ICY SCIENCE: SCIENCE SPACE ASTRONOMY Spring 2014 | Page 74

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A corona is produced when light is diffracted by small particles. These can be water droplets, dust particles,
even pollen. In this case it was the small water droplets caused by the fog. You often see this effect when
the Sun or Moon passes behind a thin cloud. The fog was drifting up and down the hill at this point so it was
perfect for playing with different effects!
It wasn’t long before the Sun had scaled the trees behind and the fog started to retreat down the hillside…this
was my chance. Fogbows, or cloudbows, are pretty much formed like rainbows and are created when light
passes through water droplets when you are facing opposite the sun. The difference is that water droplets in
rainbows are much larger so light is refracted when entering a droplet of water, then reflected inside on the
back of the droplet and refracted again when leaving it. In a fogbow the water droplets are much smaller so
when light hits them they are too small to refract light so light is diffracted by the tiny droplets themselves
to form a much broader and paler bow. Light merges into white instead of being separated into the colours.
To observe a fogbow you need to have the light source (the sun) behind you. The fogbow is then seen at
around an angle of 35 to 40 degrees from your shadow -this is called the antisolar point. The fog has to be
below you with the sun strong enough to break through the fog behind. This was why I was so excited to
have the fog so high up the hill. I could still have the sun behind and the fog just below, instead of it being
right down in the valley!
I drove halfway down the track and waited for the sun to break through the fog. I wasn’t disappointed. The
fogbow was every bit as beautiful as I imagined it to be. It was an eerie feeling with the fog drowning any
sounds. Complete silence, cold damp air and what looked like the ghost of a rainbow in front of me.

ICY SCIENCE | QTR 2 SPRING 2014