Page 77 - BBC Focus - August 2017
P. 77
EVERYDAY SCIENCE
HELEN CZERSKI ON… CAUSTICS
“THIS MEANS THE POT OF GOLD AT THE END OF
THE RAINBOW IS UNATTAINABLE”
roper summer has finally You’ve probably also seen caustics on the
arrived, along with the bottom of a swimming pool, where waves
P opportunity for sitting outside travelling across the surface focus light into
and basking like a lizard criss-crossing bands, and direct it away from the
through long and lazy spaces in between.
afternoons. On a sunny day, while eating The surface where you see a caustic is acting
al fresco, intense sunlight glints off like a screen, giving you a sneak peek into where
plates and glasses, and the most light was travelling. If you move the screen, you’ll
important question of the afternoon is see the pattern change, but the three-dimensional
whether clotted cream or jam should be pattern itself was there all along. The light field
spread on scones first. But take a around us is lumpy and bumpy like this
moment away from these ponderings to all the time, especially on sunny days. But
look at the table – really look – and we need to put an obstacle in the way to
you’ll see that it’s covered in bright make the lumps and bumps visible.
lines and odd dark patches. The surface The final stage in the beauty of caustics
of your tea is decorated with two sharp brings us to the rainbow. Light paths are
semicircles on the side furthest from bent as they cross from one transparent
the Sun. Your transparent glass of water object to the next, violet bends more than
casts a weird dark shadow (even red. You may well see some coloured lines
though it is transparent), cut in half in the shadow from your water glass
with a blaze of white light. Inside because each colour generates its own
the rim of a plate, there are thread- caustic, and they don’t line up. And so we
like lines which roll across the come to the search for a pot of gold. A
surface as you tilt it. All of these rainbow is formed as light from the Sun
features are called caustics, and a reflects and refracts its way around the
sunny day offers the perfect inside of tiny water droplets in the sky.
opportunity to admire them. Afterwards, the light field for each colour is
Caustics are the sharp dividing lines between lumpy, with caustics dividing the most and least
bright and dark regions, and they turn up wherever intense regions. You intercept that pattern with your
parallel light rays meet curved surfaces. Because the own eyes, and you see the rainbow – different caustics
Sun is effectively a point source a long way away, the for red, orange, yellow, green, blue, indigo and violet,
light rays coming from it are almost perfectly parallel. glittering in a giant arc across the sky. The lumpy light
When they reach the inside of a teacup, they bounce field exists at every point between you and those water
off at different angles, depending on which part of the droplets, and your eyes detect the unique pattern at
inside of the cup they hit. But there are some regions your viewpoint. But this means the pot of gold at the
that many different rays get directed to (those are the end of the rainbow is unattainable,
bright semicircles) and some regions which are Dr Helen Czerski is a physicist because all that’s there is a three-
dimensional pattern. However, the
effectively forbidden – they can’t be reached by any
ILLUSTRATION: KYLE SMART meet at a point called the cusp, which makes a and BBC presenter. Her latest dish of butter on the table is right
reflected ray. In your cup, the two intense semicircles
here, so maybe I’ll admire the
book is Storm In A Teacup.
caustics inside the dish and make
distinctive pattern known as a nephroid caustic,
NEXT ISSUE: FESTIVAL
do with that instead.
because it’s vaguely kidney-shaped.
VIBRATIONS
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