“Solar images formed by pinholes, crossed fingers, patches between leaves, all occur because of diffraction–a wave property of light. In the case of a pinhole, the light rays do not shoot straight by the rim of the hole, but bend around the edge. This wave effect creates a diffraction pattern of rings on the screen which resembles a bull’s eye. That’s for a flat wave single light source. If the aperture is illuminated by a scene, it acts as a lens to image the scene on a screen. With the right size hole relative to the right distance to a screen, a clear image is formed. That’s the general principle of a pinhole camera.”
“Applying this to an eclipse observation, the sun becomes the object to view. Point the pinhole camera at the sun and you see a solar image (projected on a screen) dim enough your eyes can enjoy.”
“But the pinhole effect doesn’t need a designed aperture. The solar image can be formed by any aperture if the shadow is the right distance away. The sunrays though tree leaves work to make a solar image on the ground below. Blinds on the window will covert a square opening into a round sun on the wall.”
“The marvel is that diffraction doesn’t need a round hole to form an image. A square pinhole will also work if its area is the same. Even for a random edged shape, the wave bending will average out to form an image of the scene contained in the incident light. That’s why the spots of light through the trees are round; the gaps in the foliage are imaging the sun. ” NASA on solar eclipse shadows
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