Written by Sarah Kellett
Safety: This activity uses boiling water, children must be supervised by an adult.
First aid: If you burn yourself, run the burn under cold water for 20 minutes.
When you first drop the fluid gelatine on the lid, the shape of the droplet is due to two things – surface tension and gravity. Gravity pulls the droplet down, making it spread out a little. Surface tension holds the droplet together, it’s like the droplet has a skin holding it in. That’s because molecules from the water and gelatine tend to stick to themselves.
When you flip the lid, gravity pulls the fluid gelatine down, but surface tension still works to make the smallest possible area between the water and the air. In a drop from a tap, that shape is a sphere. On our upside-down lid, the symmetrical curved shape is called a circular paraboloid.
Comparison – the jelly lens is on the left and the polymer lens made by Steve Lee at the Australian National University is on the right.
As light moves through the jelly lens, it bends at the surface between the jelly and the air. When light hits the curved sides of the paraboloid, it bends towards the middle. This focuses light onto a small point, allowing the camera to make out fine details.
This activity was inspired by news of a polymer droplet lens made by Steve Lee at the Australian National University. As jelly isn’t completely transparent and smooth, it is not as good as this polymer droplet lens made by Steve, but it uses the same concept.
In the 17th century, Antonie van Leeuwenhoek made such incredible microscopes that we know him as the ‘father of microbiology’. He gazed at organisms in water and was the first to observe bacteria. His method of making the finest microscopes was kept secret, but some suspect that he made them by chipping away glass droplets from the bottom of a blown-glass bulb.
Glass lenses can also be made by grinding glass using fine diamond sandpaper, or pouring hot, fluid glass into a mould. Optical microscopes still use lenses to see tiny organisms, and have made huge advances in health and medicine possible. Many telescopes also use lenses to see the stars and planets. Much of the universe would remain unseen and unknown without these marvellous light-focusing lenses.
Read more about the invention of these lenses and how they’re being used in real science!
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