DIY · Card 76 · Physics · Light
A shoebox, a square of foil and a needle. Prick one tiny hole and the world outside appears on a paper screen, in full colour, moving, and upside down.
01The brief
A camera with no lens at all.
0.6 mm
the sharpest pinhole for a box 20 cm long
180°
the picture is turned: upside down and back to front
f/320
how dim it is: hundreds of times darker than a camera lens
Every camera you have ever used has a lens. The very first ones didn’t. All it takes to make a picture is darkness and a small hole: light from each point of the scene shoots through in a straight line and lands at exactly one place on the far wall.
Make one from a shoebox and you can watch the world, upside down and in colour, from inside a box. It’s a cousin of the periscope and the paper microscope.
02Kit
A box, some foil and a needle.
Tick off what you have
0 of 5 ready
03Method
Black it out, add a pinhole and a screen, then hide under a towel.
Step 1 of 8 · Minute 0
You need a shoebox or any long cardboard box with a lid, black paint or black paper, kitchen foil, a needle, tracing paper or baking paper, tape, a craft knife, and a dark towel or blanket.
The screen looks completely dark.
Why
Your eyes haven’t adjusted, or the scene is too dim.
Fix
Point it at a sunlit view, wait under the towel for half a minute, and make sure no daylight leaks in.
The picture is a bright blur.
Why
The hole is too big or torn.
Fix
Tape fresh foil over the window and prick a new, smaller hole with just the tip of the needle.
The picture is washed out.
Why
Light is leaking in round the lid or the hood.
Fix
Tape the lid’s edges and wrap the towel more tightly around the screen end.
04Light and holes
A big hole lets each point of the scene spread into a blob; a tiny one makes the light spread out by diffraction. Somewhere between, the picture is as sharp as a pinhole can make it.
Find the best hole for your box:
Blur on screen
0.84 mm
Sharpest hole
0.55 mm
f-number
f/188
Light vs f/16 lens
137× dimmer
The hole is too big. Every point of the scene shines through all 0.80 mm of it, making a blob that wide on the screen. Make it smaller.
And here is why the picture lands upside down, and how big it will be:
Picture of the tree
133 mm
Fits on screen
too tall
Angle of view
28°
The ray from the top of the tree and the ray from its foot cross at the pinhole, so the picture lands upside down and back to front. Its size follows from similar triangles: 4 m × 200 mm ⁄ 6 m = 133 mm. A longer box makes a bigger, dimmer picture and a narrower view; a shorter one squeezes more of the world onto the screen.
05The big idea
Light goes
straight.
Every point of a sunlit scene sends light out in all directions. A small hole picks out just one thin ray from each point, and because rays travel in straight lines, each lands in its own place on the screen: a picture.
Lenses do the same job with a much bigger opening, bending many rays from each point back to one spot. But the pinhole proves the basic idea: a picture is just light, sorted by direction.
06The math
1 · Picture size
image = h · f ⁄ u
h the object’s height, u its distance, f the box length.
2 · Sharpest hole
d ≈ 1.9 √(f λ)
λ ≈ 0.00055 mm for yellow-green light.
3 · Brightness
N = f ⁄ d
The f-number; each doubling costs four times the light.
A 4 m tree 6 m away, through a box 200 mm long, makes a picture 4000 × 200 ⁄ 6000 ≈ 133 mm tall: too big for a shoebox screen, so step back. At 10 m it shrinks to 80 mm.
For that box, the sharpest pinhole is 1.9 × √(200 × 0.00055) ≈ 0.63 mm, giving N = 200 ⁄ 0.63 ≈ 320. A phone camera works at about f/2, so the pinhole lets in about (320 ⁄ 2)² ≈ 25,000 times less light per square of screen. No wonder you need a towel.
07Real life
In your eye, under trees during an eclipse, and in the sea.
Your eye
The pupil is a hole and the retina is a screen. Your eye has a lens to make the picture bright and sharp, but it still lands upside down; your brain turns it the right way up.
Eclipse crescents
During a partial eclipse, the gaps between leaves act as thousands of pinholes, and the ground under a tree fills with little crescent suns. A colander does the same.
The nautilus eye
The nautilus, a sea creature related to the octopus, has eyes that are true pinhole cameras: open cups with a small hole and no lens, letting in seawater along with the light.
Room-sized cameras
Black out a room, cover the window except for a small hole, and the view outside appears upside down across the far wall. The photographer Abelardo Morell has made famous pictures this way.
Solargraphs
A drinks can with a pinhole and a sheet of photo paper inside, fixed to a post for six months, records the Sun’s path across the sky every day, from the low arcs of winter to the high ones of summer.
Painters’ helpers
Some historians think painters such as Johannes Vermeer used a camera obscura, with a lens, to study light and detail in a scene. The idea is still debated, but the device was well known in his time.
1021
People had noticed upside-down pictures through small holes since ancient times: Chinese texts of the Mohist school described one over two thousand years ago. But it was the scholar Ibn al-Haytham, working in Cairo, who explained it.
In his Book of Optics, completed around 1021, he described a dark chamber with a small opening and argued that light travels from objects to the eye in straight lines, overturning the old idea that eyes send out rays. His camera obscura, Latin for “dark room”, became the ancestor of every camera since.
08Is and isn’t
The picture is upside down because of the lens.
A pinhole camera has no lens at all. Straight lines of light crossing at the hole are enough to turn the picture over.
The smaller the hole, the sharper the picture.
Only down to a point. Smaller than about 1.9 √(f λ), diffraction spreads the light and the picture gets blurry again.
You need film to use a pinhole camera.
A viewing screen works perfectly: you see the live, moving picture. Film or photo paper is only needed to keep it.
A pinhole makes it safe to look at the Sun.
Never. Pinhole viewers are safe because you look at the projected picture, not through the hole.
09Go further
A
Two pinholes
Prick a second hole next to the first. How many pictures appear? What happens with ten holes?
B
Add a lens
Widen the hole to a centimetre and hold a magnifying glass over it. Move the screen until the picture is sharp and bright.
C
A camera room
Black out a bedroom window with card, leave one small hole, and watch the street appear upside down on the wall.
D
Measure a tree
Measure the tree’s picture on the screen and the box length. Pace out the distance to the tree, then use similar triangles to find its height.
E
Watch an eclipse
Next time there is a partial solar eclipse, use your camera with your back to the Sun to watch the crescent safely.
10Questions
Paint the inside of a shoebox black, cut a small window in one end and tape foil over it, then prick a pinhole in the foil with a needle. Replace the other end with tracing paper, point the pinhole at a bright scene, and look at the paper with a dark towel over your head.
Light travels in straight lines. A ray from the top of a tree passes through the hole and keeps going downwards, while a ray from its foot goes upwards. They cross at the pinhole, so the picture lands upside down and back to front.
Lord Rayleigh’s rule of thumb is d ≈ 1.9 √(f λ), where f is the distance from the pinhole to the screen and λ the wavelength of light. For a box 20 cm long that is about 0.6 mm: roughly the hole a fine needle makes.
A smaller hole makes each point of the scene a smaller spot, but below a certain size the light starts to spread out as it squeezes through, an effect called diffraction. The sharpest picture comes from the size where the two blurs balance.
Never look at the Sun directly or through any hole or tube. A pinhole camera is safe for watching an eclipse only because you look at the projected picture on the screen, with your back to the Sun.
Safety · Never look directly at the Sun, or at it through the pinhole or any tube. A grown-up should handle the craft knife and the needle, and paint in a ventilated room.
All DIY cards →Keep wandering
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