Glossary

Diffraction: divergent nature of a wave due to its limited size
Effective focal length: algebraic distance from the primary/secondary principal plane of an optical system to its front/back focal plane
Lens: optical component with two refractive surfaces axially symmetrical for redirecting the light in a controlled manner
Back focal plane: plane conjugate by an optical system of an object plane perpendicular to the optical axis and located at infinity
Wavelength ( of a light wave ): distance between two consecutive identical states of the electric ( or magnetic ) field