The speed of light in gases is slightly slower: It is about 0.05% lower in air at sea level.The speed of light in typical glass is about 2/3's of 'c'.In typical glass, the speed of light varies a bit with wavelength.For many decades, diffraction gratings have been used to separate colors of light.The shiny surface of a CDROM or DVD approximates that of a diffraction grating,You can aim the beam from a laser at a CDROM or DVD to see different reflections.If you start with a light beam generated with a LED (Light Emitting Diode; which has a much wider spectrum than a laser) you could see the spectrum it possesses.
by Ecole Polytechnique Federale de Lausanne
In Jules Verne's famous classic 20,000 Leagues Under the Sea,
the iconic submarine Nautilus disappears into the Moskenstraumen, a
massive whirlpool off the coast of Norway. In space, stars spiral around
black holes; on Earth, swirling cyclones, tornadoes and dust devils rip
across the land.
All
these phenomena have a vortex shape, which is commonly found in nature,
from galaxies to milk stirred into coffee. In the subatomic world, a
stream of elementary particles or energy will spiral around a fixed axis
like the tip of a corkscrew. When particles move like this, they form
what we call "vortex beams."
These beams imply that the particle has a well-defined orbital angular
momentum, which describes the rotation of a particle around a fixed
point.
Thus, vortex beams can give us new ways of interacting with matter,
e.g. enhanced sensitivity to magnetic fields in sensors, or generating
new absorption channels for the interaction between radiation and tissue
in medical treatments (e.g. radiotherapy). But vortex beams also enable
new channels in basic interactions among elementary particles,
promising new insights into the inner structure of particles such as
neutrons, protons or ions.
Matter exhibits wave-particle duality. This means that scientists can
make massive particles form vortex beams simply by modulating their
wave function. This can be done with a device called a "passive phase
mask," which can be thought of as a standing obstacle in the sea. When
waves at sea crash into it, their "wave-ness" shifts and they form
whirlpools. Physicists have been using the passive phase mask method to
make vortex beams of electrons and neutrons.
But now, scientists from the lab of Fabrizio Carbone at EPFL have
demonstrated that it is possible to use light to dynamically twist an
individual electron's wave function. They were able to generate an
ultrashort vortex electron beam and actively switch its vorticity on the
attosecond (10-18 seconds) timescale.
To do this, the team exploited one of the fundamental rules governing
the interaction of particles on the nanoscale level: energy and
momentum conservation. What this means is that the sum of the energies,
masses and velocities of two particles before and after their collision
must be the same. This constraint causes an electron to gain orbital
angular momentum during its interaction with an ad hoc prepared light
field, i.e. a chiral plasmon.
In experimental terms, the scientists fired circularly polarized,
ultrashort laser pulses through a nano-hole in a metallic film. This
induced a strong, localized electromagnetic field (the chiral plasmon),
and individual electrons were made to interact with it. The scientists
used an ultrafast transmission electron microscope to monitor the
resulting phase profiles of the electrons. What they discovered was that
during the interaction of the electrons with the field, the wave function of the electrons took on a chiral modulation, a right- or left-handed
movement whose "handedness" can be actively controlled by adjusting the
polarization of the laser pulses.
"There are many practical applications from these experiments," says Fabrizio Carbone. "Ultrafast vortex electron beams can be used to encode and manipulate quantum information; the electrons' orbital angular momentum can be transferred to the spins of magnetic materials to control the
topological charge in new devices for data storage. But even more
intriguingly, using light to dynamically twist matter waves offers a new
perspective in shaping protons or ion beams such as those used in
medical therapy, possibly enabling new radiation-matter interaction
mechanisms that can be very useful for selective tissue ablation
techniques."