What it shows
A tiny drop of water held in a hole acts like a strong magnifying lens, and with it the brick-like cells of an onion peel can be seen. It shows that all living things are made of cells.
The science
A drop of water has a curved surface and bends light inward like a convex lens. The smaller and rounder the drop, the shorter its focal length and the larger the magnification, which can reach about 30 to 50 times for a very small drop. The onion peel is one cell thick and transparent, so light passes through and shows the cell walls as a pattern of rectangles. Staining makes the walls and the nucleus easier to see.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A strip of stiff card or plastic 15 cm by 3 cm
- A thin plastic sheet from a packet, about 3 cm square
- A pin and a small straw for placing the drop
- A fresh onion
- A torch or window light
- Two glass slides or a clear plastic slip
- A drop of diluted tea or ink for staining
Safety
For this project: An adult must cut the onion and help with the pin. Do not look at the sun through the lens, and keep the stain away from your eyes.
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.
Make a hole of about 3 mm in the middle of the card strip, a bit above one end, with a pencil point.
Place a drop of water in the hole using the straw or a pin so that it stays rounded on top. Making the drop small gives a stronger lens.
Peel the thin transparent skin from the inner side of an onion scale with the help of an adult, and spread it flat on a slide or a clear plastic slip.
Add a little diluted tea on the peel and wait a minute, then rinse off the extra stain with water.
Place the slide under the card on a table near a window and put your eye very close to the drop.
Move the card up and down slowly until the picture becomes sharp. Make the water drop rounder if it is flat.
Draw the cells you see and count the number of cells that fit in the width of the field.
Label the cell wall, cytoplasm and nucleus in your drawing, and write that no chloroplasts are present in the onion.
What you should see
The rectangular cells of the onion peel appear lined up like bricks, each with a thin wall and a dark spot of the nucleus, much larger than in the naked-eye view.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How does a drop of water magnify?
Its curved surface bends light like a convex lens, so the object looks larger.
2Why is the onion peel thin?
Light must pass through it, and one layer of cells is clear enough to see through.
3Why does a smaller drop magnify more?
A smaller, rounder drop has a shorter focal length.
4Who first saw cells and in what?
Robert Hooke in 1665, in thin slices of cork.
Take it further
Compare onion cells with cells from a cheek swab seen under a school microscope and note the differences.
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.
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