@xris,
XRIS
I did not switch the lights off. "I am the body electric"
When you get to know me better you will find out that physics is one of my strengths
Here is an article on slow light only one amongst many
Jul 10, 2003
Fast and slow light made easy
Physicists have created "slow" and "fast" light in a crystal at room temperature for the first time. The team at the University of Rochester in the US used an 'alexandrite' crystal to reduce the speed of light to just 91 metres per second, and also to make a laser pulse travel faster than the speed of light.
Previously these effects - which are not in conflict with special relativity - had only been observed at cryogenic temperatures or in complicated experimental set-ups. The new technique could be used for applications such as optical data storage, optical memories and quantum information devices (M Bigelow
et al. 2003
Science 301 200).
Light travels at a speed of 300 million metres per second in a vacuum, but in recent years physicists have managed to slow laser pulses down to speeds of metres per second - or to bring them to a complete halt - in ultracold gases.
In similar experiments physicists have observed superluminal or faster-than-light pulse propagation. These effects have also been observed in crystals at cryogenic temperatures and in "hot" gases. Now Matthew Bigelow, Nick Lepeshkin and Robert Boyd have observed the same effects in a much simpler system - a crystal at room temperature.
All the experiments exploit changes in the refractive index of an optical medium caused by quantum interference effects. Whereas previous experiments relied on a process known as "electromagnetically induced transparency", the Rochester team exploited "coherent population oscillations" in the crystal.
This involves shining two lasers - a pump beam and a weaker probe beam - at the crystal. Under certain conditions the probe beam experiences reduced absorption over a narrow range of wavelengths. The refractive index also increases rapidly in this "spectral hole", which leads to a much reduced group velocity - the velocity at which a laser pulse travels - for the probe beam.
Earlier this year, the Rochester team used this technique to reduce the group velocity of a laser pulse to 58 metres per second in a ruby crystal at room temperature. Bigelow and co-workers have now repeated this feat in a crystal of alexandrite. Moreover, by using different wavelengths they can make a spectral "antihole" in which the absorption is higher, and which leads to superluminal propagation.
They observed light speeds of 91 metres per second for a laser with a wavelength of 488 nanometres, and minus 800 metres per second for wavelengths of 476 nanometres. Negative speeds indicate superluminal velocities because the pulses appear to leave the crystal before they enter it under these conditions.
"Our technique is applicable to many solid materials, not just alexandrite," Lepeshkin told
PhysicsWeb. "Another important feature of our approach is the ability to cover a fairly broad range of optical frequencies." The researchers will now investigate solid state materials with higher bandwidth to use in their system that are suitable for communication applications.
About the authorPhysicsWeb
Justin,
You are correct that light can only be seen by its reflection. Of course you are able to see a light beam in a smoke filled room etc
A common Physics demonstration involves the directing of a laser beam across the room. With the room lights off, the laser is turned on and its beam is directed towards a plane mirror. The presence of the light beam cannot be detected as it travels towards the mirror. Furthermore, the light beam cannot be detected after reflecting off the mirror and traveling through the air towards a wall in the room. The only locations where the presence of the light beam can be detected are at the location where the light beam strikes the mirror and at the location where the light beam strikes a wall. At these two locations, a portion of the light in the beam is reflecting off the objects (the mirror and the wall) and traveling towards the students' eyes. And since the detection of objects is dependent upon light traveling from that object to the eye, these are the only two locations where one can detect the light beam. But in between the laser and the mirror, the light beam cannot be detected. There is nothing present in the region between the laser and the mirror which is capable of reflecting the light of the beam to students' eyes.
But then the phenomenal occurred (as it often does in a Physics class). A mister is used to spray water into the air in the region where the light beam is moving. Small suspended droplets of water are capable of reflecting light from the beam to your eye.
It is only due to the presence of the suspended water droplets that the light path from the laser to the mirror could be detected. When light from the laser (a luminous object) strikes the suspended water droplets (the illuminated object), the light is reflected to students' eyes. The path of the light beam can now be seen. With light, there can be sight. But without light, there would be no sight.
None of us are light-generating objects. We are not brilliant objects (please take no offense) like the sun; rather, we are illuminated objects like the moon. We make our presence visibly known by reflecting light to the eyes of those who look our way. It is only by reflection that we, as well as most of the other objects in our physical world, can be seen.
And if reflected light is so essential to sight, then the very nature of light reflection is a worthy topic of study among students of physics.And in this lesson and the several which follow, we will undertake a study of the way light reflects off objects and travels to our eyes in order to allow us to view them.
Next Section: The Line of Sight
Jump To Lesson 2: Image Formation in Plane Mirrors
Here is more , but I am a bit perplexed why in ended up in this thread??
http://physicsworld.com/cws/article/print/20
A very exciting possibility for optical communication concerns the recent discovery that light can be slowed down by many orders of magnitude - and even be brought to a complete standstill - in a cloud of ultracold atoms. The ability to halt light in its tracks could lead to remarkable new ways of storing and manipulating optical signals and even to new techniques for quantum computers and communications.
Go to the link very interesting?
