Geometrical optics

Geometrical optics (or ray optics) treats light as rays that travel in straight lines through uniform media and change direction only at surfaces. This approximation works whenever objects and apertures are much larger than light's wavelength, and it is the framework for analyzing mirrors, lenses, and image formation — wave effects like diffraction and interference belong to physical optics instead.

Two laws govern ray behavior. The law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal to the surface. The law of refraction (Snell's law), n₁ sin θ₁ = n₂ sin θ₂, describes how rays bend when passing between media with different indices of refraction: light entering a denser medium bends toward the normal. When light travels from a denser to a less dense medium beyond a critical angle, it undergoes total internal reflection — the principle behind fiber optics.

Mirrors and lenses form images predictably from these laws. The thin lens and mirror equation, 1/f = 1/o + 1/i, relates focal length to object and image distances, and magnification m = −i/o gives image size and orientation. Sign conventions determine whether images are real or virtual, upright or inverted: converging (convex) lenses and concave mirrors can form real images, while diverging lenses and convex mirrors always form reduced virtual images. The same machinery explains the human eye and its refractive errors — myopia corrected with diverging lenses, hyperopia with converging ones.

The MCAT tests geometrical optics in its Chemical and Physical Foundations section: applying Snell's law, using the thin lens equation with correct signs, identifying image types, and connecting optics to vision correction.