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"

Funny webs for funny science

Surfing into the net today, I have found two very amusing web sites that I strongly recommend you to visit.

• The first one, "The Naked Scientists", is, in their own words, "group of physicians and researchers from Cambridge University who use radio, live lectures, and the Internet to strip science down to its bare essentials, and promote it to the general public". Simply wonderful.
• The second one is "Cuaderno de bitácora estelar", by D. Barrado and B. Montesinos; it is a blog where interesting topics related with science and astrophysics fare presented so that everybody can understand them.

Enjoy them!

Laser cooling

If you want to know what laser cooling is, Nobel Laureate Steven Chu delivers a Lecture at UC Berkeley under the title "Laser cooling: from atomic clocks to watching biomolecules" that you can watch here:

The dancer

In which way is the woman dancing? Clockwise or anti-clockwise? Sure?
It spins in the way you want to!
Don't you trust us?
Think about the fact that the image is not a tridimensional (since it is on your PC screen, it can just have two dimensions), so it can not have a rotational movement. If you want to see it spinning on the other way, have a look for one moment to the other side of the screen.
But why do you see in one or the other way?
The image has been extracted from an austrilian newspaper:
http://www.news.com.au/heraldsun/story/0,21985,22556281-661,00.html
As it is explained here, it could depend on which side of your brain you use more.

Transverse modes

Have a look at this video:


What you are watching here are acoustic harmonics (pay attention to the way they change depending on the sound frequency). The acoustic waves produce vibrations on the table which are proportional to the the intensity pattern.

In a very simple model, a laser can be described as a cavity with an active medium inside. The field pattern measured in a perpendicular plane to the cavity is the so called transverse mode. They depend on the cavity geometry.

Some examples:



They are denoted by TEMplq, where p is the number of radial zero fields, l is the number of angular zero fields and q is the number of longitudinal fields. The TEMoo is usually preferred as it is easier to be focused.