Optics encyclopedia
Look up the quantity, see the equation and work through an example with units.
Rayleigh criterion
Estimate the angular separation of two distant points imaged through a circular pupil.
Numerical aperture
Describe a cone of light using its half-angle and the refractive index on the same side of the optical system.
Overlapping diffraction orders
A grating can send different wavelengths in different orders toward the same detector position.