

### rollcast

If you can measure it accurately enough or can gain the information you could use the lensmakers equation.

3. Jan 6, 2012

### arcanebop

Hmm...I've heard of that + looked a bit into it, but it wasn't a method taught in class so I doubt it'll be accepted for the project...furthermore, I don't have measurements for the required factors of R1 and R2.

Just to clarify, I already have the focal lengths of both double convex lenses that I happen to be using for the construction of a refracting telescope. To calculate the focal length within the telescope, I simply have to subtract the eyepiece focal length from the objective focal length, is that right? In my case;

f(e) - f(o) = f(t), where f = focal length, e = eyepiece, o = objective, t = telescope
30cm-5cm=25cm ?

I think drawing a ray diagram to scale and stating the above equation satisfies the conditions of the project, but I just want to confirm that what I am doing is right. Kind of unsure if subtracting the two focal lengths is the right equation.

4. Jan 6, 2012

### arcanebop

I just thought of something else, would it be a viable option to substitute the value of object distance as something radically far-off, such as 50m, which is 5000cm? Apply this to the thin lens equation for focal length for one of the lenses, and I would get almost the exact value of the actual focal length. Can I just round it to the closest whole number value, thus proving the focal length?