Glossary — Key Ray Optics Terms — Notes

Term	Definition
Pole (P)	The geometrical center of the spherical mirror surface. Origin for all mirror measurements.
Center of Curvature (C)	The center of the sphere of which the mirror is a part. Distance = R from pole.
Radius of Curvature (R)	The radius of the sphere. R = 2f for a spherical mirror.
Principal Axis	Line passing through the pole and center of curvature. The optical x-axis.
Principal Focus (F)	Point where paraxial rays parallel to the axis converge (concave) or appear to diverge from (convex). f = R/2.
Paraxial Rays	Rays close to and nearly parallel to the principal axis. Mirror and lens formulas are valid only for paraxial rays.
Virtual Image	Image formed by apparent intersection of extended rays. Cannot be projected on screen.
Real Image	Image formed by actual intersection of reflected/refracted rays. Can be projected on screen.
Lateral Magnification	Ratio of image height to object height: m = $\frac{h_i}{h_o}$ .
Optical Center (O)	Point within a lens through which rays pass without deviation. All measurements for lenses from O.
Refractive Index (n)	n = $\frac{c}{v}$ ; ratio of speed of light in vacuum to speed in the medium. Dimensionless, always ≥ 1.
Critical Angle (θ_c)	Angle of incidence (in denser medium) at which the refracted ray travels along the interface (θ_r = 90°). sin θ_c = $n_{2}$ / $n_{1}$ .
TIR	Total Internal Reflection: complete reflection of light at an interface when angle > θ_c (denser to rarer).
Dispersion	Splitting of white light into colors due to different refractive indices for different wavelengths.
Dispersive Power (ω)	ω = $\frac{n_v − n_r}{(n_y − 1)}$ . Measure of how much a prism separates colors. Material property.
Dioptre (D)	SI unit of power of a lens. 1 D = 1 $m^{-1}$ . P = 1/f (f in metres).
Normal Adjustment	Telescope configuration where the final image is at infinity (relaxed eye). L = $f_o$ + $f_e$ .
Near Point (D)	Shortest distance at which the eye can see clearly. Normal = 25 cm. Used in magnification formulas.
Accommodation	Ability of the eye to change its focal length to focus on objects at different distances.