Law of reflection: the angle of incidence equals the angle of reflection, both measured from the surface normal, and the incident ray, normal and reflected ray lie in one plane. For a plane mirror, the image is the object reflected across the mirror line: virtual, upright, same distance behind the mirror. Rotating the mirror by θ turns the reflected ray by 2θ.
Spherical mirrors: with focal length f = R/2 the paraxial mirror equation is 1/d_o + 1/d_i = 1/f and magnification m = −d_i/d_o. Concave mirrors (f > 0) make real images for d_o > f; convex mirrors (f < 0) always make smaller virtual images. Rays far from the axis miss the paraxial focus: spherical aberration.
Two mirrors at angle α form N = 360°/α − 1 images when 360°/α is an integer. Each reflection multiplies intensity by the reflectivity ρ, so the k-th image has brightness ρ^k.
Simplifications: geometric optics only (no diffraction, no wavelength dependence, no polarization or Fresnel angle dependence: reflectivity is a constant), ideal mirrors with no thickness or glass layer, 2D top-down geometry, object treated as a point or small arrow.