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Lockdown Challenge: Polarised light effects and protecting pets

Image credit: Bravissimos | Dreamstime

This week we get creative and experimental with polarised light and turn to our caring side as we look to shelter our pets from the spring storms.

In the latest in our series on Lockdown Challenges, Neil Downie kicks off with ways to experiment with polarised light, to understand how it works and its effects and then how you can use these effects to create imaginative artwork. In our second challenge this week, inspired by some changeable weather, Crispin Andrews encourages the family to make a weatherproof ‘nest’ for your pet.

Lockdown Challenge #8 - Polarised Light: Art and Engineering Meet

Polarised light is the secret of a lot of display screens. Light is a transverse – sideways – vibration in the electromagnetic fields that exist in space. Although phones and some TVs don’t use it, most displays are LCDs (liquid crystal displays) and this means that they emit polarised light.* And we can use that polarisation for things if we have a pair of 3D cinema glasses, polarised sunglasses, fishing glasses, or just a piece of polariser plastic stripped from a broken LCD, like an old calculator.

To get a flavour, get your polarising glasses and find an LCD screen, and get a blank white image up. Now put something made of clear plastic like a bag or a ruler in front. Whoah! Where did all that colour come from? With luck, you will see colour along the plastic. Thick plastic tends to have thin bands across it, while thin plastic like a bag or a piece of clear tape might be pretty much all the same colour. Now twist the plastic along an axis parallel to the screen, at right angles to the screen.  If you are using 3D cinema glasses, try one eye, and then the other eye, as well as both. 

What’s happening is that the colours reflect that different wavelengths of light – different colours – rotate the polarisation differently, and so one colour will get through your polarised glasses and others won’t. Added to this is that particularly thicker plastic sheets are not the same thickness all the way across. So some parts do different amounts of polarisation rotation and get different colours. Try moulded objects like clear plastic cutlery. Put one object in front of another and you can add the thicknesses and see how the variations in the thicknesses of each interact. 

Polarised light picture

Image credit: Neil Downes

Seeing forces inside things

There is more magic too: you can see strain – you can see the forces inside a clear object. Take a piece of clear plastic 10 or 15mm wide, like a 150mm clear plastic ruler cut in half lengthways, and file a notch in it halfway or so along the length. Hold it in front of the LCD screen and don your polarising glasses. Now bend it across the notch, looking carefully at the coloured bands. Look how the bands shift and concentrate around the notch. You are ‘seeing’ force! Where the plastic is compressed, it gets a bit thicker. And thicker plastic means more light rotation, which means a shift in the colours.  Similarly, where the plastic is stretched, it gets thinner, and you see that too.  And because strain, the squeezing or stretching movement, is proportional to stress, you are ‘seeing’ stress.  You can see why a notch is a weak point – you can see all the stress.

Studying stress in things that we make is important so we can make things don’t break. We can make them a bit thicker, a bit stronger, in places where they need to be. And studying stress also means that we can avoid wasting money on making things stronger than they need to be in other places.


  • Find all the plastics that show strain well in the polarised light rig.
  • Which ones are most sensitive? What thicknesses work?  What about objects which aren’t sheets?  How do they show up in your rig?
  • Look to see stress in objects when you bend or stretch them. What happens at corners? What happens at holes?

And finally… Engineer some Art

Find a roll of clear tape like Sellotape. And start sticking pieces of it on a sheet of clear plastic like an acetate sheet, or a strong plastic bag. Overlap the pieces 1, 2, or 3 times. What to create? Maybe a cityscape: Rainbow City, anyone? Or an abstract set of rectangles: it will probably end up looking like a Piet Mondrian. But you can trim the tape into interesting shapes too. Leaves? Dinosaurs? A polarised birthday card? Try rotating the picture around an axis at right angles to the screen as you create it, to get the best effect.

Now test your artistic powers on other people. Show them that dull piece of plastic bag or acetate.  Then give them the glasses, switch on the screen and unleash the magic of your polarised light painting. 

For more, search on ‘photoelasticity’ on Wikipedia and elsewhere on the internet. And try my books like 'The Ultimate Book of Saturday Science', and for lots of other things (and a free copy of Exploding Disk Cannons book),

* It is complicated, but basically, they work by the voltage across a liquid crystal in a layer between sheets of glass twisting the plane of vibration of the light. Light from behind is polarised by a sheet of polariser. This polarised light (say up-down polarised) is then converted into a light valve by a sheet of polariser, which lets only, say left-right polarised light through. If the liquid crystal does not twist, then no light comes through, if the liquid crystal twists it through 90 degrees, then the light comes through. And, by the way, 3D cinema uses circular polarisation and this is even more complicated!

Lockdown Challenge #9 - Make a weatherproof indoor shelter for your cat

I’m just about to sit down and write the next challenge, when thunder starts rolling in from over the Chiltern Hills. Nothing major, Thor isn’t heralding Ragnarok or anything like that, but enough to spook Lexi, our beloved little black and white, who had been minding her own business, curled up on her cushion and blanket in a corner of the living room.

Cats, like most pets, hate bad weather, whether it’s thunder, rain, wind, sudden cold spells, or our very occasional moments of extreme heat. It scares them, stresses them, makes them want to run and hide.

And even though we’re in the middle of the biggest single upheaval in living memory, the one thing that we can be sure that won’t change is the good old British weather. And this is one time of year when the weather is known to be at its most fickle.

Not wanting to move Lexi, I quickly hitched up some more cushions around her sleeping place, some against the wall, turned her scratching post sideways, as a fourth wall and then draped a large piece of cloth over the top. Result, cat bed becomes a cat nest into which she can retreat if the thunder comes back. If it doesn’t she can continue her rest. Either way, she doesn’t have to run and hide under the bed.

But that’s just me, a journalist not an engineer, thinking on the hop with an immediate problem to solve. Lexi’s nest will only last as long as she stays in it (either until the fire goes on or when she’s hungry). Or until we need the cushions for something else. Given some engineering wherewithal and a bit of childlike creativity, you and your kids should be able to come up with a structure that’s a lot more solid and lasting.

A few catty pointers

Any structure should have soft underlay or the cat will not want to stay there.

It should have enough room for your cat to move around in, but not too much room, so they feel safe and secure.

Their view should be restricted, but not blotted out altogether, some sort of roof is a good idea and an easy way in or out of the structure is essential.

Make it multi-use so they can use it when the weather is hot or cold, sunny or stormy. Lexi could snuggle into her new nest if she wanted, but also sit on top of it, or just outside it, if she preferred. As soon as it's sunny, she can push down the walls herself and bask in the sunshine.

There’s one more problem, though. And that’s how to get your cat to actually use the structure you’ve built. Cats are not known as the most cooperative of animals, but there are ways.

Build it in a place that they already use. Cats like routine and familiarity, so if you put it down in the basement or out in the kitchen or garage, expect them to turn their nose up.

Use their own stuff. Cats like familiar smells and sensations. They’ll likely reject a dusty old box that smells of oranges, or a musty old throw. If you have to wash something, don’t use too much wash powder, as cats often don’t like the smell of that or the feel of it against their skin.

Have patience. If your cat doesn’t use what you’ve made, think why that is and perhaps change the design or the location of the structure to more closely meet their needs

Know your cat, their likes and dislikes. For instance do they like being covered up?

Update: an hour and a half later, the thunder has stopped and Lexi is still in her new nest, fast asleep.

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