Ray optics treats light as travelling in straight lines called rays, ignoring the wave nature of light. The subject covers reflection at plane and spherical mirrors, refraction at flat and curved surfaces, total internal reflection, thin lenses, prisms, and optical instruments — collectively responsible for 3–4 questions per year in NEET.
Reflection and Spherical Mirrors. The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal at the point of incidence. A plane mirror produces a virtual, erect, laterally inverted image of equal size at the same distance behind the mirror. Spherical mirrors are sections of a sphere. A concave mirror is converging with focal length f < 0 (Cartesian convention); a convex mirror is diverging with f > 0. The focal length f = R/2, where R is the radius of curvature. The mirror formula is 1/v + 1/u = 1/f (note the plus sign between the terms). All distances are measured from the pole P of the mirror; object distance u is always negative (object always to the left); f is negative for concave, positive for convex mirrors. Magnification m = −v/u = h_i/h_o; positive m indicates an erect image and negative m an inverted image; |m| > 1 means magnified, |m| < 1 means diminished.
Cartesian Sign Convention. The Cartesian sign convention is the most critical concept for NEET numerical questions. All distances are measured from the pole (mirror) or optical center (lens). Incident light travels from left to right — this is the positive x-direction. Distances to the right of the reference point are positive; to the left are negative. Heights above the principal axis are positive; below are negative. Consequence: u is always negative for real objects; concave mirror f < 0; convex mirror f > 0; convex lens f > 0; concave lens f < 0.
Refraction and Snell's Law. When light crosses the boundary between two media, it obeys Snell's law: n_{1} sin θ_{1} = n_{2} sin θ_{2}. The refractive index n = c/v, where c is the speed of light in vacuum. Light entering a denser medium (higher n) bends toward the normal; entering a rarer medium it bends away. Total internal reflection (TIR) occurs when light travels from a denser medium to a rarer medium and the angle of incidence exceeds the critical angle θ_c where sin θ_c = n_{2}/n_{1}. Both conditions are mandatory: (1) denser to rarer medium, and (2) angle exceeds θ_c. Applications include optical fibres, mirage formation, diamond brilliance (n = 2.42, θ_c = 24.4°), and totally reflecting prisms.
Thin Lenses. The thin lens formula is 1/v − 1/u = 1/f (note the minus sign — unlike the mirror formula). Object distance u is always negative for real objects; convex lens f > 0; concave lens f < 0. Lens magnification m = v/u (no negative sign, unlike mirrors). This distinction between mirror and lens magnification formulae is one of the most tested NEET topics. Power of a lens P = 1/f (f in metres), measured in dioptres (D). Convex lenses have positive power; concave lenses have negative power. For thin lenses in contact: P = + . The lensmaker's equation 1/f = (n − 1)(1/ − 1/) relates focal length to the geometry and material of the lens. For a biconvex lens with equal radii R: 1/f = 2(n−1)/R.
Prisms. A prism deviates light toward its base. Deviation δ = (i + e) − A, where i = angle of incidence, e = angle of emergence, and A = apex angle. At minimum deviation, i = e, the refracted ray inside the prism is parallel to the base, r = A/2, and the refractive index is given by n = sin((A + δ_m)/2)/sin(A/2). For a thin prism (A < 10°), the approximate formula δ = (n − 1)A may be used. Dispersion — the splitting of white light — occurs because different wavelengths have different refractive indices. Dispersive power ω = (n_v − n_r)/(n_y − 1).
Optical Instruments. A simple magnifier has M = 1 + D/f (image at near point D = 25 cm) or M = D/f (image at infinity). In a compound microscope, the objective lens has a short focal length (creates a real, magnified intermediate image) and the eyepiece (longer focal length) magnifies this image further. Magnification at infinity: M = −(L/f_o)(D/f_e), where L is the tube length. In an astronomical telescope operating in normal adjustment (image at infinity), M = −f_o/f_e and tube length L = f_o + f_e. The objective of a telescope has a long focal length; the objective of a microscope has a short focal length — these are opposite, and confusing them is a common NEET error.
NEET Strategy. The most commonly tested elements are: (1) mirror and lens formula calculations with sign convention, (2) TIR conditions and critical angle, (3) prism minimum deviation, (4) power of lens combinations, and (5) optical instrument magnification. The most dangerous errors are using a positive focal length for a concave mirror, applying the mirror magnification formula to a lens, and using the thin prism formula for large prism angles.