A litre round bottom flask is best used for this. Some people recommend a 5 litre one but in our experience not only is it difficult to find a lens which sufficiently focuses the light beam onto the back of the ‘eye’ but also there is so much liquid in these that visibility of the beam can be a problem.
The liquid used to fill the 'eye' can be anything that is opaque enough so you can see a beam through it. Whatever you use needs to be a solution. Lemon juice and water is popular but over time the lemon settles at the bottom. A small amount of food colouring may suffice. If the solution does separate or settle, a gentle shake will remix the liquids making it usable. A small torch with a narrow beam is required to provide the light beam.
Attach a small convex lens to one side using blu-tak or plasticine and have a selection of lenses available to show short or long sightedness. Additional lenses can be used to demonstrate how glasses or contact lenses compensate for these optical problems (see diagram).
If the torch beam is not clearly visible, making a card aperture to place in front of the lens and pointing a slide projector at it gives good results. The power of the beam is strong enough to clearly see the focused effect. Slide projectors are very good at providing a bright enough beam.