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Ray Optics and Optical Instruments – Class 12 Physics notes, important questions and MCQs

NCERT Class 12 Physics, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Ray optics treats light as rays that reflect and refract. Mirror, lens and prism formulas explain image formation and optical instruments.

Mirrors, refraction and lenses

Mirror: 1/v + 1/u = 1/f, f = R/2; lens: 1/v − 1/u = 1/f; m = v/u (lens).
Snell's law n₁ sin i = n₂ sin r; total internal reflection when i > critical angle, sin C = 1/n.
Lens maker's formula: 1/f = (n − 1)(1/R₁ − 1/R₂); power P = 1/f (dioptre); lenses in contact P = P₁ + P₂.

Prism and instruments

Prism: A + δ = i + e; minimum deviation n = sin[(A + δm)/2]/sin(A/2).
Simple microscope m = 1 + D/f; compound microscope m ≈ (L/f₀)(D/fₑ).
Astronomical telescope (normal adjustment) m = f₀/fₑ, length L = f₀ + fₑ.

Key terms

Critical angle: angle of incidence in the denser medium for which refraction angle is 90°
Total internal reflection: complete reflection of light back into the denser medium
Power of a lens: P = 1/f (f in metres), unit dioptre (D)

Formulas

1/v − 1/u = 1/f
sin C = 1/n
1/f = (n − 1)(1/R₁ − 1/R₂)
m = f₀/fₑ (telescope)

Important questions with answers

Why do diamonds sparkle?2 marks

Diamond has a very high refractive index (2.42), so its critical angle is small (about 24°). Light entering it undergoes repeated total internal reflection before coming out, making it sparkle.

Derive the relation between the critical angle and refractive index, and state the two conditions for total internal reflection.3 marks

At the critical angle C the refracted ray grazes the surface (r = 90°). Snell's law from denser (n) to air: n sin C = 1 × sin 90°, so sin C = 1/n. Conditions: light must travel from a denser to a rarer medium, and the angle of incidence must exceed the critical angle.

Draw (describe) the ray diagram of a compound microscope and derive its magnifying power when the final image is at the near point. An objective of focal length 1 cm and eyepiece of 5 cm are 20 cm apart; estimate its magnifying power.5 marks

The objective forms a real, inverted, magnified image of a close object; the eyepiece acts as a magnifier and forms the final virtual image at D = 25 cm. Magnification m = mₒ × mₑ, where mₒ = vₒ/uₒ and mₑ = 1 + D/fₑ. For an object near the focus of the objective, mₒ ≈ L/fₒ (L ≈ tube length). Approximate estimate: image by eyepiece at D means uₑ = fₑD/(D + fₑ) = 125/30 ≈ 4.17 cm, so vₒ ≈ 20 − 4.17 = 15.83 cm; then 1/uₒ = 1/vₒ − 1/fₒ gives uₒ ≈ −1.07 cm, mₒ ≈ 14.8. mₑ = 1 + 25/5 = 6. M ≈ 14.8 × 6 ≈ 89.

Quick revision

TIR needs light going from denser to rarer medium.
Telescope objective has a large focal length.
Power adds for lenses in contact.

Exam tips

Optical fibres and sparkling diamonds work on total internal reflection.
Sign convention: distances measured from the pole/optical centre; along incident light positive, against it negative.

Practice MCQs

Unit of power of a lens is
1. watt
2. dioptre
3. metre
4. joule
Show answer
Answer: (b) dioptre. Dioptre.
Optical fibres work on
1. refraction only
2. dispersion
3. total internal reflection
4. diffraction
Show answer
Answer: (c) total internal reflection. TIR.
Critical angle for glass (n = 1.5) is about
1. 42°
2. 30°
3. 60°
4. 49°
Show answer
Answer: (a) 42°. sin C = 2/3.
Two lenses of power +2 D and −1 D in contact give
1. +3 D
2. −1 D
3. +1 D
4. +2 D
Show answer
Answer: (c) +1 D. Powers add.
Two thin lenses of power +3 D and −1 D in contact have combined focal length
1. 25 cm
2. 50 cm
3. 33 cm
4. 100 cm
Show answer
Answer: (b) 50 cm. P = 2 D.
A telescope has f₀ = 100 cm and fₑ = 5 cm. Magnifying power (normal adjustment) is
1. 105
2. 500
3. 20
4. 0.05
Show answer
Answer: (c) 20. 100/5.
A convex lens (n = 1.5) has R₁ = R₂ = 20 cm. Its focal length is
1. 10 cm
2. 20 cm
3. 40 cm
4. 30 cm
Show answer
Answer: (b) 20 cm. 1/f = 0.5(2/20) = 1/20.
Critical angle for glass of refractive index 1.5 is about
1. 30°
2. 42°
3. 48°
4. 60°
Show answer
Answer: (b) 42°. sin C = 1/1.5.

All chapters of Class 12 Physics

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.