What it shows
The back of a shiny spoon is a convex mirror and always shows a small, upright, wide view. The bowl side is a concave mirror: it shows your face upside down from far away and large and upright when very close.
The science
A flat mirror reflects light rays in the same pattern they arrive in, so the image is the same size as the object. A convex mirror bulges outwards and spreads the reflected rays apart, so the image is smaller and upright but takes in a wide view; that is why they are used as rear-view and shop security mirrors. A concave mirror curves inwards and brings the rays together at a focus. An object beyond the focus gives an upside-down image, while an object closer than the focus gives a large upright image, which is why shaving and make-up mirrors are concave.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A large polished steel serving spoon
- A small plane mirror for comparison
- A pencil or a small toy to look at
- A ruler
- A sunny window or a torch
Safety
For this project: Never use a concave spoon or mirror to focus sunlight on skin or paper, and do not look at the Sun in it.
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.
Look at your face in the back of the spoon from arm’s length and describe what you see: small, right way up, with a wide view behind you.
Bring the spoon closer and notice that the image stays upright and small, only growing a little.
Turn the spoon over and look into the bowl from arm’s length; your face appears upside down.
Move the spoon slowly towards your eye and find the point where the image flips to upright and becomes large.
Measure the distance from your eye at which the flip happens; this is roughly the focal length of the spoon.
Hold a pencil in front of each side and sketch the image you see on each: size, orientation and how much background is visible.
Compare both with the flat mirror, where the image is always the same size and upright.
Make a chart with three columns for plane, convex and concave, listing what each shows and where it is used.
What you should see
The convex back of the spoon always gives a small upright wide-angle image. The concave bowl gives an inverted image from far away that flips to a large upright one close up, at a distance of a few centimetres.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which side of the spoon is a concave mirror?
The bowl side, which curves inwards.
2Why is your face upside down in the bowl?
The concave mirror brings the light rays together and they cross over before reaching your eye.
3Where are convex mirrors used?
Rear-view mirrors on vehicles and security mirrors in shops, because they show a wide view.
4Why is a shaving mirror concave?
Close up, a concave mirror gives a large upright image, so you can see detail.
Take it further
Use a concave shaving mirror to form an image of a window on a sheet of paper held at the right distance.
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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