Fundamental concepts about Optics

Book: Optics F2f From Fourier to Fresnel

A particularly useful solution of the wave equation is a wave with a particular wavelength λ. This is known as the harmonic wave solution and corresponds to the case of monochromatic light

A phasor is a unit vector in a fictional plane representing the phase of a wave, φ. The axes are directions in virtual space, not real space.
A phasor vector is only a graphical representation of phase and has nothing to do with the electric field (or polarization) vector
A convenient way to represent the phase of any wave at a particular position and time is using a phasor—a unit vector that rotates anticlockwise in a fictional plane with an angle φ relative to the positive horizontal axis

frequency—the number of waves passing a point in unit time
define a spatial frequency as the number of waves per unit length—or wave ‘density’.
spatial frequency is the number of waves per unit length in a particular direction
The density of wave fronts—number of wave crests per unit length—is know as the spatial frequency

For a harmonic wave—monochromatic light— the components of the wave vector (kx, ky, and kz) are the phase change per unit length in a particular direction (x, y, and z, respectively).

Often in optics it is possible to consider only one component of the field. This is known as the scalar approximation

All light fields can be described in terms of a superposition of waves with either planar or curved wave fronts.

what is meant by a wave front. A wave front is a surface of constant phase.
The main distinction between plane and spherical (or cylindrical) waves is whether the wave fronts are planar or curved in the region of interest