The science idea
The pinhole camera, or camera obscura (Latin for “dark chamber”), is the oldest camera. Scholars were using dark rooms with a small hole to watch the Sun and to trace scenes more than a thousand years ago, and every modern camera is a descendant of it. It has no lens, no battery and no moving parts, and yet it makes a real picture.
The science is the rectilinear propagation of light: in a uniform medium such as air, light travels in straight lines. Every point of a bright object (a tree, a window, a building) sends light out in all directions. The pinhole lets through only a very thin pencil of rays from each point, and each of these rays continues in a straight line to the screen. A ray from the top of the object passes through the hole and goes downwards, landing on the lower part of the screen. A ray from the bottom goes upwards. Left and right are swapped in the same way. So the picture on the screen is upside down and also reversed left to right, which is the same as the scene turned through 180 degrees.
Because the picture is made by straight lines through one point, you can predict its size with similar triangles. If the object is H tall and is a distance d from the pinhole, and the screen is a distance f behind the pinhole (the depth of the box), then image height h divided by object height H equals f divided by d. A deeper box makes a bigger, dimmer image; moving closer to the object also makes the image bigger. In this project you will test that rule with your own measurements, which is what turns it from a toy into a physics investigation.
The size of the hole is a compromise. A bigger hole lets in more light, but each point of the object then makes a small circle of light instead of a tiny dot, so the picture is brighter but blurry. A smaller hole gives a sharper picture but a dimmer one, and if it is made too small the light bends around its edge (diffraction) and blurs the image again. For a box 20 to 30 cm deep the best hole is about half a millimetre across, about the size made by pushing the point of a sewing needle through foil.
You have already seen a pinhole camera outdoors: the round bright spots of light under a tree on a sunny day are not holes shaped like circles. Each gap between leaves is a pinhole, and each spot is a round picture of the Sun. During a partial solar eclipse these spots turn into crescents for the same reason.
Materials and tools
Cost: Free
A shoe box, kitchen foil, butter paper and a sewing needle are usually in the house. If you have to buy tracing paper or foil, it costs about Rs 10 to Rs 20.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Box with a lid, at least 20 cm deep (shoe box, a long sweets box or tea box) | 1 | Any cardboard box that closes. It must be strong enough to hold its shape. |
| Black paper or black chart paper for the lining | Enough to cover the inside | Black paper from an old photo album or a black polythene bag cut open. Colour the inside with a black marker if nothing else is available. |
| Kitchen aluminium foil | A piece about 5 cm × 5 cm, and a second one for a test | Foil from a roti wrapper or a medicine strip (use the shiny aluminium side). |
| Tracing paper, butter paper or baking paper for the screen | 1 piece, about 15 cm × 10 cm | The greaseproof paper from a bakery box, or a thin white page rubbed with a little cooking oil. |
| Sewing needle (and a larger nail tip for a test hole) | 1 needle | From the household sewing kit. |
| Black tape or masking tape painted black | 1 roll | Plain tape is fine if you colour the visible strips black with a marker. |
| A dark cloth, towel or dupatta | 1 | From the house. It keeps room light off the screen. |
| Ruler and a long measuring tape | 1 each | A ruler and a piece of string marked in centimetres. |
Tools
- Scissors
- A craft cutter for the windows (an adult cuts the cardboard)
- Pencil and ruler
- A soft pad of newspaper or an eraser to press the needle into
- Glue or tape
Safety first
Read this before you start
- Never look straight at the Sun, not even through the pinhole. Only look at the picture on the screen. Do not use the camera to look at a lamp or flame closely.
- An adult must use the craft cutter. Cut on a board, away from your hands.
- Handle the needle by the eye end and push it into an eraser or pad, never towards your fingers.
- Work in a place with good light, and do not climb on stools or window sills to point the camera.
- Do not use a candle or any flame as the bright object. Use a window, a sunlit wall or a torch.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Choose the box and measure its depth
Pick a box with a lid. Measure the inside length from one short end to the other, and write it down as f (for example 28 cm). Decide that one short end will be the front and the other end the back.
Tip: A box 20 to 30 cm long gives a good picture size. A very short box gives a tiny image.
