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Science Grade XII Physics

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Chapter 10: Wave Optics

Grade 12 Science | Chapter 10

Wave Optics

Light behaves as a wave. This chapter develops the wave model, interference and Young’s double slit experiment, diffraction, and what these reveal about the nature of light.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Light as a Wave

Interference of Light

Young’s Double Slit Experiment

Bright and Dark Fringes

Diffraction

What Wave Optics Reveals

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Light as a Wave

Ray optics treats light as straight lines, but some effects can only be explained if light is a wave. When two beams of light overlap they can brighten or cancel, and light can bend around edges. These behaviours, interference and diffraction, are the hallmarks of waves, and they gave the first strong evidence that light is a wave. This chapter explores the wave nature of light.

Core idea

Light is a wave. When two light waves overlap they interfere: in step they brighten, out of step they cancel. Light also diffracts, bending around edges and through narrow openings.

2. Interference of Light

When two waves meet, they add together, an effect called interference. If their crests arrive together, in step, they reinforce to give a brighter result, called constructive interference. If a crest of one meets a trough of the other, out of step, they cancel to give darkness, called destructive interference. This adding of waves is the key idea of wave optics.

Diagram 1 – Constructive and Destructive

Two waves interfering constructively when in phase and destructively when out of phase

Fig 1. Waves in step add to a brighter wave; waves out of step cancel to darkness.

3. Young’s Double Slit Experiment

Thomas Young showed light’s wave nature with the double slit experiment. Light from a single source passes through two narrow slits, and the two beams overlap on a screen beyond. Instead of just two bright patches, the screen shows a pattern of evenly spaced bright and dark bands, called fringes. Only waves that interfere can explain this, so the experiment proved light is a wave.

Diagram 2 – Young’s Double Slit

Young's double slit experiment forming interference fringes on a screen

Fig 2. Light through two slits overlaps on a screen, forming a pattern of bright and dark fringes.

4. Bright and Dark Fringes

The fringes arise from the different distances the two beams travel to each point on the screen, the path difference. Where the path difference is a whole number of wavelengths, the waves arrive in step and give a bright fringe; where it is a half wavelength out, they cancel and give a dark fringe. The fringes are evenly spaced, and the spacing grows with the wavelength and with the distance to the screen.

5. Diffraction

Light also bends as it passes through a narrow opening or around an edge, an effect called diffraction. Passing through a single narrow slit, light spreads out and forms a pattern with a wide, bright central band and weaker bands on either side. Diffraction is more noticeable when the opening is comparable in size to the wavelength, which is why it is small for everyday light through wide gaps.

Diagram 3 – Single Slit Diffraction

$Single slit diffraction with a central maximum and weaker side bands$

Fig 3. Light spreading through a single slit forms a wide central bright band with weaker side bands.

6. What Wave Optics Reveals

Interference and diffraction can only happen with waves, so together they show that light is a wave. They also let us measure light: the fringe spacing in the double slit experiment can be used to find the wavelength of light. Wave optics thus does more than explain effects; it gives a precise tool for studying light and the things it passes through.

7. Key Reasoning (Principles)

Principle 1: Overlapping waves interfere

When two light waves meet they add, brightening where they are in step and cancelling where they are out of step.

Principle 2: Path difference decides the fringe

A whole number of wavelengths of path difference gives a bright fringe, and a half wavelength out gives a dark one.

Principle 3: Only waves diffract and interfere

Because interference and diffraction are wave behaviours, observing them proves that light travels as a wave.

8. Worked Examples

Example 1

Q: What two effects show that light is a wave?

▶ Show Solution

Interference and diffraction.

Answer: Interference and diffraction.

Example 2

Q: What is interference?

▶ Show Solution

The adding together of two overlapping waves.

Answer: Two waves adding together.

Example 3

Q: When is interference constructive?

▶ Show Solution

When the waves arrive in step, crest with crest, giving a brighter result.

Answer: When waves are in step.

Example 4

Q: When is interference destructive?

▶ Show Solution

When a crest meets a trough, out of step, so they cancel.

Answer: When waves are out of step.

Example 5

Q: What did Young’s double slit experiment show?

▶ Show Solution

A pattern of bright and dark fringes, proving light is a wave.

Answer: Light is a wave.

Example 6

Q: What causes the bright and dark fringes?

▶ Show Solution

The path difference between the two beams to each point on the screen.

Answer: The path difference.

Example 7

Q: When is a fringe bright?

▶ Show Solution

When the path difference is a whole number of wavelengths.

Answer: At whole wavelengths.

Example 8

Q: What is diffraction?

▶ Show Solution

The bending of light through a narrow opening or around an edge.

Answer: Bending around edges.

