Biodegradable mems. Vocabbox. A lot of machines made by physicists are used in medicine now, страница 2

A free-electron laser is a cross between a synchrotron radiation source and a conventional laser. Relativistic electrons from an accelerator are injected into a straight magnetic structure that causes them to "wiggle" as they pass through it. This motion causes the electrons to emit synchrotron radiation, which then bounces back and forth between mirrors at either end of the device, just like the light in a laser.

However, mirrors that reflect light at wavelengths below about 150 nm are not available, so the DESY team had to rely on the process of self-amplified spontaneous emission, in which the radiation emitted by the electrons interacts with the beam again, causing it to emit more radiation with the same wavelength. The team found that spontaneous emission around 100 nm had been amplified 150 times above its normal level by the laser, proving that the technique works at short wavelengths.

“DESY's work to date has been first rate,” says Todd Smith, who works on free-electron lasers at Stanford University in the US. “I expect to see it continue.”

The ultimate goal for the DESY free-electron laser is to produce X-rays with wave-lengths of about 0.1 nm. However, shorter wavelengths require higher electron energies, and any X-ray free-electron laser at DESY will depend on the TeV-Energy Superconducting Linear Accelerator (TESLA) being approved. This 3.3 km long machine would collide electrons and positrons for particle-physics experiments. It is accepted that there is only scope for one "next linear collider" in the world, and DESY is competing with Stanford in the US and KEK in Japan to host it.

Comprehension check

Say whether these sentences are true or false:

1.  A free-electron laser is a cross between a synchrotron radiation source and a conventional laser.

2.  Electrons emit X-rays just like the light in a laser.

3.  The DESY team developed mirrors that reflect light at wavelengths below 150 nm.

4.  Shorter wavelengths require lower electron energies.

Discussion

Comment on the text using the following prompts: first of all... , secondly…, as I see it…, I’m not sure about...

Give your own project of the new possibilities of improving lasers.

UNIT 3

BEYOND  THE  SPEED  OF  LIGHT

Vocabbox

noun collocations

§  superluminal effects/ speeds

§  ring-shaped slit

§  rectangular pulses

§  chamber

§  phase velocity

verb collocations

§  persist for distances

§  modulate with

§  plot the arrival time

§  submit to Nature

§  violate causality

Pre-reading task

Nothing can travel faster than the speed of light. Do you agree?

Reading

Read the text. Find some information about substances that can travel faster than light.

One of the most fundamental laws in physics is that nothing can travel faster than the speed of light. However, two teams of researchers have not shown experimentally that electromagnetic pulses can be made to travel faster than c, the velocity of light in vacuum, and over relatively long distances.

Such superluminal effects had previously been demonstrated in "tunnelling" experiments with optical and microwave radiation. However, these effects only occur over distances comparable to the wavelength of the radiation.

In one of the new experiments, Anedio Ranfagni and co-workers at the Institute for Research on Electromagnetic Waves in Florence, Italy, have shown that microwaves can be made to travel faster than c through air. Moreover, the superluminal effect persists for distances of around one metre - much further than in any previous experiment.