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Shining a very bright light into very dark corners

What are our eyes for?

Well, it depends who you ask! If you ask a biologist this question you will probably get an answer along the lines of:

‘They are used to gather information about the world around us’.

If you ask a physicist you may get an answer along the lines of:

‘They are used to reduce the information we receive from the world around us to a manageable level’.

Ahh!!!!

So what’s going on?

Our eyes see only a tiny part of the information that the universe continually sends us.

The electromagnetic spectrum is vast, and until about 150 years ago we had no idea that nearly all of it even existed. Our eyes are sensitive to about a thousand billionth of one per cent of what is actually there.

When you look at a diagram of the electromagnetic spectrum, notice that the scale is logarithmic. That means each point marked shows a wavelength ten times bigger than the next. If the diagram was shown on a normal linear scale, the whole thing would be 100km (about 62 miles) long. The bit we can see with our eyes would be thinner than a human hair!

The electromagnetic spectrum

The whole spectrum is composed of the same phenomenon, electromagnetic waves. The only difference between them is their wavelength. The longest wavelengths found are around 2500Km (about 1553 miles long) and the shortest about 10-15m. What’s amazing is whatever their length, they are made in the same way.

Take a charged particle, any charged particle,an electron or a proton for example. Now accelerate it! That is, change its velocity. As it accelerates it will lose energy as electromagnetic waves! It’s a very simple recipe and it works every time! Delia would be proud!

Want to make a radio wave? Simple!

Just take a conductor (a piece of copper wire is ideal) and put an alternating voltage across it. The charged particles in the wire vibrate back and forth as the voltage changes direction. This change in velocity causes them to lose energy as electromagnetic waves. This is how you make a radio transmitter.

Want to make a radio receiver? No problem!

If you want to make a radio receiver all you need is another conductor (the aerial ideally should be about the same length as the wavelength of the wave). The electromagnetic fields in the electromagnetic wave will grab hold of the charged particles in the aerial and cause them to vibrate at the same frequency as the electromagnetic wave. The aerial therefore has an alternating current flowing in it.

By changing the amplitude (AM radio) or the frequency (FM radio) information can be sent from place to place by electromagnetic waves. So strictly speaking, ‘Strictly’ is transmitted courtesy of dancing electrons. Didn’t they do well!

Antenna

Can you think of what charged particles are accelerating (or vibrating) to make other parts of the em spectrum?

Seeing detail

We can use electromagnetic waves to ‘see’ detail. You are using your eyes to do this now. The amount of detail any microscope sees is limited by the wavelength of the electromagnetic radiation they use. For a visible light microscope the wavelength is a bit under a millionth of a metre, so the smallest thing you can see is about a millionth of a meter across.

This sounds fantastic but if you think of a millionth of a meter being only about a thousandth of a millimetre is doesn’t seem quite so fabulous. A typical cell membrane is a about a tenth of a millionth of a metre across, so can only just be detected and is not really seen with any clarity at all.

The highest usable magnification for a visible light microscope is about X1500. What would be nice would be to have a microscope which works at much shorter wavelengths and therefore can see much more detail. In fact, it would be really nice to have a microscope that works at such a short wavelength that we could see individual molecules.

Doughnuts and the diamond light source

Deep in the heart of darkest Oxfordshire lives a doughnut that even Homer would be proud of. Its job is to make very short wavelength electromagnetic radiation that is incredibly bright. It is, in short, a very, very powerful microscope.

The Doughnut

It makes the light by firing electrons around a circular racetrack. Because anything moving in a circle is accelerating towards the centre, the electrons lose energy. In this case, because they are accelerating towards the centre so quickly, the electromagnetic radiation emitted is in the X-ray part of the spectrum and it is over one thousand times brighter than a hospital X-ray machine.

So what can you do with a very pure, very bright source of X-rays?

Lots!

Medical researchers can look at the structure of individual molecules and design drugs that will ‘key’ into specific sites on cells. This will allow more effective treatments.

The structure of individual protein molecules can be seen. Some proteins become ‘folded’ and cause infectious diseases such as Creutzfeld-Jacob-Disease (CJD), Alzheimer's Disease and BSE. By studying these mechanisms, vaccines may one day be developed.

DNA

What the future holds

The structure of all living cells can be revealed at a fundamental level. This means that the possibilities for practical biological research are endless. The understanding of allergic responses, pathogens, gene sequences, and drug design are all massively enhanced.

Chemists can study the structure of polymers and see how they change when put under stress or degrade when exposed to different environments. This will allow the design of even better, stronger materials.

Nanomaterials can be studied allowing the next generation of semi-conductor devices to be developed. The beams are used to study the structure of superconductors with a view to developing new high-temperature superconducting materials for the future. Many of these are likely be polymer films and will be used in machines that have not even been invented yet, but you will be carrying around in your pocket in a few years time!

Project

Can you use an old CD to make a spectroscope? Google ‘CD spectrometer’ for help! If you get it right you should be able to show the very different spectra made by florescent tubes, LEDs, and filament lamps. You may even be able to see the dark lines in the spectrum of the sun.

BE VERY CAREFUL and ask your teacher for help if you want to try this. The light from the sun is easily bright enough to permanently damage your eyes. Send us some pictures of your device and the spectra you produce.

The big questions:

What would the universe look like if our eyes could see in different parts of the electromagnetic spectrum?

Can other creatures see different parts of the spectrum from us?

Could there be other ways of explaining the gravitational effects of dark matter?