Make something from two planoconvex lenses?

November 26, 2018 1:27 PM

Can I make something (telescope, microscope, whatever) from the two 4-1/2" x 9" planoconvex lenses in this old Q4M spotlight?

The lenses are each like 10cm across, and it seems a shame to throw them away. They will be fine as amusements, but I would much prefer to make something with them, and learn something along the way.

Here's a picture of the light in question: thread.

I have been reading about Keplerian telescopes, and have built a Galileoscope myself some years ago. But those seem to need a convex lens at one end. Are there any devices that can use this pair of planoconvex lenses?

posted by wenestvedt to Science & Nature (6 answers total)

To first order, planoconvex lenses are equivalent to convex lenses: they take a bundle of parallel rays and focus them down to a point a certain distance away. That distance is called the focal length of the lens. You can get an estimate of the focal length of your lenses by putting a sheet of white paper on a table, turning on a light source directly above it, and then moving the lens up and down (with its plane parallel to the table) until you get a clear image of the light source. (Apologies if this is all basic, but I wasn't quite sure where to start.)

The magnification of a Keplerian telescope is the ratio of the focal lengths of the two lenses. (You want the lens with the shorter focal length to be closer to your eye.) This means that building a telescope out of two identical lenses wouldn't do anything except turn the image upside down. You could always still build a Keplerian telescope out of the lenses, though, and you'd probably learn something along the way; so if that's your only goal, I'd encourage you to do it anyhow.

If the focal lengths are short (less than a few inches), it might be possible to build a compound microscope out of them. But I assume by ' 4 1/2" x 9" ' that you mean that these have a 9" focal length; and given the pictures, I wouldn't expect their focal lengths to be terribly small. (I'm mentally using the lensmaker's equation to estimate the focal lengths.)

There might be other things you can do with them, though; I'll have to ponder this some more.
posted by Johnny Assay at 2:28 PM on November 26, 2018 [5 favorites]

Sounds like fun! The classic application, of course, is using sunlight to set things on fire. It gets old after a minute or so if you're not nine years old. But, if you know any nine year olds, it's a neat party trick.

One idea would be to make a shadow projector that takes a silhouette cutout or a colored gel and projects it onto distant walls. (Look up the optical arrangement of old school overhead transparency projectors to get started.) You'll probably spend at least as much on a bright light source and mounting hardware as the lenses are worth, but it could be an excuse to make a fun thing and play with some optics. If you wanted to go nuts with it, you could replace the silhouette with an LCD screen and make a digital version.

For tinkering around with geometric optics ideas, this online simulator is great, though the user interface is a little bit clunky.
posted by eotvos at 3:00 PM on November 26, 2018 [3 favorites]

I have not fully internalized the lensmakers equation, but I think you might be able to use these as focal pieces for a pair of eye stilts.

I made a temporary pair once with only mirrors mounted to a balcony overlooking San Francisco and it was great, even though our ad-hoc system barely worked.
posted by SaltySalticid at 4:54 PM on November 26, 2018

Response by poster: The classic application, of course, is using sunlight to set things on fire

OOOH, YEEEAAAHHHH

ObDisc: was nine year-old boy, some years ago.
posted by wenestvedt at 5:36 PM on November 26, 2018 [1 favorite]

Response by poster: I have not fully internalized the lensmakers equation, but I think you might be able to use these as focal pieces for a pair of eye stilts.

Luckily I also have a tall stack of mirrors, 3" on a side, and this would be a fan-freakin'-tastic project for Christmas vacation. Thank you!!!

(My workbench, let me show it to you! It's...over here...somewhere...behind this pile and these...things...and...)
posted by wenestvedt at 5:38 PM on November 26, 2018 [4 favorites]

What about a kaleidoscope? I have no idea what lenses might be needed so it might not be a project for you. But I spent a day volunteering at the children's zoo last year and my assignment was to monitor the giant kaleidoscope so the kids wouldn't jump on it too much. OMG they loved putting their whole face into the front and looking at the patterns as they turned the wheels.
posted by CathyG at 8:57 AM on November 27, 2018 [1 favorite]