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REFLECTION OF LIGHT

Class 7 ICSE Physics

Discover how light bounces off surfaces, how mirrors form images and why we can see the world around us.

Concept Clarity Learning Objectives Science Story Thinking MCQs Interactive Quiz

1. Learning Objectives

After studying this lesson, students will be able to:

• Define reflection of light.
• State the approximate speed of light in vacuum.
• Identify the incident ray, reflected ray, normal and point of incidence.
• Explain the angle of incidence and the angle of reflection.
• State and explain the two laws of reflection.
• Differentiate between regular and irregular reflection.
• Identify reflecting surfaces in everyday life.
• Explain why smooth surfaces can produce glare.
• Perform an activity to verify the laws of reflection.
• Apply the laws of reflection to solve simple problems.

2. A Small Story: Riya and the Mysterious Mirror

One sunny morning, Riya stood in front of a plane mirror and adjusted her school uniform. She noticed that her image moved whenever she moved.

Later, while walking to school, she saw sunlight reflecting from a polished car. The bright flash made her squint. She wondered, "Why does a mirror form a clear image, while a wall does not?"

Her science teacher explained that light travels in straight lines in a uniform medium and can bounce back when it strikes a reflecting surface. This phenomenon is called reflection of light.

A smooth mirror reflects light in an orderly manner, allowing a clear image to form. A rough wall scatters reflected light in different directions. This is why we can see the wall but do not normally see a clear image of ourselves in it.

Riya realised that the same scientific principle helps us use mirrors, see objects around us and understand glare on wet roads.

Think about it:
If a wall reflects light, why does it not normally form a clear image like a plane mirror?
Answer: A wall has a microscopically uneven surface. It scatters reflected light in many directions instead of reflecting it in an orderly manner as a smooth mirror does.

3. Speed of Light: Nature's Speed Champion

Light travels extremely fast. In vacuum, its speed is approximately 300,000 kilometres per second.

Speed of light in vacuum = 3 × 10⁸ m/s

Light from the Sun takes about 8 minutes 20 seconds to reach Earth. This is because the Sun is approximately 150 million kilometres away.

The speed of light in air is very close to its speed in vacuum. Light travels more slowly in materials such as water and glass.

Compare these speeds

Usain Bolt

His top running speed is around 37–45 km/h, depending on the measurement.

Cheetah

A cheetah can reach roughly 100 km/h or more in short bursts.

Speed of Sound

Approximately 340 m/s in air at room temperature.

Speed of Light

Approximately 300,000 km/s in vacuum.

Interesting fact: Light travels about 300,000 kilometres in just one second. That is fast enough to circle Earth roughly 7.5 times in one second.

4. What Is Reflection of Light?

Reflection of light is the bouncing back of light into the same medium after it strikes a surface.

When light falls on a surface, some of it may be absorbed, some may pass through the material and some may be reflected. The proportions depend on the material and the surface.

Examples of reflecting surfaces

• Plane mirrors
• Polished metal surfaces
• Still water
• Glass surfaces
• Walls, books and other everyday objects

Remember: We do not see objects only because they are shiny. We see most objects because light from a source falls on them and reflected light reaches our eyes.

5. Important Terms Related to Reflection

1. Incident Ray

The ray of light that strikes a reflecting surface is called the incident ray.

2. Point of Incidence

The point at which the incident ray strikes the reflecting surface is called the point of incidence.

3. Reflected Ray

The ray of light that bounces back from the reflecting surface is called the reflected ray.

4. Normal

The normal is an imaginary line drawn perpendicular to the reflecting surface at the point of incidence.

5. Angle of Incidence

The angle between the incident ray and the normal is called the angle of incidence.

6. Angle of Reflection

The angle between the reflected ray and the normal is called the angle of reflection.

Common mistake:
Angles of incidence and reflection are measured from the normal, not from the mirror surface.

6. The Two Laws of Reflection

First Law of Reflection

The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

In simple words, these three lines can be represented on the same flat sheet of paper.

Second Law of Reflection

The angle of incidence is equal to the angle of reflection.

∠i = ∠r

Here:

• i represents the angle of incidence.
• r represents the angle of reflection.

Example:
If the angle of incidence is 35°, the angle of reflection is also 35°.

If the angle of incidence is 50°, the angle of reflection is also 50°.

Try This Thinking Challenge

A ray of light makes an angle of 30° with the mirror surface. What is the angle of incidence?

Solution: The normal is perpendicular to the mirror, so:

Angle of incidence = 90° − 30° = 60°

Therefore, the angle of reflection is also 60°.

7. Types of Mirrors

Plane Mirror

A plane mirror has a flat reflecting surface. It is commonly used in dressing mirrors and other everyday applications.

Spherical Mirrors

Spherical mirrors have curved reflecting surfaces. They are broadly classified into concave and convex mirrors.

A concave mirror has a reflecting surface curved inward, while a convex mirror has a reflecting surface curved outward. Their image-forming properties are studied in greater detail in later classes.

