Wavefronts on Anglesey

Looking out over the sea from South Stack near Holyhead on Anglesey, I was able to see waves being reflected. Looking down, you see the ripple pattern of the peaks of the waves. This is called the wave front. I have drawn them on in black in case you can't pick them out. Huygens favoured the wave front analysis. Isaac Newton prefered to draw rays which show the direction of motion. You can't see motion in a still photograph so I've had to draw them in for you as arrows. Now you can see that plane waves (flat ones) are coming down from the top of the object and reflecting from a circular promontary. If plane waves reflect from a curved object, the reflected wave fronts are curved too.

Wrong again!

The blue sky is definitely polarised but not because of the ash cloud. The blue colour is caused by scattering. You can look at the post about this on this blog last September. The scattering process polarises light. That explains a thing that I noticed - the polarised light was in every direction excpet that close to the Sun. Light close to the Sun is less scattered.
Also, I was surprised by how quickly Mercury disappeared. I was confidently telling a group on Friday night that it was just behind a cloud, but it clearly wasn't. It must have moved a long way across the sky in a week and have changed to a position that is below the horizon. Mercury was the speedy messenger of the gods in mythology. That's why the fast planet got its name. Now I have seen it for myself.

Ash cloud and polarised sunlight

I heard a suggestion that the ash cloud from the Icelandic volcano might polarise sunlight. Ordinarily, light waves from the sun are unpolarised. This means that some vibrate up and down, some vibrate side to side and others vibrate diagonally. If the ash cloud polarises the light, it would mean that it would vibrate in one direction only. You can check this with a polarising filter. Point it at the sky and rotate it. If it goes alternately light, dark, light, dark every quarter of a turn then the light is polarised. I've just been out to try it and this has turned out to be the case. Of course, I need to try again when the ash cloud has gone to be fair. You could try it if your sunglasses are polarising - some are and others are just thick filters.

Planets in the night sky

I watched The Sky At Night on BBC4 last week. It's almost the longest running programme on TV and is, shall we say, a bit dry. However, they explained where to look for 4 planets that are visible before 10pm at the moment. I went looking. At 9pm, look in the direction of the sunset. There is a really bright star. That's actually the panet Venus. Down and right of it, maybe about the width of your hand held at arm's length, there is a much fainter dot. That is the planet Mercury. I'd never seen Mercury before. Last night I picked it out from the car park in the middle of Wigton. Mercury is closest to the Sun so it is only visible in the half light at sunset (and just before sunrise at other times). This makes it hard to see because it is not very bright. It is also close to the horizon as well and it has gone before it is properly dark. All this makes it hard to see. So I photographed Venus. It is clearly a disk and not a point. That makes it a planet. My next ambition is to see Venus go through phases like the Moon (Eg full disk, half disk, quarter disk etc) Galileo observed this. My telesope is really for wildlife not stars, but last night I picked out the rings of Saturn with the 40x lens. Life doesn't get much better than that! By the way, Saturn is roughly South West at 10pm and about 30 degrees up. Mars is almost above your head and clearly redder. You have a day to watch the second of the two programmes which tells you where to see the planets if you whizz through to nearer the end. http://www.bbc.co.uk/programmes/b006mk7h

Double rainbows

We had a very wet holiday on Anglesey over Easter but I did manage to catch up with a double rainbow. I read about these years ago but pulled up short a year or so ago when I finally noticed that the colours are reversed on the outer rainbow. Enlarge this picture and take a good look at it.
The colours of a rainbow are caused by total internal reflection of the light in a raindrop and the subsequent refraction as it leaves the front of the drop. For a double rainbow, the light reflects twice on the inside before exiting.
See http://en.wikipedia.org/wiki/Rainbow

Miles per litre

I'm impressed that many of my classes seem to know the fuel economy of a car in miles per gallon. Problem is that petrol is sold in litres now. As part of my quest to verify the data in David MacKay's book last summer, I started noting the amount of fuel that went into the car and the mileage. Over 5 weeks of the summer holiday, our car averaged about 17 miles per litre. Looking in the handbook, that comes between the given values for motorways and for city driving, so it sounds about right. They test their cars on rollers in a lab so I'd like to know if they factor in air resistance as well.

Join the Institute of Physics

If you are studying physics in the Sixth Form, you can join the Institute of Physics for FREE. This would look good on a UCAS form but is also a good thing in its own right. I trained as an engineer but it is through my membership of the Institute that I have come to think of myself as a physicist. They send me the monthly magazine that I put up in the lab. You would be able to access this online.
The Institute is the organisation that promotes and regulates physics in this country.
www.iop.org/16-19 to sign up!