NEET · Physics
Own the geometrical-optics half of Unit 16 with strict New Cartesian sign conventions and model conditions. It covers reflection, spherical mirrors, refraction at plane/spherical surfaces, lenses/lens maker, TIR, magnification/power, lens combinations, prism and microscope/telescope.
In short
Ray Optics in NEET UG 2026 covers reflection and refraction, spherical mirrors, plane/spherical refracting surfaces, thin lenses and lens maker, total internal reflection, magnification, lens power/combinations, prism and optical instruments. Mirror/lens equations are only safe when the chosen New Cartesian sign convention is stated and applied consistently.
Subject faculty
Also in Physics: Kumar Gandharva Saxena
Use one New Cartesian convention: measure distances from the pole/optical centre, positive along the chosen positive axis, commonly the direction of incident light, and negative opposite it; heights above the principal axis positive and below negative. Every diagram/formula example must use the same convention.
Teach image construction and the mirror formula together. Magnification sign should connect to image orientation. Avoid memorizing "concave means negative" without tying it to the chosen axis and incident-light direction.
Angles are measured from the normal. Refractive index is medium and wavelength dependent; for a fixed monochromatic case, Snell law relates incident and refracted angles. At a stationary interface, frequency remains unchanged.
The thin-lens equation and lens maker relation require paraxial/thin-lens assumptions. Power is reciprocal focal length in metres. For thin lenses in contact, powers add under the ideal model.
Two conditions: light travels from optically denser to rarer medium, and incidence angle exceeds the critical angle. The critical-angle relation must use the two medium indices, not a memorized air-only form unless the rarer medium is air.
Prism formulas must state whether minimum deviation is assumed. Optical-instrument magnifying-power formulas must identify the viewing condition, such as normal adjustment vs final image at least distance; do not show one unlabeled formula as universal.
Current Unit 20 links to focal length of convex/concave mirrors and convex lens, prism deviation graph and refractive index of a glass slab.
Mirror formula: 1/f = 1/v + 1/u. Spherical mirror, paraxial rays, New Cartesian signs. Use consistent length units.
Mirror formula
Use when — Spherical mirror, paraxial rays, New Cartesian signs. Use consistent length units.
Mirror magnification: m = h_i/h_o = -v/u. Same sign convention.
Mirror magnification
Use when — Same sign convention.
Refractive index: n = c/v. Phase speed in medium for stated wavelength/frequency.
Refractive index
Use when — Phase speed in medium for stated wavelength/frequency.
Snell law: n1 sin i = n2 sin r. Angles from normal, isotropic media.
Snell law
Use when — Angles from normal, isotropic media.
Spherical refraction: n2/v - n1/u = (n2-n1)/R. Paraxial refraction at spherical interface with consistent Cartesian signs.
Spherical refraction
Use when — Paraxial refraction at spherical interface with consistent Cartesian signs.
Thin-lens formula: 1/v - 1/u = 1/f. Thin lens, paraxial rays, New Cartesian convention.
Thin-lens formula
Use when — Thin lens, paraxial rays, New Cartesian convention.
Lens magnification: m = v/u. Thin lens with same convention.
Lens magnification
Use when — Thin lens with same convention.
Lens maker: 1/f = (n_l/n_m - 1)(1/R1 - 1/R2). Thin lens in medium, paraxial surfaces. In air n_m approx 1.
Lens maker
Use when — Thin lens in medium, paraxial surfaces. In air n_m approx 1.
Lens power: P = 1/f(m). Thin lens; P in dioptre.
Lens power
Use when — Thin lens; P in dioptre.
Thin lenses in contact: P_eq = sum P_i. Thin coaxial lenses in contact.
Thin lenses in contact
Use when — Thin coaxial lenses in contact.
Critical angle: sin i_c = n2/n1. n1 > n2, ray from denser medium 1 to rarer medium 2.
Critical angle
Use when — n1 > n2, ray from denser medium 1 to rarer medium 2.
Prism deviation: delta = i + e - A. Geometric prism relation.
Prism deviation
Use when — Geometric prism relation.
Prism at minimum deviation: n = sin[(A+delta_m)/2]/sin(A/2). Prism in air, symmetric minimum-deviation path.
Prism at minimum deviation
Use when — Prism in air, symmetric minimum-deviation path.
Telescope normal adjustment: M = -f_o/f_e. Astronomical telescope, final image at infinity; sign shows inversion.
Telescope normal adjustment
Use when — Astronomical telescope, final image at infinity; sign shows inversion.
Compound microscope, normal adjustment: . M
Compound microscope, normal adjustment
Use when — M
Target concept: thin-lens sign convention.
Condition: a convex lens has f=+20 cm; a real object is 30 cm to the left, so u=-30 cm.
Method selection: thin-lens formula under New Cartesian convention.
Reasoning/calculation: 1/v - 1/(-30) = 1/20, so 1/v = 1/20 - 1/30 = 1/60; v=+60 cm. m=v/u=60/(-30)=-2, so the image is real, inverted and twice the object height.
Units/sign check: all lengths use cm consistently; positive v and negative magnification match a real inverted image.
PI category: Execution Error if object-distance sign is lost.
Uses magnitudes instead of signed u,v,f
Execution errorWhy it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Draw positive axis and assign signs before formula.
Measures Snell angles from surface
Recall gapWhy it happens
Preparation Intelligence classification: Recall Gap.
How it is corrected
Angles are from the normal.
Applies TIR from rarer to denser medium
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Check medium order before critical angle.
Uses minimum-deviation prism formula for arbitrary ray path
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Verify symmetric minimum-deviation condition.
Uses one microscope/telescope formula without viewing condition
Needs reviewWhy it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
Label optical configuration first.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
Use one New Cartesian convention consistently and assign signs from the chosen axis before substitution.
Light must travel from optically denser to rarer medium, and incidence angle must exceed the critical angle.
For thin lenses in contact under the ideal coaxial model.
No. It requires the minimum-deviation symmetric condition.
Official scope is taken from the NEET (UG) 2026 syllabus and official NTA documents; concept structure follows current NCERT contents. No performance, weightage or question-frequency data is asserted.
