Outreach scientific videos

If you are interested in Physics, you will like this channel:

Youtube: El origen de la simetría

The project is run by undergrad students of the University of Salamanca. The aim of this channel is providing a source of outreach videos for the general public. The experiments presented in the videos are amusing and the explanation is very clear.

It covers many disciplines: experiments range from the operation principles of a LCD to the explanation of a storm.

Enjoy them!

Sam's Laser FAQ

The homepage of "Sam's Laser FAQ" looks on the first moment very old and strange but actually, it is a real treasure about lasers. Sam's Laser FAQ exist already since 1994 by Samuel M. Goldwasser and is updated until today. One has to understand this page as "a practical guide to lasers for experimenters and hobbyists". It is a huge page and the navigation is a bit unusual but nevertheless it is worth to spend some time on it. Have a look on the "comprehensive table of contents". There are tons of chapters and sections as well as many many pictures, covering the scope from different types of lasers to possible experiments to do by oneself. Actually, there is a certain change that you find your laser which you uses in the lab and how you can repair it ;). What is missing on the page is a section about pulsed lasers, especially fs or ps laser as well as amplifications schemes. However, Sam's Laser FAQ addresses mainly hobbiest and for them it is a true treasure... and many hearts of scientists will jump faster if they see the pictures ;).

ps: check out the laser equipment gallery

LAB2 - The Virtual Femtosecond Laboratory

LAB2 is a (for free) modular programming tool written in Labview which enables one to simulate basic experiments and devices daily used in femtosecond experiments.

LAB2

As examples they mention:

• propagation through a linear medium
• prism compressor
• second harmonic generation with depleted fundamental wave
• crystal wizard - how to find the phase matching angle

But to my knowledge there are more functions in the package.

LabII was developed in 1998 at the Institute for Optics and Quantum Electronics (IOQ) in Jena. Since Fall 2004 it is at the Institute of Applied Physics in Bern with Prof. Thomas Feurer. The download is free but you have to register. What I like is the detailed manual which will help you to use efficiently LAB2. Basics and concepts are introduced too and always worth to read it. If you have used LAB2 already, please write a comment about your experience with it.

"Your Own Laser Experiments"

Doing laser experiments at home are in most cases challenging due missing tools. Nevertheless some easy but still very interesting experiments can be done demonstrating fundamental phenomena. By chance I found a homepage which describes in detail and well explained some very interesting and easy to do experiments.

The page is the EU-HOU project: “Hands on Universe, Europe. Bringing frontline interactive astronomy to the classroom” and has the goal of renewing the teaching of science and awaking the interest of science to the young generation. The page covers mainly experiments from astronomy but some nice optics experiments (important for astronomy) are presented too.

EU-HOU, startpage

Four easy laser optics experiments can be found here:

EU-HOU, Laser Experiments
EU-HOU, home-made spectroscope

They handle of the:

• “Refraction , or bent light”
• “Determination of the index of refraction by a laser pointer”
• “Eureka, Light is a wave”
• "A home-made spectroscope"
  

My favorite is the third one but all of them are great. The easiest one to perform is the first one and very interesting for many peoples will be the last one. But let me show you some more pictures:

(bent light)

(measuring the refraction index)

(halogen bulb and its spectrum)

All the pictures are taken from the EU-HOU page. Note that the EU-HOU page is addressed for teachers but every pupil (high school) can do it too. And please,

never never look into a laser beam!

It can cause serious irreparable eye damage! Hence, take care even with a laser pointer. Enjoy the experiments!

Modelling an artificial eye with a CCD camera: see how they see!

In September I attended the "IX Reunión Nacional de Óptica" (Spanish National Optics Meeting); one of the posters presented there by the Optics Group from Zaragoza (J. Ares, V. Collados, J. Arines and A. Sánchez-Cano) was "Adiestramiento de la refracción subjetiva con ojos simulados mediante cámaras web". In this communication, the authors reported a system based on a CCD camera to simulate an artificial eye that is used to teach optics and optometry.

Let's start by explaining how the eye works.

Figure 1: Scheme of the eye

In the image an schematic view of an eye is represented; in a very simple model, we can consider the eye as a system formed by a refractive surface, the cornea, and the lens. In a normal eye, the light is focused at the fovea and the image is well-defined. When a person suffers from myopia, light is focused before the fovea; when a person suffers from long-sight, image is formed after the fovea.

Now, how can it be modelled with a web camera? Well, basically the webcam is a CCD sensor with a lens. If we replace this lens by a 35mm focal length lens (typical value for a human eye) and a diaphragm (which will act as a pupil), we can build an artificial eye.

Once, the webcam is ready you can try how people see. Ask for example for a pair of glasses and place them immediately before the webcam. If the person who has lent you the glasses suffers from myopia, you will see a blurred image: this is how this person sees without glasses (or better said, the opposite to the way this person sees: it corresponds to the vision of a long-sighted person with the same gradation):

Figure 2: Pictures from the first row represent a near-sighted eye without (left) and with (right) glasses to correct the myopia. The second row corresponds to a normal eye (ie., the webcam) without (left) and with (right) the same lenses as used in the previous case.

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"