Feynman lectures - Project Tuva

I never thought I'd be "bigging up" Cambridge University AND Bill Gates, and in the same sentence.

Anyway, the Natural Sciences site for Cambridge University lists useful reading for prospective students. http://www.natsci.tripos.cam.ac.uk/prospective-students/reading

However, the Physics section sends you to this link: http://research.microsoft.com/apps/tools/tuva/index.html#data=3%7C84edf183-7993-4b5b-9050-7ea34f236045%7C%7C

Bill Gates has acquired the rights to a set of Richard Feynman lectures and put them up so that we can all watch them. Hurrah!

You should also find out why it is called Project Tuva - it's another marvellous Feynman story.

Polarisation in Teesdale

These photographs are taken at the Cow Green reservoir, high in the Pennines in Upper Teesdale. This first picture is taken in normal light conditions.

 

 

I noticed that the view was different when I had my polarising clip-on sunglasses on. If you notice in this second photograph, you can clearly see the waves on the water.

 

Here's how I took the photograph.

 

Light is an electromagnetic wave. It is transverse, which means that the oscillation is perpendicular to the direction of travel of the light. The oscillation can be up and down, it can be side to side or any angle in between. The polarising sunglasses only allow one direction of oscillation to pass. For example, they could allow the side to side through but stop the up and down. You can tell that less light is allowed through because the second picture is dimmer. The sunglasses must be blocking the light from the main part of the water but not from the wave crests, because I think the white tops on the waves are bright reflections rather than white water. Perhaps the wave crests are polarising the waves differently to the main part of the water. I'll have to think about that.

Sunspots

 I set my telescope up to project an image of the Sun onto a piece of paper. There are about half a dozen sunspots visible at the moment. This activity on the surface of the Sun will result in more charged particles streaming towards Earth and interacting with our magnetic field around the poles. Hence the Aurora Borealis is more likely to be seen.

 

 

Whilst I was at it, I noticed the dimming of the picture if I covered over part of the lens. Casting around for something to use in the photograph, I used my finger. That is my pointing finger, in case you were wondering! Notice that you don't get a shadow of the finger in the image. You still get the full image. The reason is that light from each part of the object is refracted by each part of the lens. In other words, there are many different routes light can take from the Sun to the paper. My finger blocked off a few of these routes so less light was able to get through and the image is dimmer overall.

The fluorescein eye

This is a very "old skool" experiment that has appeared in the syllabus again. I found it in our store. The idea is that there is a round glass jar to represent your eyeball and that it is filled with a fluorescent dye called Fluorescein so that you can see the path of the light rays inside your eyeball.

 

 

If you are lucky enough to have normal sight, the lens makes the rays come together as a focussed point on your retina. You can see this happening down the middle of the picture below.

 

 

I am short-sighted, either because my lens is too strong or that my eyeball is too long. As a result, the light focuses in the middle of my eyeball and then spreads out again as shown below. I wear concave lenses to push the focal point back onto the retina.

 

If you are long sighted, the rays don't even come together in your eyeball. You need to wear convex lenses to bring the focal point in onto your retina.

Seeing through a dragon's eyes

My friend lent me the Eragon series of fantasy fiction books. In it, a boy becomes a Dragon Rider. The premise is that the Rider and the Dragon become mentally one; they live in each other's thoughts. One observation in the third book, Brisingr, caught my attention: when Eragon sees what his dragon sees, the colours look different because the dragon's eyes are physiologically different and so detect different wavelengths. This is realistic: remember that some insects can see beyond our visible range into the ultra-violet. It's details like this that have got me hooked on the series.

 

 

A similar issue came up in work I've been doing with the Upper Sixth. Any object that emits electromagnetic radiation in Physics is called a black body. It's a technical term, based on the idea that matt black objects absorb infra-red heat radiation well - so don't wear black clothes on a sunny day. Technically, the Sun is a black body. Black bodies have a characteristic curve like the one shown below.

 

They emit across a range of wavelengths. The wavelength for peak intensity is fixed by the temperature of the object. The equation under the graph is called Wein's Law. If you put in the surface temperature of the Sun, the wavelength of the peak comes out somewhere in the green/yellow part of the spectrum. That's also the colour for maximum response for your eye - ie why high visibility jackets are the colour they are. It is no surprise really that evolution has resulted in our eyes having a maximum response for the peak of the Sun though.