The problem with my simple star projector

 

This lovely device projects the northern constellations onto the ceiling of my room. However I do have difficulty working out which constellation is which. I've realised what the problem is: All the stars are equal brightness. I'm used the differences in intensity between stars in a constellation and the simple hole method has me thinking I'm looking at a different sky!

Identifying the fossils

 Thanks to Tom for telling us what our fossils were and for showing us much better samples!

Below is Siphonodendron.

This is Lithostrotion junceum

They are both corals from the Lower Carboniferous period so maybe 350 million years ago.

The circle is a cross-section through the stem of a crinoid, which is sometimes called a sea lilly.

The correct ray diagram?

 

I should have thought to draw the Ray diagram first. Looks like both my reflections have had multiple reflections from both inner glass surfaces... 

Reflections in the double glazing

 

The first thing we noticed was that beyond the first pair of reflected beads there was a second fainter pair circled in yellow. The first pair must be the reflection from the inner pane and the second pair are reflected from the outer pane. 

Close examination of the nearest bead reflection shows a double row very close together. These will be reflections from the outside and inside surfaces of the single pane.

Looking from outside in, notice the reflection of the metal spacer between the panes.

Then using my finger as a marker, I detected a second reflection. This will be the reflection from the other pane of glass. It will be in this second reflection that I saw the thin-film interference bands last week.

Thin film interference cleaning windows

 

It hasn't photographed as well as I saw it but when I was spraying window cleaner onto the pane and wiping it off, the reflection of the double-glazing centre showed the sort of colour bands I've been thinking about. It seems that we might also be talking about multiple reflections from the inner surfaces of the glass as well so this is going to take some thinking about.

More ideas about thin film interference

 

I might have worked out why the colours seem more like secondary colours with thin film interference. Here's the ray diagram:

I am sure that the red ray that reflects from the lower surface ends up parallel to the red ray that reflects from the top surface. That means that they can interfere. Now the blue ray should have the same but having had a bigger refraction inside the thin film. This must mean that the red and blue rays can interfere with each other. Some wavelengths of colour will cancel so that not all will constructively superpose so we don't get a white reflection but if a few colours do constructively superpose, that would account for the strange colours.

Highlighters and fluorescence

 Thanks to Ben for this brilliant book. I have raced through the first 50 pages and found out a lot of stuff I didn't already know.

The first to grab my attention was highlighter pens.

Now I know that the idea is that ultraviolet photons that we can't see are absorbed, sending electrons up energy levels. The electrons de-excite but don't fall all the way down to the original level in one go. The photons released have less energy and are therefore visible. What I hadn't figured was that the amount of UV in ordinary sunlight coming into my room is enough to release extra photons that make the yellow colour stand out as brighter. Mrs B is knitting with fluorescent yellow for a hi-vis jacket. I was interested to find that you can define a QUANTUM YIELD as the ratio of visible photons released to UV photons absorbed and that 0.1 still counts as fluorescent.

Solar radiometer

 

This lovely present spins very fast in the sunshine but more slowly in the shade. I had an idea that it worked due to photons changing momentum on reflection but that couldn't be correct because it would mean that the shiny sides move away from you but in fact the black ones do. It works because it is not a full vacuum inside. There is a tenuous amount of air. The black absorbs infra-red radiation from the Sun best and becomes warm. This thermal energy transfers to the air in the bulb which expands next to the black surface and pushes it.

Iron ore in Dry Gill?

 

This rock in Dry Gill on Carrock Fell looked distinctly rusty. A magnet pulled off a few grains so I suspect it was a millimetre thick layer of some kind of iron ore. 

Getting the clock to keep time

The clock has been temperamental over the years so I have been loath to correct it. It has been running slow all that time. The pendulum keeps it in time and time period is directly proportional to the square root of the length. In this case it means the position of the mass. The position can be adjusted up or down by altering the screw. It is trial and error. First go made it run fast. When it is correct I shall be able to take a measurement and see if it does obey the formula.
 