Example 9

Q: What does single slit diffraction look like?

▶ Show Solution

A wide central bright band with weaker bands on either side.

Answer: A central bright band with side bands.

Example 10

Q: What can the fringe spacing be used to find?

▶ Show Solution

The wavelength of the light.

Answer: The wavelength of light.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. Two effects proving light is a wave are interference and: (a) reflection (b) diffraction (c) refraction (d) absorption

2. Constructive interference happens when waves are: (a) in step (b) out of step (c) absent (d) reflected

3. Young’s experiment uses: (a) one slit (b) two slits (c) a mirror (d) a prism

4. A bright fringe forms at a path difference of: (a) half a wavelength (b) a whole number of wavelengths (c) zero only (d) any value

5. Diffraction is the bending of light through a: (a) wide gap (b) narrow opening (c) mirror (d) lens

▶ Reveal Answers

1. (b) diffraction.

2. (a) in step.

3. (b) two slits.

4. (b) a whole number of wavelengths.

5. (b) narrow opening.

Set B – Short Answer (Understanding)

1. What is interference, and name its two kinds?

2. Describe Young’s double slit experiment and its result.

3. What decides whether a fringe is bright or dark?

4. What is diffraction?

5. What does observing interference and diffraction prove?

▶ Reveal Answers

1. The adding of two overlapping waves; constructive (in step, brighter) and destructive (out of step, cancel).

2. Light from one source passes through two slits and overlaps on a screen, forming bright and dark fringes.

3. The path difference: a whole number of wavelengths gives bright, a half wavelength out gives dark.

4. The bending of light through a narrow opening or around an edge.

5. That light travels as a wave, since only waves interfere and diffract.

Set C – Application and Reasoning

1. Why does the double slit give many bands, not two?

2. Why does a crest meeting a trough give darkness?

3. Why is diffraction small for light through a wide doorway?

4. Why does longer wavelength light give wider fringe spacing?

5. Why can the experiment measure the wavelength of light?

▶ Reveal Answers

1. Because the two beams interfere across the screen, brightening and cancelling at different points to give many fringes.

2. Because the two waves are out of step and cancel each other, leaving no light.

3. Because the gap is far larger than the wavelength, so the bending is too small to notice.

4. Because the fringe spacing grows with wavelength, so longer waves spread the fringes further apart.

5. Because the fringe spacing depends on the wavelength, so measuring the spacing gives the wavelength.

Set D – Higher Order (Challenge)

1. Explain why interference cannot be explained by treating light as particles travelling in straight lines.

2. Explain how path difference produces a regular fringe pattern.

3. Explain why diffraction is more obvious for sound than for light in everyday life.

4. Explain how the same experiment both proves light is a wave and measures it.

5. Explain why a single slit gives a central band much wider than the side bands.

▶ Reveal Answers

1. Because two straight beams of particles would give just two patches, whereas the dark fringes show waves cancelling, which particles cannot do.

2. As the path difference grows steadily across the screen, it passes through whole and half wavelengths in turn, giving alternating bright and dark fringes.

3. Because sound has a much longer wavelength, comparable to everyday openings, so it bends noticeably, while light’s tiny wavelength bends very little.

4. The fringes prove interference, a wave effect, and their measured spacing depends on the wavelength, so it can be calculated.

5. Because most of the light passes nearly straight through to form a strong central band, while only smaller amounts reinforce off to the sides.

Chapter Summary

Wave Nature

Interference and diffraction show light is a wave.

Interference

Overlapping waves add: in step bright, out of step dark.

Young’s Experiment

Two slits give bright and dark fringes.

Fringes

Path difference decides bright or dark.

Diffraction

Light bends through narrow openings; central bright band.

Use

Fringe spacing measures the wavelength of light.

Quantity

Unit

Symbol

Wave proof

interference, diffraction

–

Bright fringe

whole wavelengths

–

Diffraction

central bright band

–

8-Point Exam Quick-Check

Interference and diffraction show that light is a wave.

Interference: overlapping waves add together.

In step gives bright (constructive); out of step gives dark (destructive).

Young’s double slit gives a pattern of bright and dark fringes.

Path difference decides whether a fringe is bright or dark.

A whole number of wavelengths gives a bright fringe.

Diffraction bends light through narrow openings, giving a central bright band.

Fringe spacing can be used to measure the wavelength of light.

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Class 12 Physics Chapter 10: Wave Optics, Complete Notes and Practice

This revision guide follows the current NCERT Class 12 Physics syllabus and develops wave optics, covering the wave nature of light, interference and its constructive and destructive forms, Young’s double slit experiment and the bright and dark fringes set by path difference, diffraction through a single slit, and how these effects prove light is a wave and let us measure its wavelength, with three diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.