First light from world's first hard X-ray laser

A couple of days ago, it was published that the the world's first X-ray laser (LCLS) has achieved "first light".

When fine tuning is complete, the LCLS will provide the world's brightest, shortest pulses of laser X-rays for scientific study. As tool for studying the arrangement of atoms in materials, this source will find a wide range of applications in science.

(I love this video, you can really feel the passion of these scientists for their work)

But do you know how it works?

Imagine an accelerated electron bunch which goes through a sinusoidal transverse magnetic field. It will experience a force given by F=q(E+vxB) (Lorentz's force) and thus the trajectory will be sinusoidal too; if the trajectory is a curve, the electrons suffer centripetal acceleration. As it is well known, a charged accelerated particle emits radiation (Lienard-Wiechert). In the Spring-8 website, you can download a program to simulate the radiation emitted by an electron in different magnetic fields.

Due to the electron-radiation interaction, the electrons form electron bunches which emit coherently and the intensity is increased.

These systems are called free electron lasers (FEL); they consist on an accelerator (formed by klystrons which deliver microwave radiation to accelerate the electron and a resonator) and a ondulator (a periodic structure of magnets to produce the sinusoidal transverse magnetic field).

The main problem of these facilities is the size and cost.
Another approach to get X-rays is based on high power lasers: instead of the accelerator, a high-power laser accelerates the electrons which go later through the ondulator. This implementation leads to a reduction of energy and size (in fact, these systems are called Table-Top XFEL).

Prof. Sánchez Ramos awarded Best Invention of the Year

Spanish researcher Professor Celia Sánchez Ramos, has been awarded in Geneva with a prize for the Best Invention of the Year given by the World Intellectual Property Organisation.
Prof. Sánchez Ramos, who is a researcher at the Complutense University in Madrid (UCM), invented a light filter for contact lenses to protect the retina and prevent blindness.
Between 15% and 20% of the light spectrum consists of harmful colours for us (blue and violet), which destroy the retina neurons leading to a degeneration into the macula (DMAE); the fovea, a part of the retina structure, protects us from these colours. Prof. Sánchez Ramos proved that this part of the spectrum could be blocked by inserting a yellow filter into the lenses.
In spite of being yellow, the lens does not vary the perception of the person.
To test the efficiency of the lens, the researcher carried out a experiment with rats exposed to different light types and rabbits which had been previously operated for cataracs.
Nowadays, the UCM is carrying out a clinic trial in 23 hospitals with people operated for cataracs.

Michelson interferometer

Maybe you have ever heard that "light+light=darkness". Today, we will try to explain this and build a home-made Michelson interferometer to prove it.

Interferences

In physics, we define an interference as the superposition of two or more waves. Depending on the kind of wave, the pattern is different: for example, the interference of two spherical waves is a pattern of rings, while the pattern for plane waves are fringes.

But, why don't we see this phenomenon if we switch on two lamps?

Well, it has to do with the "coherence": for a simple explanation, we could translate this in waves coming from the same source or having nearly the same frequency. It is also possible to get them with non-monochromatic waves, but in these case they have to have the same range of wavelengths and same phase differences for each wavelenght (for instance, you can see interferences from sunlight when there is oil on a road).

Michelson interferometer

In the picture below, you can see what is called Michelson interferomter.

Let's explain how it works: coherent light emitted from a source (for example, a laser or a sodium lamp) travels up to a beamsplitter (BS), a device capable of dividing a beam into two beams (it is basically a glass with one of its surfaces partially reflective). From there, one of the beams goes to a first mirror (M1) where it is reflected; the same happens to the second beam. In the beamsplitter they meet again and travel together to the observation screen. Notice that there is an adittional glass (the blue one in the figure) to compensate the paths (the first beam goes twice through the beamsplitter and the second one just once).

In the following film from the Celtic Mad Scientist you can learn how to build your own Michelson interferometer, as well as applications and a detailed description of the setup:

For students of advanced levels, you should notice that if the interferometer was perfectly align you would get the interference of two plane waves with the same propagation vector (as they should be pararell) and thus there should not be interference fringes! Obviously, it is not the case here: since it is not perfectly aligned, there is a slighty angle between the two beams that allows you to see the fringes.

As curiosity you should know that, together with Edward Morley, Abraham Michelson tried to demonstrate with this system the existence of the "ether" in 1887. In the 19th century, it was believed that the Earth was surronded by a gas called ether. Michelson and Morley wanted to measure the speed of the Earth with respect to the ether. This movement will produce a wind and, since the light propagated through the ether, the speed of light would be different depending on the propagation direction. In one of the interferometer arms, the light would propagate in the same direction as the wind, but in the other one would propagate against the wind and thus more slowly.

However, they did not find any differences; the "failure" of the experiment showed the non-existance of ether and the propagation of light in vacuum.

In this link, you can find an interactive movie where you can reproduce the experiment and in the AIP Center for History of Physics you can download the original article.

How to measure the speed of light with a liquorice stick

Try the following experiment at home (all you need is a liquorice stick and a microwave oven):

Can you guess why the value you have calculated is the speed of light?
First of all, let's explain how the microwave oven works.

The oven acts as a cavity for the waves; thus, the waves are no longer travelling waves, but standing waves.

As you can see in the video, the amplitude remains zero at some points which are separated half wavelenght. For this reason, the stick is not burnt at these points.

Since the speed of light is the product of the frequency (given by the oven specifications) by the wavelength, which can be deduced from the distance between two non-burnt points, it is easy to get a good approximation for it.

This experiment has been adapted from "Cuaderno de bitácora estalar"