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CBSE Class 12 · Physics

Ray Optics and Optical Instruments

Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Ray Optics and Optical Instruments” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 8 concepts

1Reflection of Light by Spherical Mirrors

Spherical mirrors: 1/v + 1/u = 1/f as taught, with the school sign rule. A real image can sit on a screen; a virtual cannot. Mirror is a bounce-map. One concave-object case is enough.

Mixing a lens-sign into a mirror write is a miss.

Figure. The spherical surface is not drawn. For a concave mirror, C and F sit on the incoming side. PF equals FC, so f = R/2. Object between F and C; image beyond C, inverted.

How it works

  1. Copy u and f with the taught signsThe given.
  2. Form 1/v=1/f−1/uThe image.
  3. Keep real/virtual as the lookThe extra.

Concave numbers

f=20 cm, u=−30 cm (school signs as framed). Find v.

  • 1/v1/20 − 1/(−30)=1/20+1/30
  • 1/v1/12
  • v12 cm as the write

Pro tip. Copy the lesson’s sign rule first.

2Refraction

Refraction: n = c/v as taught; Snell n1 sin i = n2 sin r. A ray bends toward the normal into a slower medium. Refraction is a bend at a surface, not a bounce.

Using reflection’s i=r as Snell is a miss.

Figure. Incident 60° in air becomes 35.3° in glass with n = 1.5. The ray bends toward the normal. Angles are measured from the dashed normal, not from the surface.

How it works

  1. Copy n1, i, n2The given.
  2. Form n1 sin i = n2 sin rSnell.
  3. Keep toward-normal into slowerThe bend.

3Total Internal Reflection

TIR: when i > critical angle as taught, the ray bounces inside the denser look. sin c = n2/n1 as framed. TIR is a bounce-because-too-steep, not an ordinary mirror-coat. Optical fibre if named.

A small i as TIR is a miss.

Figure. Three rays leave one point in glass. At 25° the ray escapes, bent away from the normal. At 41.8° it grazes — that is θc for n = 1.5. At 58° nothing leaves; the bounce is ordinary reflection.

How it works

  1. Need denser-to-rarer and i > cThe gate.
  2. Write sin c as taughtThe critical.
  3. Keep fibre as a use if askedHonest extra.

4Refraction at Spherical Surfaces and by Lenses

Lenses and spherical surfaces: the lens formula 1/v − 1/u = 1/f as taught (signs as framed). A convex (converging) has f>0 in the usual convention. Lens is a through-bend, not a mirror.

Using 1/v+1/u=1/f on a lens without checking the lesson’s write is a swap.

Figure. Thin-lens construction, object at 2F. Parallel ray through F', undeviated centre ray, and the ray through F then parallel. They meet at 2F' as a real inverted image of the same height.

How it works

  1. Copy u and f with lens-signsThe given.
  2. Use the taught lens writeThe image.
  3. Keep convex/concave as the typeThe extra.

5Refraction through a Prism

Prism: deviation δ = (μ−1)A for small A as taught, or the full i+i′−A. A spectrum-split if named (dispersion). Prism is a two-surface bend, not a lens.

A mirror-bounce as the prism-job is a miss.

Figure. A ray enters a prism, bends toward the normal, then leaves bending away from the normal. The two face-bends add to the deviation. The triangle is a straight-edge prism, not a measured angle set.

How it works

  1. Name A and μ (or the two i)The given.
  2. Form the taught δDeviation.
  3. Keep dispersion as a colour-split if askedHonest extra.

6Optical Instruments

Optical instruments: microscope and telescope as taught — objective and eyepiece, a magnification write. An instrument is a two-lens job, not a single f. One school m-write is enough.

A lone magnifying glass as “the telescope” is a miss.

Figure. A compound microscope is two thin lenses on one axis. The objective makes a real intermediate image; the eyepiece uses that image as its object and magnifies it for the eye.

How it works

  1. Name objective and eyepieceThe pair.
  2. Write the taught MThe instrument.
  3. Keep microscope ≠ telescope as the useTwo jobs.

7A definition is a test you can run

Ray-word is a test: a mirror-formula, Snell, TIR, a lens, a prism, or an instrument. If you only say “light”, you have a heading.

A sun-sticker is not the test.

Figure. A definition in this chapter is a test you can run: name the quantity, give its SI unit, and write the one relation that owns it. Mixing two sign conventions fails the test.

How it works

  1. Name the formula or the bend-lookThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.

8Name the given before the unknown

The given is u and f, or n and i. The unknown is v or r. Copy the sign rule before you treat a real object as +u.

Using a lens write on a mirror is a silent swap.

Figure. Write every given with its Cartesian sign before you touch 1/v − 1/u = 1/f. Here u is negative and f is positive; v is the unknown. A missing sign is a new, false language.

How it works

  1. Copy u, f, or n,iThe given.
  2. Name v, r, or δThe unknown.
  3. Then computeGiven first.
TIR needs
  1. i greater than the critical angle as taught
  2. Any small i
  3. A mirror-coat always

Too-steep in the denser.

Notes

  • Mapped to the official NCERT chapter “Ray Optics and Optical Instruments”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • mirror: 1/v+1/u=1/f (signs as taught)
  • Snell: n1 sin i = n2 sin r
  • lens: 1/v−1/u=1/f as taught

Recap

Hold these pegs from the official chapter “Ray Optics and Optical Instruments”. The wording is ExamMaster’s teaching, not a textbook recap.

Reflection of Light by Spherical Mirrors
Spherical mirrors: 1/v + 1/u = 1/f as taught, with the school sign rule.
Refraction
Refraction: n = c/v as taught; Snell n1 sin i = n2 sin r.
Total Internal Reflection
TIR: when i > critical angle as taught, the ray bounces inside the denser look.
Refraction at Spherical Surfaces and by Lenses
Lenses and spherical surfaces: the lens formula 1/v − 1/u = 1/f as taught (signs as framed).
Refraction through a Prism
Prism: deviation δ = (μ−1)A for small A as taught, or the full i+i′−A.
Optical Instruments
Optical instruments: microscope and telescope as taught — objective and eyepiece, a magnification write.

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