large cheap convex lens

[jnb]

I'm looking for a large cheap convex lens for demonstrations. Quality really doesn't matter. Currently I use a large magnifying glass but that is only 100mm and perhaps 500 mm focal length. Does anyone know where I could get something larger without spending much?

[pete_l]

fresnel?

[saac]

Try surplus shed. American supplier of various optics and associated parts - very reliable.

Jim

[Stub Mandrel]

I bought a 100mm acrylic lens from Astromedia, for a few quid, am going to use it in a bench magnifier.

Looks like they may have sold out now but you could ask.

http://www.astromediashop.co.uk/Components.html

 

[Peter Drew]

I have a collection of large lenses, if you could be more specific with your requirement I could make a judgement as to whether any of them would be suitable.

[Astrobits]

If all else fails you could make your own. When I was starting to make mirrors I looked for cheaper tools other than the usual thick glass. One option that I tried was to use thin (10mm thick ) float glass from the local glass suppliers. This had to be mounted onto a plaster tool to prevent flexing and give height for the working surface. When I had finished I then polished the ground surface ( for no particular reason, I just wanted to ). This has left me with a 200mm dia. lens of approx 1400mm focal length that I find useful for a few jobs. It's not brilliant as there are some scratches on the raw flat surface from the original sheet and I didn't bother to check the figure but it does what I want. If you don't/can't grind it yourself perhaps a local mirror making group will oblige.

Nigel

 

[Merlin66]

Surplus Shed currently have 1000mm focal length singlet lenses available.

 

[jnb]

13 hours ago, Astrobits said:

If all else fails you could make your own. When I was starting to make mirrors I looked for cheaper tools other than the usual thick glass. One option that I tried was to use thin (10mm thick ) float glass from the local glass suppliers. This had to be mounted onto a plaster tool to prevent flexing and give height for the working surface. When I had finished I then polished the ground surface ( for no particular reason, I just wanted to ). This has left me with a 200mm dia. lens of approx 1400mm focal length that I find useful for a few jobs. It's not brilliant as there are some scratches on the raw flat surface from the original sheet and I didn't bother to check the figure but it does what I want. If you don't/can't grind it yourself perhaps a local mirror making group will oblige.

Nigel

 

Roughly how long did that take you to do?

What I want is one or more large lenses to demonstrate creating an inverted image on a screen in one of our observatory domes. I'm usually showing this to 30 or so people but with a magnifying glass it's only really practical for a few people to see at a time.

 

[Astrobits]

It didn't take too long but this was over 20 years ago. An experienced mirror maker can make a finished 6" mirror within a day and that includes figuring to a parabola. Polishing the tool for use as a lens would be just as easy and without the figuring stage, somewhat quicker.  

What space have you to do this demo? That will define the limit on focal length of your lens.

Can you not move the object nearer to the lens you have and produce a larger image at a greater distance? It doesn't change the principle you are demonstrating. With the object at twice the focal length from your lens the image will also be at twice the focal length away and at 1:1 size ratio.  That will consume 2000mm of your available space ( with a lens of f= 500mm ). Move the object nearer the lens and the image appears even further away and larger ( with the object at the focal length the image will be at infinity ). Use the lens formula to work out the distances:   1/u +1/v = 1/f        u= object distance     v= image distance     f= lens focal length

Nigel

[Stub Mandrel]

Nigel is right, if the lens is close to the object and the object is brightly lit it will work. Think of a projector lens.

[StuartJPP]

I may have a projector lens kicking about somewhere...I will see if I can find it.

[jnb]

On 5/27/2017 at 10:16, Astrobits said:

It didn't take too long but this was over 20 years ago. An experienced mirror maker can make a finished 6" mirror within a day and that includes figuring to a parabola. Polishing the tool for use as a lens would be just as easy and without the figuring stage, somewhat quicker.  

What space have you to do this demo? That will define the limit on focal length of your lens.

Can you not move the object nearer to the lens you have and produce a larger image at a greater distance? It doesn't change the principle you are demonstrating. With the object at twice the focal length from your lens the image will also be at twice the focal length away and at 1:1 size ratio.  That will consume 2000mm of your available space ( with a lens of f= 500mm ). Move the object nearer the lens and the image appears even further away and larger ( with the object at the focal length the image will be at infinity ). Use the lens formula to work out the distances:   1/u +1/v = 1/f        u= object distance     v= image distance     f= lens focal length

Nigel

For practicality and scale of the demonstration the object I use is the doorway to the observatory. That gives an object distance of about 5m so a long focal length is needed for the lens.

 

[Merlin66]

Just a reminder the singlet lenses from Surplus Shed also come in 2500mm focal length (we were looking at this one for a spectrohelioscope application)

 

[Astrobits]

42 minutes ago, jnb said:

For practicality and scale of the demonstration the object I use is the doorway to the observatory. That gives an object distance of about 5m so a long focal length is needed for the lens.

 

You can still use the lens formula to determine the three parameters given your space available.  If your object to lens distance has to be 5M you can determine the image distance for various focal lengths of lens.  eg. if you use a lens of 2500mm focal length then the image will be 5M from the lens and be the same size as the object. Is your observatory 10M long? Use the formula to find the optimum positioning and focal length of your lens.

The classical object to show lens  "rules" is a candle. Not suitable?

Nigel