Ray Optics and Optical Instruments — Class 12 Physics NCERT Solutions (Free)

Free step-by-step NCERT solutions for Class 12 Physics chapter "Ray Optics and Optical Instruments" — 8 important questions with detailed answers for CBSE board exam preparation.

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TL;DR: Free step-by-step NCERT solutions for Class 12 Physics chapter "Ray Optics and Optical Instruments" — 8 important questions with detailed answers for…

Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated Jul 23, 2026

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Key Questions Covered:

What is refraction? State Snell's law and derive it using Huygens' principle.
Define critical angle and total internal reflection. For glass (n = 1.5), fin…
Derive lens maker's formula and show how it relates object distance, image di…
An object 5 cm tall is placed 20 cm from a convex lens of focal length 10 cm.…
What is power of a lens? A person has near point at 50 cm. What power of lens…
Explain the construction and working of a microscope. Derive the magnifying p…
+ 2 more questions in the full chapter

Solutions Summary:

Question	Status
What is refraction? State Snell's law and derive it using…	✓ Solved
Define critical angle and total internal reflection. For …	✓ Solved
Derive lens maker's formula and show how it relates objec…	✓ Solved
An object 5 cm tall is placed 20 cm from a convex lens of…	✓ Solved
What is power of a lens? A person has near point at 50 cm…	✓ Solved
Explain the construction and working of a microscope. Der…	✓ Solved

Showing 6 of 8 questions

Q1: What is refraction? State Snell's law and derive it using Huygens' principle.

Refraction: Bending of light ray at the interface between two media of different optical densities. Snell's Law: n₁ sin θ₁ = n₂ sin θ₂ Where n₁, n₂ are refractive indices; θ₁, θ₂ are angles of incidence and refraction. Derivation using Huygens' Principle: Consider plane wavefronts AB and A'B' incident on interface at angle θ₁. Let speed of light: v₁ in medium 1, v₂ in medium 2 Time taken from B to B': t = BB'/v₁ = AB'/v₂ In triangle ABB': BB' = AB' sin θ₁ In triangle AA'B': AA' = AB' sin θ₂...

Q2: Define critical angle and total internal reflection. For glass (n = 1.5), find critical angle.

Critical Angle: Angle of incidence in denser medium for which angle of refraction is 90°. Total Internal Reflection: Complete reflection of light at interface from denser to rarer medium when angle of incidence exceeds critical angle. Derivation: At critical angle θ_c, refraction angle = 90° Using Snell's law: n₁ sin θ_c = n₂ sin 90° n₁ sin θ_c = n₂ × 1 sin θ_c = n₂/n₁ = 1/n (for denser to rarer medium) θ_c = sin⁻¹(1/n) For glass-air interface (n = 1.5): sin θ_c = 1/1.5 = 2/3 θ_c = sin⁻¹(0.66...

Q3: Derive lens maker's formula and show how it relates object distance, image distance, and focal length.

Lens Maker's Formula: 1/f = (n - 1)[1/R₁ - 1/R₂] Where n = refractive index, R₁, R₂ = radii of curvature Derivation: Consider a thin lens with radii R₁ (first surface) and R₂ (second surface). For refraction at first surface (air to glass): 1/s₁ + n/s₁' = (n-1)/R₁ For refraction at second surface (glass to air): n/(-s₁' + t) + 1/s₂' = (1-n)/R₂ For thin lens, t → 0: n/(-s₁') + 1/s₂' = (1-n)/R₂ Using object distance s = s₁ and image distance s' = s₂': 1/s + 1/s' = (n-1)[1/R₁ - 1/R₂] Lens Equ...

Q4: An object 5 cm tall is placed 20 cm from a convex lens of focal length 10 cm. Find image position, size, and nature.

Given: Object height h₁ = 5 cm Object distance u = 20 cm Focal length f = 10 cm (convex lens) Using lens equation: 1/f = 1/u + 1/v 1/10 = 1/20 + 1/v 1/v = 1/10 - 1/20 = 2/20 - 1/20 = 1/20 v = 20 cm Image is at 20 cm from lens (on opposite side of object) Magnification: m = -v/u = -20/20 = -1 Image height: h₂ = m × h₁ = (-1) × 5 = -5 cm (Negative sign indicates inverted image) Nature of image: - Real image (v > 0) - Inverted (m < 0) - Same size as object (|m| = 1) - At 2f position (u =...

Q5: What is power of a lens? A person has near point at 50 cm. What power of lens is required to correct this vision defect?

Power of Lens: Reciprocal of focal length in meters. P = 1/f (in diopters, D) f in meters Vision Defect: Hypermetropia (far-sightedness) Near point at 50 cm instead of normal 25 cm Correction Required: For comfortable reading at D = 25 cm (normal near point), the lens should form virtual image at 50 cm when object is at 25 cm. Using lens equation: 1/f = 1/u + 1/v Here: u = 25 cm = 0.25 m, v = -50 cm = -0.5 m (virtual image, negative) 1/f = 1/0.25 + 1/(-0.5) 1/f = 4 - 2 = 2 f = 0.5 m = +50 cm...

Q6: Explain the construction and working of a microscope. Derive the magnifying power.

Microscope: Optical instrument for viewing small objects magnified. Construction: - Objective lens: small focal length (f_o ≈ 1-5 mm), high power - Eyepiece (ocular): moderate focal length (f_e ≈ 2-3 cm), acts as magnifying glass - Tube length: distance between focal points of objective and eyepiece - Lenses placed close together: separation ≈ L (tube length) Working: 1. Object placed just beyond focal point of objective 2. Objective forms real, magnified, inverted image 3. This image acts as ...

Showing 6 of 8 questions. Visit the full page for complete solutions.