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Moment of truth
The fastest man-made object that ever leave the Earth is the New Horizon spacecraft, launched in January 2006, and attained a speed of more than 36000 mph.

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reciprocal lattice Chem., Phys.
See reciprocal space.

reciprocal space Chem., Phys.
In crystallography, reciprocal space, also called Fourier space, momentum space or k-space, is an 'encoded' version of real space - the actual geometrical positions of atoms arranged in a real crystalline material. Reciprocal space can be defined as a set of encoded, or imaginary points arranged in such a way that the direction of a vector from one point to another is the direction of a normal to the real space planes, while the separation of those imaginary points is equal to the reciprocal of the real interplannar distance.

In diffraction analysis, the X-ray is diffracted by the crystal lattice of a sample material (real lattice) to produce an image of diffracted pattern, which consists of a series of spots arranged in the reciprocal space. Such an image is also called the reciprocal lattice. Diagram below illustrates the relationship between the real and reciprocal lattices.

reciprocal and real lattices

Diagram on the left shows two sets of crystal planes, outlined in black and red lines, the corresponding reciprocal lattice is shown on the right. Note that lattice spacing with larger d value has a corresponding smaller vector value. Normally, the reciprocal vector scales as 2p/d.

rectified spirit Chem.
Aqueous ethanol (ethyl alcohol) containing 4.4% water and 95.6% of the alcohol. It is an azeotropic mixture, boiling at 78C.

red shift Astron.
Change in the wavelength of light emitted from a source moving away from an observer. In the visible light range, red light is less energetic (longer wavelength) than blue light. The relative recessional motion (Doppler effect) causes the wave to have an observed wavelength longer (and hence redder) than it would if it were not moving. It is thought that the cosmological red shift is caused by the stretching of space as the universe expands. Distant galaxies and quasars have long redshifted spectra and it is thought that the Universe expands at a faster rate near to its edge.

refining Min. Ext., Chem.
The process of removing impurities from substances such as oil, metals and sugars or extracting substances from mixtures.

reflux Chem.
A technique for carrying out a slow chemical reaction over long periods in order to increase yield in which the reaction mixture is boiled in a flask attached to a condenser. The technique is usually applied in organic synthesis involving volatile chemical substances. The reflux condenser, usually a Liebig condenser ensures the volatile chemical substances are recondensed back into the reaction flask while the mixture is boiling.

refractive index Phys.
The ratio of electromagnetic waves propagation in vacuum to that of a medium. Since electromagnetic waves (e.g. light) always travel faster in vacuum than any medium (solid, liquid or gas), the value is always greater than 1. The value usually increases with frequency of the electromagnetic waves. Some example of the refractive index: water = 1.30; sodium chloride (salt) crystals = 1.5.

regolith Astron.
The layer of material deposited on the surface of the moon and other planets and asteroids. Made by meteoritic impact, the material can be of anything from rocky debris to fine powders.

regolith Earth Sci.
The fragment of rock material that overlies bedrock and surface of land. These materials can be residual of local samples or being transported elsewhere.

relative density Chem.
The ratio of the density of a substance to the density of some reference substance. For liquids and solids, the reference substance is usually pure water with a density of 1.0 g/cm3. For gases, the reference density can be that of air at 0C and 1 atm pressure. Sometimes relative density is abbreviated as r.d. and is also called specific gravity.

relative atomic mass Chem.
The ratio of the mass per atom of an element, averaged over all of the naturally occurring isotopes of the element to 1/12 of the mass of a carbon-12 atom. It is also called the atomic weight.

resistivity Elec. Eng.
Intrinsic electrical resistance of a conductor, describes in terms of its dimencions. It is defined as Ra/l where R is the resistance, l is the length of the conductor and a is the cross-section of the conductor. Resistivity is in unit ohm meter (Wm) and is also defined as the reciprocal of the electrical conductivity.

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