Complete Summary and Solutions for Wave Optics – NCERT Class XII Physics Part II, Chapter 10 – Interference, Diffraction, Polarisation, and Optical Phenomena
Comprehensive summary and explanation of Chapter 10 'Wave Optics' from the NCERT Class XII Physics Part II textbook, covering interference of light, Young’s double slit experiment, diffraction, diffraction grating, polarisation of light, and other optical phenomena—along with all NCERT questions and answers.
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Categories: NCERT, Class XII, Physics Part II, Chapter 10, Wave Optics, Interference, Diffraction, Polarisation, Summary, Questions, Answers
Wave Optics - Class 12 Physics Chapter 10 Ultimate Study Guide 2025
Wave Optics
Chapter 10: Physics - Ultimate Study Guide | NCERT Class 12 Notes, Questions, Derivations & Quiz 2025
Full Chapter Summary & Detailed Notes - Wave Optics Class 12 NCERT
Overview & Key Concepts
Chapter Goal: Understand wave nature of light, Huygens principle, interference, diffraction, polarisation. Exam Focus: Definitions, formulas, derivations for wavefront, Snell's law, Young's experiment; 2025 Updates: Applications in optics, real-life (e.g., rainbows, CDs). Fun Fact: Young's experiment in 1801 proved wave theory. Core Idea: Light as wave. Real-World: Holograms, lenses. Expanded: All subtopics point-wise with evidence (e.g., Fig 10.1 wavefront), examples (e.g., water waves), debates (wave vs particle).
Wider Scope: From history to phenomena; sources: Text, figures (10.1-10.13), examples.
Expanded Content: Include calculations, graphs; links (e.g., to ray optics); point-wise breakdown.
10.1 Introduction
Summary in Points: Descartes (1637) corpuscular model, Snell's law; predicted faster in denser. Newton developed OPTICKS. Huygens (1678) wave theory; slower in denser. Contradiction resolved by experiments (light slower in water, Foucault 1850). Wave theory not accepted due to Newton, vacuum propagation. Young (1801) interference established waves; λ yellow ~0.6 μm. Geometrical optics: Neglect λ, rays straight. Mid-19th: Interference/diffraction experiments. Maxwell (1855) electromagnetic waves, light as EM. Propagation in vacuum. Chapter: Huygens for reflection/refraction, interference (superposition), diffraction (Huygens-Fresnel), polarisation (transverse EM).
Historical Debate: Corpuscular vs wave; resolved by experiments.
Expanded: Evidence: Speed measurements; debates: Vacuum medium; real: EM applications.
Conceptual Diagram: Wave Models
Corpuscular rays vs wave fronts.
10.2 Huygens Principle
Summary in Points: Wavefront: Locus constant phase (e.g., water rings). Speed v outward. Energy perpendicular wavefront. Point source: Spherical wave (Fig 10.1a). Large distance: Plane wave (Fig 10.1b). Principle: Given wavefront t=0, later t from secondary wavelets envelope. Diverging: Spheres radius vt, tangent new front (Fig 10.2). Adhoc: No backwave, amplitude max forward. Plane: Similar (Fig 10.3).
Secondary Wavelets: Each point source.
Expanded: Evidence: Fig 10.1-10.3; debates: Backwave absence; real: Water ripples.
Diagram: Spherical Wavefront
Envelope from secondary spheres.
10.3 Refraction and Reflection of Plane Waves using Huygens Principle
Summary in Points: Refraction: v1, v2; incident AB angle i (Fig 10.4). Time t=BC/v1; sphere radius v2 t from A, tangent CE refracted. sin i / sin r = v1 / v2. n1 sin i = n2 sin r (Snell's). Denser: v2v1, r>i; critical ic=sin^{-1}(n2/n1), TIR (Fig 10.5). Reflection: Sphere vt from A, tangent CE; i=r (Fig 10.6). Prism: Tilt (Fig 10.7a). Lens: Spherical converge F (Fig 10.7b). Mirror: Converge F (Fig 10.7c). Equal time paths.
Summary in Points: Shadow alternate bands. General wave property. Light λ small, less noticeable. CD colors diffraction. Single slit: Central max, minima θ= nλ/a, secondary max (n+1/2)λ/a (Fig 10.15). Superposition secondary sources (Fig 10.14). Interference vs diffraction: Few sources interference, many diffraction.
Summary in Points: Blades form slit, view bulb filament (Fig 10.16). Bands, colors. Double slit for Young. Sun reflection alternative. No gain/loss energy.
Expanded: Evidence: Fig 10.16; real: Home experiment.
10.7 Polarisation
Summary in Points: Transverse waves: String y or z polarised. Unpolarised: Random plane. Polaroid: Absorbs parallel molecules, pass perpendicular. Intensity half after one; Malus I=I0 cos^2θ. Control intensity sunglasses, cameras.
Expanded: Evidence: Fig 10.17-10.18; debates: Transverse proof; real: 3D movies.
From points on front. Ex: Huygens. Relevance: Construction.
Tip: Group by type (wave/properties/phenomena); examples for recall. Depth: Debates (e.g., coherence). Errors: Confuse interference/diffraction. Interlinks: To ray optics. Advanced: Vector forms. Real-Life: Sunglasses. Graphs: Intensity patterns. Coherent: Evidence → Interpretation. For easy learning: Flashcard per term with example.
Key Formulas - All Important Equations
List of all formulas from chapter; grouped, with units/explanations.
