Schooling · Class 12 · Physics · Ray Optics and Optical Instruments · Chapter on a page
CBSE Class 12 · Physics · NCERT Physics Part-II
Ray Optics and Optical Instruments: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Laws of reflection
- Laws of refraction (Snell's law)
- Mirror formula
- Lens-maker's equation
- Total internal reflection
- Power of a lens
- Magnifying power
- Formulas and facts
- Mirror formula
- Refraction at a single spherical…
- Thin-lens formula
- Lens-maker's equation
- Power
- Critical angle
- Prism deviation
- Worked examples
- Image by a concave mirror
- Refraction through a glass slab
- Magnifying power of a compound…
- Watch out for
- Forgetting the sign convention and…
- Using the thin-lens formula for a…
- Confusing magnification (ratio of…
- Applying total internal reflection…
- Adding focal lengths instead of…
What this chapter is about
Ray optics treats light as straight lines called rays, valid when the objects and apertures involved are much larger than the wavelength of light (about 400–700 nm). Under this approximation you can explain reflection, refraction, image formation by mirrors and lenses, and the working of common optical instruments without invoking wave effects such as diffraction or interference.
Key ideas
- Laws of reflection: The incident ray, the reflected ray and the normal lie in one plane; the angle of incidence equals the angle of reflection.
- Laws of refraction (Snell's law): n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index and θ is measured from the normal.
- Mirror formula: 1/v + 1/u = 1/f, with the sign convention that distances measured against the incident light are negative.
- Lens-maker's equation: 1/f = (n − 1)(1/R₁ − 1/R₂), connecting the focal length of a thin lens to its radii of curvature and refractive index.
- Total internal reflection occurs when light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle θ_c given by sin θ_c = n₂/n₁.
- Power of a lens P = 1/f (in metres), expressed in dioptres (D); powers add for thin lenses in contact.
- Magnifying power of an optical instrument is the ratio of the angle subtended at the eye with the instrument to the angle subtended at the unaided eye at the near point (usually 25 cm).
- Angular magnification of a telescope equals the ratio of the focal length of the objective to that of the eyepiece: m = f_o / f_e.
Formulas and facts to remember
- Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u.
- Refraction at a single spherical surface: n₂/v − n₁/u = (n₂ − n₁)/R.
- Thin-lens formula: 1/v − 1/u = 1/f; magnification m = v/u.
- Lens-maker's equation: 1/f = (n − 1)(1/R₁ − 1/R₂).
- Power: P = 1/f (f in metres, P in dioptres). For lenses in contact, P_net = P₁ + P₂ + …
- Critical angle: sin θ_c = 1/n when light goes from medium of index n to air.
- Prism deviation: At minimum deviation D_m, the ray inside the prism is parallel to the base; n = sin[(A + D_m)/2] / sin(A/2), where A is the prism angle.
- Simple microscope magnifying power: m = 1 + D/f (image at near point) or m = D/f (image at infinity), where D = 25 cm.
- Compound microscope magnifying power: m ≈ (L/f_o)(D/f_e), with L the tube length.
- Astronomical telescope magnifying power: m = f_o/f_e; tube length ≈ f_o + f_e for normal adjustment.
Common mistakes
- Forgetting the sign convention and mixing up real/virtual image positions → always assign signs before substituting in mirror or lens formulas.
- Using the thin-lens formula for a single refracting surface → the single-surface formula has n₁ and n₂ explicitly; the thin-lens formula is derived for a lens in air.
- Confusing magnification (ratio of image to object size) with magnifying power (ratio of angles subtended at the eye) → magnifying power is used for instruments like microscopes; linear magnification for simple mirror/lens problems.
- Applying total internal reflection when light goes from rarer to denser medium → TIR only occurs when light travels from higher to lower refractive index.
- Adding focal lengths instead of powers when combining lenses → powers (in dioptres) add; focal lengths do not add directly.
Quick revision
- Light rays reflect and refract obeying angle rules; use sign conventions consistently.
- Mirror formula: 1/v + 1/u = 1/f; lens formula: 1/v − 1/u = 1/f.
- Total internal reflection needs light going from denser to rarer and angle > critical angle.
- Power of a lens P = 1/f (dioptres); powers add for thin lenses in contact.
- Microscope magnifying power ≈ (L/f_o)(D/f_e); telescope magnifying power = f_o/f_e.
- Critical angle sin θ_c = 1/n links refractive index to the onset of TIR.