You will need
  • Plastic bottle, plasticine, glycerol, or water, syringe with needle, plastic bag.
Take a-liter plastic bottle. From the top of the bottle cut out two identical, convex circle (for convenience, cut out one circle and then it cut out the second diameter).
Then, take an ordinary clay or some other sticky material. This play-DOH glue both halves of the lens so that both sides of it was convex.
After that, in two places, between the halves of the lens make holes. In one of these holes with the syringe and pour the liquid (you can pour ordinary water, but it is better to use a liquid with a large refractive index, try to fill the lens with glycerin, the effect is better), fill it to the brim and it should not be air bubbles. The second hole is for air outlet, so as not to create pressure. After filling the lens with liquid paper over the holes.
In extreme conditions, for example, being in the forest in winter, not having matches, you can make a lens for a bonfire. For this, take an ordinary plastic bag and fill it with water. Give the package a spherical shape and let the water freeze. After freezing tear the package and get the lens. The lens can also be cut from the frozen ice.