Line the inside with black paper
Stick black paper or colour the whole inside black, including the lid’s underside. A matt black surface stops light bouncing around inside and washing out the picture.
Common mistake: Leaving the inside white. Stray light then fills the box and the picture looks pale.
Cut the front window
On the front end wall, draw a 3 cm × 3 cm square in the middle and cut it out. An adult uses the cutter.
Tip: The window only needs to be big enough to hold the foil plate; the pinhole itself is tiny.
Cut the back window for the screen
On the back wall, cut a rectangle about 12 cm × 8 cm in the middle, leaving a border of at least 3 cm. If the end wall of your box is smaller, make the window about two thirds of its width and height.
Common mistake: Cutting the window too close to the edge. The wall tears and the box loses its stiffness.
Fix the tracing-paper screen
Cut a piece of tracing paper a little bigger than the back window and tape it over the window on the inside, pulled flat and tight.
Tip: Wrinkles scatter light and make the picture look patchy. Tape the four sides one after the other, pulling gently.
Make the pinhole plate
Lay a 5 cm square of foil on a soft pad. Press the tip of the needle straight down through the middle, turn it once, and take it out. Hold the foil up to the light: you should see a single small, neat round hole.
Common mistake: Making a large or torn hole with a nail or a compass point. The picture becomes a bright blur.
Fix the foil over the front window
Tape the foil to the inside of the front wall so that the pinhole is in the middle of the 3 cm window. Tape all four edges with black tape so that no light can come in except through the pinhole.
Tip: The foil should lie flat. Crumples and gaps let in light round the edges.
Close the lid and seal the seams
Put the lid on and tape all along the lid, corners and joints. Then check for leaks: cover your head and the back of the box with the dark cloth, point the front at the sunny side of the room or at a window, and look at the screen for bright lines or spots, which are leaks.
Common mistake: Skipping the leak check. Even a thin crack lets in enough light to ruin a dim picture.
View a bright scene
Stand in a dim part of the room or in the shade and point the pinhole at a bright scene such as a window with a sunlit tree outside. Put the dark cloth over your head and the back of the box. Wait for about half a minute, then look at the screen. You should see the scene upside down and left-right reversed.
Tip: The scene must be much brighter than the room. A cloudy sky or a dark room gives almost no picture.
Measure the image and test the rule
Choose a bright object with a known height, such as a window that is 1.2 m high. Measure the distance d from the pinhole to the window and the height h of its image on the screen (hold a ruler against the paper). Calculate h = H × f ÷ d and compare it with your measurement. Do this three times, changing d each time.
Tip: Measure d with the tape or string from the pinhole, not from the back of the box. Keep f the same each time.
Test the size of the hole
Make a second foil plate with a larger hole using the tip of a nail. Swap it in and compare the picture with the needle plate: note which is brighter and which is sharper.
Common mistake: Judging brightness from memory. Swap the plates one after the other and compare the pictures straight away.
Finish and decorate
Add a small flap of black card over the pinhole as a shutter, label the parts, and paint the outside. Write your name, class and the value of f on the box.
Tip: A flap you can lift on and off keeps dust out of the hole when the camera is stored.
How it should look when it is finished
A neat closed box, black on the inside, with a tiny foil-covered window at one end with a needle-sized hole in the middle, and a milky tracing-paper window at the other end. Under a dark cloth, on a bright day, a small, dim, upside-down colour picture of the scene outside appears on the paper, and trees and clouds can be seen swaying in it.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
Light from every point on a bright object spreads out in all directions. Only the rays that pass through the pinhole get into the box; all the others are blocked by the front wall. Because light travels in straight lines, a ray that comes down from the top of the object continues going down after the hole and hits the lower part of the screen. A ray from the base goes up and lands on the upper part. So the picture is turned upside down (and left-right reversed).
Each point on the object makes one small spot on the screen, and all of the spots together form the picture. Because the hole is tiny the spots are small, so the picture is sharp. If the hole is wide, each point makes a larger circle and the circles overlap, which makes the picture blurry.
