A Scientific Ramble

Posted by Met Shop on

On a recent family tramp through some beautiful woodland in the foothills of the Tararua range, the early Spring sunshine filtering through the trees, I noticed the curious dappling of the sunlight on the footpath in front of us.

I had seen this before, but this was one of the best examples yet. The effect is shown in the first image below.  Instead of the leafy shadows that you might expect, there was a collection of circles and ellipses of varying intensity dancing about as the wind jostled the branches.

circular dappled light on the ground from the sun shining through trees

There are many good descriptions online explaining that the circles are in fact images of the sun and appear when the height of the leaf canopy and contrast conditions are just right.  Observing the phenomena during an eclipse will show crescent-shaped light spots or images of the partially obscured sun.

Despite being quite a common occurrence, the effect still ceases to fascinate the observer who likes to know the science behind the everyday nature of things. 

tree branch shadows on a path

By contrast, a second example is shown in the next image where the light and shadows are more what you might expect, the shadows and light form much crisper boundaries and actually look like the branches and leaves of a tree. In this case the distance from the branches to the path is much smaller and the surrounding light hitting the footpath is much brighter.

There is a detailed description of the conditions required and the physics involved in this phenomenon described by the astronomer M. G. J. Minnaert who is referenced on Edward Tufte's fascinating web site: Edward Tufte forum: Dappled light

This "pinhole camera" effect can also be demonstrated with a simple piece of card on a sunny day.  Making a small hole in the card and projecting an image into the shadowed area of the card a metre or so away shows how the sun's image can be easily generated on the ground.  If you try this, however tempting it is, never look directly back at the sun when trying to line up your card.

In practice, a much better image can be formed by using a pin-pricked hole in a small piece of foil taped across a cut-out in the card.  Additionally making a box to block out as much extraneous light as possible allows a far better viewing experience.

Ultimately this can lead you (as it did me) to digging deeper into the fascinating area of pinhole photography and camera obscura which are both recommended topics for further investigation.

Happy rambling!


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