Sitting outside with my back to the sun, I noticed that the shadow cast by each clear lens of my glasses was equally as dark as that cast by the frame and my head. Why? Surely the clear lens would let the light through rather than casting a shadow?
• The purpose of each lens in the spectacles is to bend the light rays passing through it, and assist the wearer in focusing clearly. This may also mean that light passing through the lenses is bent away from the area where the shadow forms, and this is as effective at forming a shadow as blocking light completely.
The questioner may, however, notice a brighter patch elsewhere, in the place where the diverted light ends up. It is interesting to see what happens when you start with the spectacles close to a wall, and slowly move them away towards the light, to show how the patterns of light and shade change.
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John Wood, Winster, Derbyshire, UK
• There are more ways of creating shadows than simply by stopping light. Any time you redirect light, it illuminates the new place it strikes, decreasing the brightness where it had fallen before – in effect creating shadows as well as bright areas.
Spectacle lenses work by redirecting light, so if a pair of glasses has convex lenses and you hold them in sunlight, you can focus the image of the sun on the ground, collecting all the light that would have landed evenly over the area behind each lens into a smaller, blindingly bright spot. The rest of the area is as darkly shadowed as if by a sheet of cardboard, despite the fact that practically all the light entering the lens passes through.
Next, if you gradually move the lens several metres back, you will see the unfocused image spread out with hardly any of its light falling directly behind the lens.
Once again, the lens casts a shadow even though virtually all of the light passes through. A concave lens for correcting myopia will not form a concentrated image, but it still redirects light, spreading it outwards so it forms a shadow much like a convex lens that is too far from the surface to focus.
Jon Richfield, Somerset West, South Africa
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