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glossary

Diffraction: divergent nature of a wave due to its limited size
Numerical aperture ( optical system ): dimensionless number characterizing the aperture in the object space or in the image space and defined as the product of the refraction index by the sine of half the aperture angle
Image space: space conjugate of the object space by an optical system
Wavelength ( of a light wave ): distance between two consecutive identical states of the electric ( or magnetic ) field
Airy pattern: pattern representing the intensity distribution of a plane monochromatic light wave diffracted by a circular aperture