What it shows
A drop of water on clear plastic has a curved top, like a convex lens, and makes small print look bigger. A rounder, smaller drop magnifies more. It shows how lenses in magnifying glasses, spectacles and microscopes work.
The science
Water pulls itself into a rounded drop because of surface tension, and a rounded transparent shape is a lens. Light rays from the print pass through the curved surface and are bent inwards (refracted), so they reach your eye as if they came from a bigger object. The more strongly curved the drop, the more it bends the light and the bigger the magnification, which is why a small round bead of water magnifies more than a large flat puddle. Glass magnifying lenses are shaped the same way but keep their curve.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear plastic sheet, a CD case or a piece of cling film stretched over a cup (free)
- Water and a dropper or a straw
- A newspaper with small print
- A piece of wire bent into a small loop for the extension
- A ruler
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Lay the clear plastic sheet over the newspaper print.
Put a single small drop of water on the plastic above a word and look straight down through it; the letters look bigger.
Add more water to make a wider, flatter drop and notice the magnification drops.
Lift the plastic a little way above the paper and find the height where the letters look biggest and sharpest.
Measure a letter under the drop and a letter beside it with the ruler and work out roughly how many times bigger it looks.
Try a drop of cooking oil beside the water drop and compare the magnification.
Dip the wire loop in water so a film of water fills it, and use it as a hand-held magnifier.
Draw a diagram of the drop as a lens with rays bending inwards towards your eye.
What you should see
Print under the drop looks two to three times bigger. Small, rounded drops magnify more than large flat ones, and the wire loop makes a magnifier you can hold.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What shape is the water drop, as a lens?
Convex: thicker in the middle than at the edges.
2Why does the drop magnify?
Its curved surface bends light rays inwards so the object appears bigger.
3Which drop magnifies more, a small round one or a wide flat one?
The small round one, because it is more strongly curved.
4Where are convex lenses used?
In magnifying glasses, cameras, microscopes and spectacles for long sight.
Take it further
Make a microscope with a water drop over a pinhole in a card and look at an onion skin or a hair.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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