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.
Optical wedge : is a strip of material, clear at one end and gradually increasing in opacity, which is used to determine the effect of light intensities on sensitized materials.
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.