Thin film interference on mother of pearl wardrobe

 

The lovely mother of pearl details on this wardrobe produced interesting pastel colours which changed as I moved. The theory is called thin film interference. Basically, there is partial reflection of light from the top surface so some light is transmitted to hit the bottom surface. Again, some of the light is reflectd back up from the bottom surface and this will interfere with the reflected light from the top surface. Because they have gone different distances, their waves will be at different points in the cycle. If they line up so that peak meets peak, there is constructive interference are we see a colour. The colour will change with movement because a different angle will mean that the light has travelled a different distance through the mother of pearl and with each colour having a different wavelength, that change in distance will affect whether peak meets peak. What has been bothering me is that you don't see the true colours of the classic rainbow spectrum. This article lists browns, golds, turquoises, teals, magentas. Some of those are secondary colours so more than one wavelength must be experiencing constructive interference. The article talks about bandwidth.

Update on the hammock

 If you look at a hammock from side on, the view is the top one. I was foxed by this view and thought the string length was important. But the end-on view below is the correct analysis. The mathematical analysis of the time period of a pendulum can be found here. It is height h that is important and that will be the 80cm I calculated from the time period.

Time period of a hammock

 

I was laid up in this very comfortable hammock. I timed 10 complete oscillations at 18 seconds. Can this be modelled as a pendulum? Using the formula T=2.pi Root(L/g) would give L=80cm. Modelling for a point mass in the middle I'd estimate that the length should be twice that. One thought is whether it is the length of the pendulum string that matters or the vertical height below the suspension point. If it is the latter, then the estimate will be about right. I need to do some maths!

Rainbows and droplet size

There was a fabulous rainbow this morning. You could see the supernumerary bow underneath it. This is produced by interference whereas the main rainbow is by refraction. Turns out it was explained by Thomas Young by noting that a single reflection in a droplet could produce different path lengths and thus interference effects. See here. I also found this which says that strong red at the top of the rainbow means heavy rain but that a supernumerary bow means small droplets. This site shows how droplet size affects what you see. Looks like the droplets might have been 800 microns today.
 

Bottle top that flew off

 

We were sat at table when the blue top just flew off. It was a very hot day. My first thought was that the milk had gone off and a build up of gas from the bacteria had increased the pressure enough to blow off the lid. But the bottle had not long come out of the fridge and was cold. So my next thought was that the cold air in the bottle had lower pressure than the warm air in the room. As thermal energy conducted into the air in the bottle, the extra energy made the air particles move faster, increasing the pressure until it was greater than atmospheric and the lid blew off. But now that makes no sense because the temperature in the air in the bottle cannot be greater than that in the air. Maybe the colder air, being more compact, had more particles in the same volume which could lead to a higher pressure than the external when thermal equilibrium was reached.

Dalwhinnie Winter Gold: why doesn't it freeze?

 

Thanks to Paul for this treat! It is kept in the freezer for 24 hours before serving but doesn't melt. I found a graph of freezing temperature versus % alcohol here and this would need -30 degrees Celsius to freeze, far below any domestic freezer. This has raised a load of questions. Firstly, what is a eutectic point? That's going to need some research. Secondly, what is it about alcohol that means that it has a different freezing point in the first place? More work to be done.

Sousaphone: Old School Funky Family at Greenbelt

 

Old School Funky Family from France were a complete surprise to me. I was wandering past the main stage; they pulled me in and got me moving. I could hear the bass line and then noticed that they didn't have a bass guitar. It turns out that the bass lines were being played by a sousaphone. I don't know much about brass instruments but I think that they are pipes with two open ends that produce a stationary wave with an antinode at each end and a node in the middle for the fundamental frequency. This means that the length of the tube is half of one wavelength of the fundamental. It turns out that a sousaphone is a variant on a tuba and is tuned to BBflat. So I looked up the data for such a tuba: the tube should be 5.5 metres long and will have a fundamental frequency of 29Hz. Now I'll check the data using v = f.lambda. If the tube length is half the wavelength, then lambda = 11m If speed of sound = 340m/s, then frequency = 340/11 = 31 metres. Not bad. It seems to work.

Thomas Trilby and Steve Kaos at Greenbelt: modified gravity

 

We were fortunate to find out about this juggling show that was a late replacement for a science show. Thomas Trilby makes me so happy. He was joking that the science content was "gravity": he's a juggler. I've been listening to one of Sean Carroll's Mindscape podcasts from earlier in the year about the problems that galaxies and dark matter cause with the accepted laws of Physics and that some theorists are pursuing "modified gravity". Here's one such idea. Were it to be true then we could do away with the need for dark matter halos, which would be handy because no one has yet found dark matter.