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a. a multiplet that has two members
b. a closely spaced pair of related spectral lines
2. two dice each showing the same number of spots on one throw
3. Physics two simple lenses designed to be used together, the optical distortion in one being balanced by that in the other
Two stationary states which have the same orbital and spin angular momentum but which have different total angular momenta, and therefore have slightly different energies due to spin-orbit coupling.
A source and a sink separated by an infinitesimal distance, each having an infinitely large strength so that the product of this strength and the separation is finite.
A lens made up of two components, especially an achromat.
Two elementary particles which have slightly differing masses and the same baryon number, spin, parity, and charge conjugation parity (if self-conjugate), but have different charges.
Two electrons which are shared between two atoms and give rise to a nonpolar valence bond.
Two closely separated spectral lines arising from a transition between a single state and a pair of states forming a doublet as described in the atomic physics definition.