The two similar triangles (object, pinhole, and image, pinhole) give h ÷ H = f ÷ d. The black lining and the dark cloth help because the picture on the screen is dim: the less stray light there is, the better you can see it.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| I can see no picture at all | The scene is too dim, there is a light leak, or the pinhole is blocked. | Face a bright scene, cover your head with the cloth, wait for your eyes to adjust, seal leaks with tape and re-make the hole if it is blocked. |
| The picture is very blurry | The hole is too large or torn, or the screen paper is wrinkled. | Make a new plate with a clean needle hole, and re-stick the paper flat. |
| The picture is sharp but too dim to see | The scene is not bright enough, or the hole is too small. | Use a sunnier scene, or try a slightly larger hole (a thick needle). |
| The picture is upside down | This is correct. It shows that light travels in straight lines. | Nothing to fix. Draw the ray diagram to explain it. |
| Bright lines or patches appear on the screen | Light leaks through gaps in the lid or window edges. | Tape every seam with black tape and repeat the leak check. |
| Only part of the scene is visible | The screen window is small or the object is too large and too close. | Step back so the object looks smaller, or cut a larger screen window. |
Test and record
Use the same bright object each time (for example a window of height H). Keep f the same and change the distance d. Check your measured h against the predicted h.
| Trial | Object height H (cm) | Distance d (cm) | Box depth f (cm) | Predicted h = H × f ÷ d (cm) | Measured h (cm) |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- Why is the image in a pinhole camera upside down?
Light travels in straight lines. Rays from the top of the object pass through the pinhole and go downwards to the bottom of the screen, and rays from the bottom go to the top, so the image is inverted.
- What does this project prove about light?
It shows the rectilinear propagation of light: in a uniform medium light travels in straight lines.
- Is the image real or virtual?
Real, because it forms on a screen where the rays actually meet and can be seen on the paper.
- What happens to the image if you make the hole bigger?
It becomes brighter but blurry, because each point of the object makes a larger circle of light on the screen.
- Why not make the hole as small as possible?
A very tiny hole lets in too little light and light bends around its edge (diffraction), so the picture becomes dim and blurred again.
- If you use a deeper box, what happens to the image size and brightness?
The image gets bigger (h = H × f ÷ d, so h grows with f) and dimmer, because the same light is spread over a larger area.
- How is a pinhole camera different from a camera with a lens?
A lens gathers a wide beam of light and focuses it, so it gives a bright, sharp picture quickly. A pinhole has no focusing at all; it only lets thin pencils of light through, so its picture is dim.
- What natural examples of a pinhole camera have you seen?
The round spots of sunlight under a tree, which are images of the Sun made by gaps between leaves, and the crescent-shaped spots seen during a partial solar eclipse.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a pinhole camera, to show that light travels in straight lines, and to test the relation h ÷ H = f ÷ d.
- Apparatus
- A box with a lid, black paper, aluminium foil, tracing paper, sewing needle, tape, cutter, scissors, ruler, measuring tape, dark cloth.
- Procedure
- Line the box with black paper, make the pinhole in foil on the front window, tape the tracing-paper screen at the back, seal the box, look at a bright scene under a dark cloth, and measure H, d, f and h for three distances. Include the labelled diagram and the ray diagram.
- Observation
- An inverted, reversed image on the screen. The table of predicted and measured image heights. A note on what happened with a larger hole.
- Result
- The measured image heights agree closely with H × f ÷ d (state the largest difference). A larger hole gave a brighter but blurrier picture.
- Conclusion
- Light travels in straight lines, so the image formed through a pinhole is inverted and its size depends on the ratio of the box depth to the object distance.
Take it further
- Plot a graph of image height h against 1/d for one object. It should be a straight line through the origin: what does its slope mean?
- Make plates with holes of three different sizes and rank them for brightness and sharpness. Which size is best for your box?
- Watch the round spots of light under a tree and measure how their size changes with the height of the gap above the ground.
- Compare the picture with the one from a lens: hold a magnifying glass in front of the screen window and focus a distant window on a white card. (Never point it at the Sun.)
- With your teacher, make a pinhole viewer that projects the Sun on a white card for the next solar eclipse, and never look at the Sun directly.
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