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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.

You will need

• Gelatine powder (plain, no flavouring)
• Boiling water
• Flat plastic lids
• A glass or container
• Teaspoon
• Small bowl

  

• Measuring spoons

What to do

1. Measure two teaspoons of boiling water into the small bowl and sprinkle a quarter of a teaspoon of gelatine powder on top.
2. Stir for four minutes, until the gelatine becomes thicker. You don’t want it to set, just to thicken so it slowly drips off the spoon.
3. Using the spoon, place a small drop on the plastic lid.
4. After five seconds, turn the lid over and balance it on the jar so that the drop is upside down. Gravity will pull the drop into a parabola – but it shouldn’t drip off the lid, if that happens, you need your jelly to be thicker.
5. Try placing other drops on different materials with flat surfaces – like the top of the gelatine container, a jam jar lid, a ruler or glass – whatever you have around.
   Turn each upside down. This way you can pick which works best.



6. Let the jelly set for 15 minutes.
7. Check all your tests and pick which one came out roundest.
8. Gently lift the jelly drop off the lid, pushing upwards with your finger.
9. Place the drop over the lens of a smartphone. If it’s not your phone, ask permission from the owner first!
10. Try taking close-up photos through your jelly lens. You need to get really close to what you’re focusing on for it to work, so the lens is almost touching the object.
11. When you’re done, remove the jelly lens and wipe the phone camera clean. Jelly contains water, so you don’t want to leave it too long on your camera.

    

What’s happening?

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.

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.

Real-life science

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!

If you’re after more science activities for kids, subscribe to Double Helix magazine!