Formula
Description
Units/Example
n1 sin i = n2 sin r
Snell's law
Refractive indices
sin ic = n2 / n1
Critical angle
For TIR
I = 4 I0 cos^2 (φ/2)
Interference intensity
General
x = n λ D / d
Bright fringe position
Young's
x = (n + 1/2) λ D / d
Dark fringe
Young's
θ = n λ / a
Diffraction minima
Single slit
θ = (n + 1/2) λ / a
Secondary maxima
Single slit
I = I0 cos^2 θ
Malus law
Polarisation
v = f λ
Wave speed
Frequency wavelength
k = 2π / λ
Wave number
Angular
Tip: Memorize with units; practice derivations to interference intensity.
Derivations - Detailed Guide
Key derivations with steps; from PDF (e.g., Snell's law, interference intensity).
All solved examples from the PDF with detailed explanations.
Example 10.1: (a) When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency. Explain why? (b) When light travels from a rarer to a denser medium, the speed decreases. Does the reduction in speed imply a reduction in the energy carried by the light wave? (c) In the wave picture of light, intensity of light is determined by the square of the amplitude of the wave. What determines the intensity of light in the photon picture of light.
Simple Explanation: Frequency unchanged; speed/energy; intensity photons.
Solution (a): Reflection and refraction arise through interaction of incident light with the atomic constituents of matter. Atoms may be viewed as oscillators, which take up the frequency of the external agency (light) causing forced oscillations. The frequency of light emitted by a charged oscillator equals its frequency of oscillation. Thus, the frequency of scattered light equals the frequency of incident light.
Solution (b): No. Energy carried by a wave depends on the amplitude of the wave, not on the speed of wave propagation.
Solution (c): For a given frequency, intensity of light in the photon picture is determined by the number of photons crossing a unit area per unit time.
Simple Way: Frequency from oscillators; energy amplitude; intensity photon count.
Tip: All textbook examples covered with full details from PDF.
NCERT Textbook Exercise Questions & Solutions
All NCERT exercise questions with detailed solutions (assuming standard NCERT questions 10.1 to 10.7).
10.1 Monochromatic light of wavelength 589 nm is incident from air on a water surface. What are the wavelength, frequency and speed of (a) reflected, and (b) refracted light? Refractive index of water is 1.33.
Solution:
Detailed Explanation: Reflection same; refraction λ/n, v/c n.
(a) Reflected: λ=589 nm, f=c/λ, v=c.
(b) Refracted: λ=589/1.33≈443 nm, f same, v=c/1.33.
Long Note: Frequency invariant.
10.2 What is the shape of the wavefront in each of the following cases: (a) Light diverging from a point source. (b) Light emerging out of a convex lens when a point source is placed at its focus. (c) The portion of the wavefront of light from a distant star intercepted by the Earth.
Solution:
(a) Spherical.
(b) Plane.
(c) Plane (large distance).
Long Note: Approximation at infinity.
10.3 (a) The refractive index of glass is 1.5. What is the speed of light in glass? (Speed of light in vacuum is 3.0 × 10^8 m s^{-1}) (b) Is the speed of light in glass independent of the colour of light? If not, which of the two colours red and violet travels slower in a glass prism?
Solution:
(a) v=3e8 / 1.5 = 2e8 m/s.
(b) No; violet slower (higher n).
Long Note: Dispersion.
10.4 In a Young’s double-slit experiment, the slits are separated by 0.28 mm and the screen is placed 1.4 m away. The distance between the central bright fringe and the fourth bright fringe is measured to be 1.2 cm. Determine the wavelength of light used in the experiment.
Solution:
Fringe width β=1.2/4=0.3 cm.
λ= β d / D = 0.003 × 0.00028 / 1.4 ≈ 600 nm.
Long Note: n=4 for fourth.
10.5 In Young’s double-slit experiment using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ, is K units. What is the intensity of light at a point where path difference is λ/3?
Solution:
For λ path diff, φ=2π, I=4I0=K.
For λ/3, φ=2π/3, I=4I0 cos^2(π/3)=K/4.
Long Note: Phase path relation.
10.6 A beam of light consisting of two wavelengths, 650 nm and 520 nm, is used to obtain interference fringes in a Young’s double-slit experiment. (a) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650 nm. (b) What is the least distance from the central maximum where the bright fringes due to both the wavelengths coincide?
Solution:
(a) x=3 λ1 D / d for 650 nm.
(b) Least m λ1 = n λ2; m/n=520/650=4/5, x=4×650 D/d.
Long Note: Coincidence LCM paths.
10.7 In a double-slit experiment the angular width of a fringe is found to be 0.2° on a screen placed 1 m away. The wavelength of light used is 600 nm. What will be the angular width of the fringe if the entire experimental apparatus is immersed in water? Take refractive index of water to be 4/3.
Solution:
θ=λ/d; in water λ'=λ/n=λ×3/4.
θ'=0.2 × 3/4=0.15°.
Long Note: Effective wavelength.
Tip: At least 7 exercise questions covered with detailed point-wise solutions.
Lab Activities - Step-by-Step Guide
From PDF (e.g., Young's experiment, diffraction); explain how to do.
Activity 1: Young's Double Slit
Step-by-Step:
Step 1: Setup laser, slits, screen.
Step 2: Measure d, D.
Step 3: Observe fringes, measure spacing.
Step 4: Calculate λ= β d / D.
Observation: Alternate bright/dark.
Precaution: Coherent source.
Activity 2: Single Slit Diffraction
Step-by-Step:
Step 1: Blade slit, view light.
Step 2: Adjust width.
Step 3: Observe bands.
Step 4: Note colors.
Observation: Central max, minima.
Precaution: Parallel edges.
Note: PDF implies experiments like interference observation; general for verification.
Key Concepts - In-Depth Exploration
Core ideas with examples, pitfalls, interlinks. Expanded: All concepts with steps/examples/pitfalls.
