Shapes of Shadows

First a correction: this post is really about the shapes of the bright patches between shadows but “shapes of shadows” sounds much better so please excuse my decision to go for artistic alliteration!

If you have ever walked under some leafy trees on a bright summer’s day, with the sun high in the sky, you are sure to have seen shadows on the ground cast by the canopy above. But how often have you thought about the shapes of the bright patches between the shadows?

The shape of each bright patch depends on the shape of the corresponding gap between the leaves but that doesn’t mean the two shapes are the same. In particular, you might sometimes see circular patches on the ground but does that mean there are circular gaps between the leaves?

The answer is no – and the reason for this discrepancy is due to the fact that small gaps behave like pinhole apertures that cast an image of the object on the other side of the hole. In this case, the object is the light source itself and the circular patches are therefore images of the sun.

This might sound unlikely but I have a piece of evidence that, I hope, will help to convince you. The picture below was taken in central London around midday on 11th August 1999 when there was a solar eclipse visible across most of central Europe. The eclipse was only partial in London, which is why the bright patches seen on the pavement with leafy trees above are partial discs.

This is a nice cross-over between physics and photography so it’s worth adding that every picture actually contains information about both the original object and the aperture that was used to image it. Most of the time, the effect of the aperture itself is minimal but it does show up fairly often in out-of-focus areas. This is often referred to as the bokeh of an image, which some people deliberately manipulate for creative effects.

I should end by admitting that not many of the bright patches you will see in everyday life are circular as this effect requires a specific set of conditions. Don’t be surprised, therefore, if circular highlights seem hard to find. But you will spot them if you are looking – and when you do, remember that you are looking at images of the Sun.