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On holiday in France, me and my family were walking along a road through a field of smooth mud/dirt. The sun was coming from the right hand side. Looking to the left hand side, the field looked a sort of clay-y orangey tan. Looking to the right, it looked dark brown. When we came to another road that was parallel to the original one, the field that had been on our right and looked dark brown was now on our left, looking orangey tan instead.

An observer in the middle of a field whose surface is slightly rough

This seemed a bit weird, because the field was very smooth and there weren’t any trees or buildings casting shadows. What (I think) explains it is that the surface of the field, although smooth-looking, was slightly rough, being made of dirt. So, on a scale of a few inches, the field’s surface had little peaks and troughs, as shown in the diagram. When the observer is looking in the direction that the sun comes from, this means that lots of tiny bits of the field are in shadow, caused by raised and depressed bits of dirt, as shown in the inset.

Because it had been made quite smooth, we couldn’t really see the actual texture of the field, but the overall reduction in the amount of sunlight reaching us from it made it look dark when viewed in this way, even though the whole field was ‘really’ the same colour. The field, viewed from the right direction, is ‘in shadow’, but on a very small length scale. The relative difference in perceived colour or brightness when you look in each direction must be related somehow to the density and characteristic size of the peaks and troughs in the field’s surface. Fun bit of maths to do?

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Here’s probably quite a well-known trick, but one that has a nice physics-y explanation. Defrosting things to cook can take quite a lot of time depending what they are, requiring forward planning, leaving the thing out overnight, etc. Failing that, there’s the option of defrosting in a microwave, which can have mixed results (e.g. the outside of a thing semi-cooks in a horrible way while the inside stays frozen), and can be dangerous for things like meat because it results in large parts being very slightly, germ-growingly warm for a long time without getting hot enough to kill any germs.

A nice way of getting around this is to put whatever’s being defrosted in a bowl full of cold water. Liquid water is much better than air at conducting heat energy away from (or in this case, towards) something else. So, even though the water is no warmer than the air around it — and in fact can’t get warmer than room temperature, reducing the chances of germs growing — the thing defrosts much more quickly than it would otherwise.

This works especially well for things like vacuum-packed frozen chicken breasts or other meats, where the thing being defrosted can be in more or less direct contact with the water, without any insulating layer of air around it. Happy defrosting and subsequent cooking!

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(Skip to the bottom for the bit after the intro)

Hello. In the course of a day we encounter lots of little tasks that need doing, observations of things which are so trifling they barely merit the word ‘observation’, and so on. This is pretty broad (!!), but hopefully what I mean will become more obvious soon. It occurred to me that lots of these things have often very very basic (and sometimes less basic but still interesting) physics behind them. Not cutting edge physics by any means, and sometimes barely even physics, just a sort of physics-leaning bit of common sense.

I’m sort of talking about the reason why the sky is blue but much more mundane. Perhaps a pretty good example is my post about how to stop the cable from 9-volt adaptors from breaking by distributing the stress around a larger region of the cable (here).

So I’m going to write posts about these things whenever they occur to me, just for somewhere to put them. Here’s the first one:

Bags

When I go shopping and get asked if I need help with packing, I manfully decline. But until I clocked this one I actually did need help, just to open the stupid bags. I’d seen people lick their fingers and then seem to have no difficulty doing it, so I imitated them but still couldn’t manage it. Where was I going wrong? I was just licking one finger. If the other one’s still dry and slippery (like mine usually are), the two faces of the bag just move together, sliding over the dry finger and not coming apart. So now I lick both my fingers and the bag doesn’t stand a chance. Erm.

There’s no way to really end a post like this……………. HAPPY BAG OPENING FOLKS!!