Day 4 – @mjbrogers

Every day until Christmas Dr Helen Czerski will be finding some cool, hidden science in pics of people’s every day lives. To get involved, tweet your pic to @helenczerski and @cosmicshambles with the hashtag #ShamblesAdvent

Another one from across the pond, from @mjbrogers .  The beautifully flat water in the pond is giving us a lovely reflection, with only a few ripples stopping it being a perfect mirror.  The thing that caught my eye here is that you can’t see into the water at all.  We have no idea how deep it is or what’s underneath the surface, even though water is transparent.   But before we get to that, let’s consider what we can see.

If you held a laser pointer over this pond, pointing directly downwards, all of the light would pass straight into the water.  But if you tilted the laser beam sideways a bit, it would hit the surface and split into two.  One of the new beams will go into the water, and one will reflect off the surface and back into the air.  As you tilt the laser beam more, the proportion that gets reflected gets higher and higher.  At the angles we can see here, around 90% is reflected, making the water a pretty good mirror (as this picture shows us).

But for us to see into the watery world beneath, light would have to travel into it and come back out again.  The light going in is everything that isn’t reflected: the water surface isn’t taking anything away here, just diverting it.  So there’s probably quite a bit of light going in, even on an autumn day when the sun is low in the sky.  But very little of it comes back out.

If you were looking up at the water surface from underneath, you would also looking at a mirror.  Light travelling straight up from the pond bottom will pass through the surface and reach the atmosphere again, but light hitting the underside at a shallow angle will be reflected back downwards.  The difference is that beyond a certain angle, it’s all reflected. The light is trapped, and can’t get back out to show us what’s down there.   A tiny fraction of it could reach us at our low viewing point, but it’s insignificant.  So the pond is effectively covered with an optical shield.  Water may be transparent, but it’s perfectly capable of hiding things.

Read the rest of Helen’s Shambles Advent entries here

Dr Helen Czerski is a physicist, first and foremost, but she’s acquired a few other labels along the way: oceanographer, presenter, author and bubble enthusiast. A regular on The Cosmic Shambles Network, she has also presented a number of acclaimed documentaries for the BBC and her first book, Storm in a Teacup, which looked at the physics of every day things, was a bestseller. Recently she was awarded the prestigious William Thomson, Lord Kelvin Medal and Prize from the Institute of Physics.

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