RM 14/09/04
We looked at spectral lines produced by different hot gases. Emission spectra are caused because of the limited number of distances that electrons can orbit their nuclei in each element. When excited by some external force the electrons "jump up" energy levels and often fall back down again. When they do this, they release energy in the form of electromagnetic radiation. The specific frequency of this radiation is proportional to the energy change that the electron has undergone. Each element has its own specific pattern of energy changes which are possible due to its unique electron structure. Therefore, when excited, each element produces a different pattern of frequencies of e-m radiation. In the visible range, we pick these up as different bands of colours. The light emitted by the excited gas must be seperated into its constituent colours by a spectrometer.
When a full spectrum is shone through a sample of an element, the element absorbs the same frequencies that it would emit if excited, leaving dark bands in the spectrum. These missing frequencies form the same pattern as they would for an emission spectrum and are known as an absorption spectrum.
These can be used by astronomers in a number of ways to discover things about the universe.
RM 14/09/04
We carried on talking about how to measure distances to other starts for a bit. Standard candles, which are objects of a known total brightness, allow you to know how far they are away by how bright they appear to us. Then we derived the red shift formula. If an object is moving away from you, any waves it emits will appear "stretched out". This is known as the Doppler effect and is familiar to us in sound waves. The same is true of the light emitted by stars. The absorbtion spectra within the starlight will also appear to have changed wavelength if their is relative motion between the star and the observer. Looking for a well known absorption spectrum and seeing how much it has moved can tell you how fast something is moving towards or away from you. Hubble did a telescopic survey in the early twentieth century of lots of objects in the sky. He catalogued their distance away, and compared it to the shift in the frequency of the light being emitted. He found that all distant objects in the universe are heading away from us. Their light is "redshifted". (Objects moving towards are "blueshifted". The amount that the light is redshifted tends to be proportional to the distance away. The further away something is, the faster it is moving away from us. This implies a universe which is expanding, and that all the matter we can see started out at one central point at some time in the past.
RM 21/09/04
We continued to look at the idea of the universe expanding. Parallax and then standard candles are methods used to determine distances out to stars whose redshifts can then be measured.
HW Talks on some aspect of cosmology in the news/popular science press.
RM 28/09/04
The balloon universe demonstrated the idea that space itself is expanding, which fits onbservations very well. We had some interesting talks on inflationary theory, the idea that the universe is actually accelerating in its expansion due to some mysterious form of energy present in the vacuum of space.
The fate of the universe probably depends on this as well as the amount that the expansion is halted by the force of gravity. This in turn depends on the average density of matter in the universe. This is not easy to find out as most matter is not lit up like stars. This matter is known as dark matter. If we can solve the problem of how much dark matter there is in the universe, we can better predict whether the universe will recollapse (Big Crunch), expand, but at an ever slowing rate (density of matter is equal to the so called "critical density) or just expand away for ever (when there is less than the critical density of matter).
If all matter were visible, there would be much less matter than needed in the universe for the critical density. However, galaxies rotate at speeds which imply that there is a lot of matter around the outside (in the halo) which is not visible.
HW 2 Cosmo questions from the back of the book.
RM 05/10/04
We practised some practical exam tasks. They proved to be fairly tough, you really have to get your skates on and work very accurately for these things. More on this next week.
RM 12/10/04
More practical exam practise.
HW Do planning a practical stage on impedance.
RM 19/10/04
We started looking at longitudunal and transverse waves. Longitudunal waves are made up of vibrations in a medium which are in the same line as the direction in which the energy of the wave is travelling. In transverse waves the vibrations are perpendicular to the direction of energy propagation.
Transverse waves can therefore have vibrations which occur in a range of 360 degrees. If transverse waves are all vibrating in the same direction they are known as polarised. Electromagnetic waves such as light can be polarised (electric field vibrating in one direction only). There are many uses for this type of light. More on this next week...
RM 02/11/04
RM absent.
RM 9/11/04
Prize day.
RM 16/11/04
The speed of waves was discussed. Wavespeed is the distance travelled by the energy of the wave divided by the time taken to do it. The distance travelled during one oscillation is the wavelength. The time taken for one oscillation is the period.
Wavespeed = Wavelength/Period
Frequency of a wave is number of oscillations per second = 1/Period
Wavespeed = Frequency times Wavelength
The speed that a wave travels at depends upon the mechanical properties of the medium it is passing through. e.g. a sound wave will travel much faster through a solid than a gas ("stiffer" material passes energy through more quickly.) The restoring force in a transverse wave will similarly alter the speed of the wave.
Light (em waves) is an oddity in that it appears to have no medium. The idea of a medium for light, the "ether", was dispelled when Michelson and Morley did an experiment to test for the Earth's motion through this mysterious substance using an interferometer.
HW Research 2 methods of determining the speed of light. 1 astronomical (using moons of Jupiter) and the other using a rotating cylinder of mirrors and explain how they work.
RM 23/11/04
Standing waves were discussed. When a wave interferes with its own reflection, standing waves can be produced. They appear not to be moving at all, with stationary points (nodes) and constantly moving points (antinodes). These kind of waves can be produced in any type of wave that will reflect. However, the length of the wavesource to reflector distance must be of a specific size to allow a standing wave to form. If reflection end is fixed, this point must be a node. The distance between nodes in a standing wave = half the wavelength of the original wave which is causing the stationary wave.
HW Qs on chapter 17 from the book.
RM 30/11/04
Only 3 in. We nevertheless looked at methods using standing waves to measure the speed of sound by finding the resonant frequency of a column of air in a plastic tube. The tubes were open at the top and so you will have an anti-node at the end with a node at the closed end of the tube. This means that when resonance occurs the tube is 1/4 of a wavelength long. Knowing the driving frequency (from a sig gen + loud speaker) you can then make a calculation of the speed of sound. We got 334m/s - not half bad.
We also looked a standing wave in microwaves. The 3cm wave emitter was put opposite a reflecting surface, the emitted waves interefered with their own reflection, and the distance between nodes could be measured using a microwave detector.
We looked at the mess left by microwaving marshmallows. The idea is that a standing wave is set up inside your microwave oven and that the antinodes will be clear, where the most cooking has gone on. A rough estimate of 6cm between well cooked areas and an operating frequency of 2450MHz gave us a decent value for the speed of light. Try this at home with chocolate, apparently this can give clearer results. (you need to disable the turntable of the microwave).
We started doing/going through the Jan 2004 paper. We'll continue this next week.