MOTION MOUNTAIN

188 4 images and the eye – optics
grass
dew
(not to
scale)
head
Sun
Ref. 141
Ref. 142
Ref. 143
Challenge 183 s
F I G U R E 138 The path of light
for the dew on grass that is
responsible for the aureole or
Heiligenschein, and a photo
showing that it is seen only
around one’s own head
(© Bernt Rostad).
the front of the retina only in the dark, and then produce black and white pictures.)
In summary, evolution has provided us with an observations system that has amazing
properties. Take good care of your eyes.
How to prove you’re holy
Light reflection and refraction are responsible for many striking effects. The originally
Indian symbol for a holy person, now used throughout most of the world, is theaureole,
also called halo or Heiligenschein: a ring of light surrounding the head. You can easily
observe it around your own head. You need only to get up early in the morning and look
into the wet grass while turning your back to the Sun. You will see an aureole around
your shadow.The effect is due to the morning dew on the grass, which reflects the light
back predominantly in the direction of the light source, as shown in Figure 138.The fun
part is that if you do this in a group, you will see the aureole around onlyyourown head.
Retroreflective paint works in the same way: it contains tiny glass spheres that play
the role of the dew. A large surface of retroreflective paint, a traffic sign for example, can
also show your halo if the light source is sufficiently far away. Also the so-called ‘glow’
of the eyes of a cat at night is due to the same effect; it is visible only if you look at the
cat with a light source behind you. By the way, do Cat’s-eyesworklike acat’seyes?
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net