# Ray Optics and Optical Instruments Notes | SwaVid Ray Optics and Optical Instruments, NCERT Class 12 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers. Canonical: https://www.swavid.com/cbse/class-12/physics/chapter/ray-optics-and-optical-instruments Source: https://www.swavid.com/cbse/class-12/physics/chapter/ray-optics-and-optical-instruments # Ray Optics and Optical Instruments ## Formulas and definitions ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 12 Physics on SwaVid ### Reflection by spherical mirrors ### Refraction and total internal reflection ### Refraction at spherical surfaces and thin lenses ### Refraction through a prism ### Optical instruments ### Next chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 12 › Physics › Ray Optics and Optical Instruments Notes written by SwaVid from the NCERT textbook. Class 12 Physics, Chapter 9. Key ideas, formulas and common mistakes. CBSE prints 18 marks for Units V-VI together, not for this chapter alone. Short answer. This chapter covers the reflection and refraction of light using the ray approximation, image formation by mirrors and lenses, and the working of prisms and optical instruments. It equips students to apply the Cartesian sign convention and calculate image locations, focal lengths, and magnifications. Source: NCERT Physics Part II (Reprint 2026-27) , Chapter 9, Ray Optics and Optical Instruments Spherical mirrors obey the laws of reflection. The focal length is half the radius of curvature, f = R / 2. Object distance u, image distance v, and focal length f are related by the mirror formula 1/v + 1/u = 1/f. Refraction at an interface satisfies Snell's law, n₁ sin i = n₂ sin r. When light travels from a denser medium to a rarer medium at an angle of incidence greater than the critical angle, it undergoes total internal reflection, satisfying sin i c = 1 / n. Refraction at a single spherical surface relates object and image distances via n₂/v - n₁/u = (n₂ - n₁)/R. For a thin lens in air, the lens maker's formula gives 1/f = (n - 1) (1/R₁ - 1/R₂), and thin lenses in contact have an effective power P = P₁ + P₂. A light ray passing through a triangular prism suffers deviation. At the angle of minimum deviation D m , the ray passes symmetrically, giving the refractive index n = sin((A + D m )/2) / sin(A/2). Microscopes and telescopes produce magnified images of near and distant objects respectively. A compound microscope uses an objective and an eyepiece to give high angular magnification, while an astronomical telescope uses an objective of large focal length and aperture. Try each question first. Then tap it to see the answer. Here u = -20 cm, f = -15 cm. Using 1/v + 1/u = 1/f gives 1/v + 1/(-20) = 1/(-15). 1/v = -1/15 + 1/20 = (-4 + 3) / 60 = -1/60. v = -60 cm. -60 cm By Cartesian convention, R₁ = +0.20 m and R₂ = -0.20 m. 1/f = (1.5 - 1) × (1/0.20 - 1/(-0.20)) = 0.5 × (5 + 5). 1/f = 0.5 × 10 = 5 m⁻¹. f = 1 / 5 = 0.20 m = 20 cm. 20 cm In normal adjustment, magnifying power is given by m = f o / f e . m = 100 / 5 = 20. 20 Wave Optics . Electromagnetic Waves . The 14 chapters of Class 12 Physics . CBSE does not give marks to one chapter. It prints 18 marks for Units V-VI together (Electromagnetic Waves; Optics). Ray Optics and Optical Instruments is part of that group. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Wave Optics. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 9 NCERT book - 18 marks, Units V-VI together - 3 quick checks - Mixing up the negative sign in the mirror formula and thin lens formula . Remember that the mirror formula has a plus sign (1/v + 1/u = 1/f) while the thin lens formula has a minus sign (1/v - 1/u = 1/f). For magnification, the mirror has m = -v/u, whereas the lens has m = +v/u. - Assigning incorrect signs to the radii of curvature in the lens maker's formula . Measure all distances from the optical centre along the direction of incident light as positive. For a standard convex lens, the first surface has R₁ > 0 and the second surface has R₂ < 0. - Calculating lens power using focal length in centimetres directly . Power in dioptres requires the focal length to be in metres. Convert f to metres first, or use P = 100 / f when f is in centimetres. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 12](https://www.swavid.com/cbse/class-12) - [Physics](https://www.swavid.com/cbse/class-12/physics/chapters) - [Start free →](https://www.swavid.com/signup) - [All 14 chapters](https://www.swavid.com/cbse/class-12/physics/chapters) - [NCERT Physics Part II (Reprint 2026-27)](https://ncert.nic.in/textbook.php?leph2=0-6) - [Open notes](https://www.swavid.com/cbse/class-12/physics/chapter/wave-optics) - [Open notes](https://www.swavid.com/cbse/class-12/physics/chapter/electromagnetic-waves) - [Class 12 Physics](https://www.swavid.com/cbse/class-12/physics/chapters) - [Start free →](https://www.swavid.com/signup)