Colour compounds

We observe color as varying frequencies of electromagnetic radiation in the visible region of the electromagnetic spectrum. Different colors result from the changed composition of light after it has been reflected, transmitted or absorbed after hitting a substance. Because of their structure, transition metals form many different colored ions and complexes. Color even varies between the different ions of a single element - MnO4− (Mn in oxidation state 7+) is a purple compound, whereas Mn2+ is pale-pink.

Coordination by ligands can play a part in determining color in a transition compound, due to changes in energy of the d orbitals. Ligands remove degeneracy of the orbitals and split them in to higher and lower energy groups. The energy gap between the lower and higher energy orbitals will determine the color of light that is absorbed, as electromagnetic radiation is only absorbed if it has energy corresponding to that gap. When a ligated ion absorbs light, some of the electrons are promoted to a higher energy orbital. Since different frequency light is absorbed, different colors are observed.

The color of a complex depends on:

* the nature of the metal ion, specifically the number of electrons in the d orbitals
* the arrangement of the ligands around the metal ion (for example geometric isomers can display different colors)
* the nature of the ligands surrounding the metal ion. The stronger the ligands then the greater the energy difference between the split high and low 3d groups.

The complex ion formed by the d block element zinc (though not strictly a transition element) is colorless, because the 3d orbitals are full - no electrons are able to move up to the higher group.
Wikipedia







Free Electron Laser instability

The free-electron laser (FEL) is in a sense an extension of the undulator radiation source that has proven so useful to the synchrotron community. An undulator is a periodic magnet array that imposes a periodic deflection on a relativistic electron beam. Interference effects enhance the probability of each electron emitting radiation at wavelengths selected by a phase match between the electron energy and the undulator period. Ordinarily, these interference effects apply independently to the radiation probability for each electron, with no inter-electron effects. However, with a very long undulator and a carefully prepared electron beam, an effect
arises that is known as the FEL instability. It introduces correlations between the electrons, and opens the possibility of greatly enhanced peak x-ray brightness. This instability produces exponential growth of the intensity of the emitted radiation at a particular wavelength. The radiation field that initiates the instability can be either the spontaneous undulator radiation or an external seed field. In the case of FEL action arising from spontaneous radiation, the process is called self-amplified spontaneous emission (SASE). If an external seed is used then the FEL is referred to as an FEL amplifier.

X ray free-electron lasers, Journal of Physics, (2005), http://iopscience.iop.org/0953-4075/38/9/023/pdf?ejredirect=.iopsciencetrial



Coriolis Effect
In physics, the Coriolis effect is an apparent deflection of moving objects when they are viewed from a rotating frame of reference.

The effect is named after Gaspard-Gustave Coriolis, a French scientist who described it in 1835, though the mathematics appeared in the tidal equations of Pierre-Simon Laplace in 1778. The Coriolis effect is caused by the Coriolis force, which appears in the equation of motion of an object in a rotating frame of reference. The Coriolis force is an example of a fictitious force (or pseudo force), because it does not appear when the motion is expressed in an inertial frame of reference, in which the motion of an object is explained by the real impressed forces, together with inertia. In a rotating frame, the Coriolis force, which depends on the velocity of the moving object, and centrifugal force, which does not depend on the velocity of the moving object, are needed in the equation to correctly describe the motion.

Perhaps the most commonly encountered rotating reference frame is the Earth. Freely moving objects on the surface of the Earth experience a Coriolis force, and appear to veer to the right in the northern hemisphere, and to the left in the southern. Movements of air in the atmosphere and water in the ocean are notable examples of this behavior: rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, winds and currents tend to flow to the right of this direction north of the equator, and to the left of this direction south of the equator. This effect is responsible for the rotation of large cyclones (see Coriolis effects in meteorology).

Wikipedia