How can I build an optics bench for a science exhibit?

[nebulinda]

I am trying to build an optics bench, similar to one you might find in a high school physics classroom. This is a prototype for a science center exhibit. Now, it's been a while since I took optics, so I'm feeling a little overwhelmed. I know what I want my result to be, but I'm having trouble figuring out what I need to start with.

I want my optics bench to be abut 10 feet long, and any resulting image to range from a few millimeters to about 3.5 feet (all these numbers are flexible). People will be able to play around, put the lenses (5 or 6 of them), the image source, and possibly the screen where ever they want.

So my questions are these:
1. What focal lengths (for a biconvex lens) will result in images of the right sizes, but also use most of the length of the table (most of the spare lenses I've been able to play around with have to be extremely close to the source to get a focused image at the right distance)?

2. What exactly is focal length? How does it relate to the location of the source and the image? I know all the equations, but I'm having trouble wrapping my brain around the concept of focal length.

3. Not physics, but maybe you have suggestions: what do you think is the best light source or method of generating an image? I was trying a projector with a picture drawn on a clear piece of plastic, and the projector is too bright, and kind of unfeasible to use anyway. Next I tried an LED flashlight, which isn't quite bright enough, especially for using in a well lit room.

4. Am I right in thinking the diameter of the lens doesn't affect the focal length? By how much does the thickness affect it? If I use the thin lens equation for my calculations, will I get answers that are way off, or is that okay, since I don't need my answers to be really precise? Does it make much of a difference if I use glass or plastic lenses?

5. Is there anything you can think of that I haven't considered? What would you do if you were building an optics bench from scratch?

Thanks so much for any help!

 

[schip666!]

There's some nice pictures on the wiki page:
http://en.wikipedia.org/wiki/Lens_(optics)

The focal length of a positive lens is the distance where an object at infinity will produce a focused image. Think of your days burning ants with a magnifier. The sun is effectively at infinity, so it focused to a point when the lens was held at it's focal length away from the poor little innocent ant.

A nice rule-o-thumb is that a 1::1 object::image setup has the lens at 2 times it's focal length from both, so you need 4 times the lens' focal length to make it work.

If your projector is too bright, put an iris just in front of the lens...a piece of cardboard with a small hole in it. Which brings up a good point, you should have a set of iris holes that you can put in various places on your bench for experiments...

For the most part the lens thickness doesn't have a big effect, although really-thick may mean that you have to measure from different non-center points on each side of the lens. Glass or plastic is a non-issue, they all work the same for visible light.

Also the image screen should be something translucent so you can see the results from either side. You'll probably get a better result by looking through the thing anyway.