Emphasized words (those in italics) in Chapter 1 of Lectures on Quantum Mechanics, by Gordon Baym:
Polaroid filter
correspondence principle exactly probability complex basis superposition
principle orthonormal basis probability amplitude transformation matrix
eigenvector eigenvalue angular momentum spin operator for the photon definitely
definitely expectation value eigenvalue eigenstate operator completeness
relation probability amplitude superposition interference never
indistinguishable probability amplitudes or amplitude amplitude exactly
interference completely unpolarized or not mixed state mixed case pure case
extraordinary ray ordinary ray optic axis transition amplitude transition
probability change unitary.