01 / WHY ROUND
The patch is the shape of the sun, not of the gap
Gaps between leaves look triangular, or long and thin, or anything else. Yet the patches on the ground are all round. What is being projected is not the shape of the gap but the shape of the sun.
When a gap is small enough relative to its distance from the ground, that gap works as a pinhole. Light from each point of the sun travels straight through it and forms a reduced image of the sun on the ground. The sun is round, so the image is round.
If the gap is too large, or the distance to the ground too short, the condition breaks down. What lands then is not an image but a bright area in the shape of the gap. That is why dappled light is crisply round on some days and vague on others.
02 / ECLIPSE
During an eclipse, dappled light turns to crescents
While a partial eclipse is under way, the moon takes a bite out of the sun and leaves a crescent. If dappled light really is an image of the sun, the patches on the ground should become crescents too. They do.
Every patch under the tree turns into a crescent facing the same way, all at once. It can only be seen during an eclipse, and it is a chance to confirm with your own eyes — not by calculation — that dappled light is a projected image of the sun.
The observation is old. Book 15 of the Greek Problems asks why sunlight through a square gap becomes circular, and why light passing through a sieve, through leaves or between interlaced fingers makes crescents on the ground during an eclipse.
03 / SAFETY
Never look at the sun directly
Do not look at the sun directly when observing an eclipse. Looking at it through a small hole is not safe either: putting your eye to an aperture and looking at the sun can injure your eyes.
The safe method is to look at the projection instead. As long as you are looking at an image cast on the ground or on white paper, you are not looking at the sun. Watching dappled light is exactly this projection method.
Take care with camera and phone lenses pointed at the sun as well. For detailed procedures, follow guidance from public bodies such as the NASA solar eclipse guide.
04 / SAME PRINCIPLE
This is the same thing a pinhole camera does
What happens in dappled light is the pinhole camera principle itself: a small aperture, light travelling straight, and a surface to receive it. The only differences are that the aperture is a gap in the leaves, the surface is the ground, and the subject is the sun.
In a pinhole camera too, a larger hole softens the image — the same reason a wide gap gives a vague patch. Change the hole diameter in the model below and you can check that relationship in both the numbers and the projected image.
05 / INTERACTIVE
Change the gap, watch the ground
Move the size of the gap between the leaves and the distance down to the ground. You can check for yourself that the gap is not round while the light is, and find the point where opening the gap stops the light being round.
INTERACTIVE / GAP SHAPE
The gap's shape is not the light's shape
The common misunderstanding“The light is round because the hole is round.”
Gaps between leaves are almost never round. The light on the ground is round anyway, because what lands there is not the shape of the gap — it is a picture of the sun.
Every point of the gap forms its own image of the sun, and those images are offset from each other by the width of the gap. Here that overlap is drawn as stamps of the gap's shape, laid out across the sun's disc.
- Size of the sun's image
- 37.0 mm
- Gap ÷ sun's image
- 0.38
- Spread of the light (approx.)
- 51.0 mm
- What you see
- Round. You are looking at the sun
So
01A small gap makes small stamps, and the arrangement as a whole reads as the sun's disc. That is round dappled light.
02A large gap makes large stamps, and the overlap comes to resemble the gap itself. It stops being round.
03Crescent dappled light during a partial eclipse has the same cause. What is projected is the sun, so when the sun is bitten into, so is the light.
06 / REFERENCES