When the sum of the radii and the distance between the centres are equal, the circles touch externally. If the difference between the radii and the distance between the centres are equal, then the circles touch internally.
Kerr cell : high speed shutter without moving parts, using two crossed polarizing filters at either end of a cylinder filled with nitrobenzine.
Deoxyribonucleic Acid [DNA]
Deoxyribonucleic acid is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. DNA and RNA are nucleic acids; alongside proteins, they compose the three major macromolecules essential for all known forms of life.