Did you know? A mirror usually consists of glass with a reflecting coating on one side. The polished reflecting coating is responsible for most of the reflection.

8. Regular Reflection

When a beam of parallel light rays falls on a smooth and polished surface, the reflected rays remain parallel. This is called regular reflection.

Examples

• Plane mirrors
• Polished metal surfaces
• Still water under suitable conditions

Characteristics

• It occurs on smooth surfaces.
• Parallel incident rays remain parallel after reflection.
• It can produce clear images when the surface is suitable.
• It may cause glare when bright light is reflected towards the eyes.

Real-life connection: Sunlight reflected from a shiny car, mirror or smooth wet road can produce uncomfortable glare.

9. Irregular or Diffused Reflection

When parallel rays of light fall on a rough surface, the reflected rays travel in different directions. This is called irregular reflection or diffused reflection.

Examples

• Walls
• Tree trunks
• Paper
• Rough ground
• Wavy water surfaces

Characteristics

• It occurs on rough surfaces.
• Parallel incident rays are reflected in different directions.
• It generally does not produce a clear image.
• It allows us to see objects from different viewing directions.

Important concept:
Irregular reflection does not mean that the laws of reflection are broken. Each ray still obeys the laws of reflection at the point where it strikes the surface. The different surface orientations cause the reflected rays to travel in different directions.

10. Regular Reflection vs Irregular Reflection

Regular Reflection	Irregular Reflection
Occurs on smooth surfaces.	Occurs on rough surfaces.
Parallel incident rays remain parallel after reflection.	Parallel incident rays are reflected in different directions.
Can produce clear images.	Normally does not produce a clear image.
Example: plane mirror.	Example: wall.

11. Hands-On Activity: Verify the Laws of Reflection

Aim

To verify the two laws of reflection using a plane mirror.

Materials Required

• Plane mirror with a stand
• White sheet of paper
• Cardboard
• Board pins
• Ruler and protractor
• Pencil

Procedure

1. Fix the white sheet of paper on the cardboard.
2. Draw a straight line LL' and mark a point O on it.
3. Draw a normal ON perpendicular to LL' at point O.
4. Draw an incident ray AO making an acute angle with the normal.
5. Place the plane mirror vertically along LL', with its reflecting surface facing the incident ray.
6. Use pins to trace the incident and reflected rays by aligning the pins with their images in the mirror.
7. Remove the mirror and pins, then draw the rays through the marked points.
8. Measure the angle of incidence and the angle of reflection with a protractor.
9. Repeat the experiment using different angles of incidence.

Observation

The angle of incidence is equal to the angle of reflection, within experimental limits.

Conclusion

• The incident ray, reflected ray and normal lie in the same plane.
• The angle of incidence equals the angle of reflection.

Perform the activity under teacher supervision. Handle the mirror and pins carefully. Never direct a powerful beam of light or a laser into anyone's eyes.

12. Reflection of Light in Everyday Life

Dressing Mirrors

Smooth mirrors reflect light in an orderly manner and form clear images.

Road Safety

Glare from headlights on wet roads can make it difficult for drivers to see clearly at night.

Photography

Reflection affects lighting, image quality and unwanted glare in photographs.

Seeing Objects

Diffuse reflection allows light to reach our eyes from objects such as books, walls and trees.

Think About This

Why can you read the printed words on a sheet of paper, even though the paper is not a mirror?

Answer: The paper reflects light diffusely. Some of this reflected light reaches our eyes, allowing us to see the printed words.

13. Thinking-Based MCQs

Choose the correct answer. Read each situation carefully.

1. A ray of light strikes a plane mirror at an angle of incidence of 40°. What is the angle of reflection? 20° 40° 50° 90°

2. A ray makes an angle of 25° with the mirror surface. What is its angle of incidence? 25° 65° 75° 90°

3. Why does a plane mirror form a clear image, while a rough wall normally does not? A wall does not reflect any light. A mirror absorbs all light. A smooth mirror reflects light in an orderly manner, while a rough wall scatters reflected light. A wall produces its own light.

4. A beam of parallel rays falls on a rough surface. What happens after reflection? All rays disappear. The reflected rays travel in different directions. The reflected rays must all travel along the normal. The laws of reflection stop working.

5. Why can bright sunlight reflected from a smooth surface cause glare? The surface produces extra sunlight. The surface absorbs every ray. A large amount of reflected light may enter the eyes. Light stops travelling in straight lines.

6. Which statement about irregular reflection is correct? It violates the laws of reflection. Each ray still obeys the laws of reflection. It occurs only in complete darkness. It occurs only in plane mirrors.

7. If the angle of incidence is 0°, what is the angle of reflection for an ideal plane mirror? 0° 30° 45° 90°

8. Why can we see a book from different positions in a classroom? The book emits its own light. The book reflects light diffusely. The book is a plane mirror. The book absorbs all the light.

9. A ray strikes a plane mirror at 60° to the normal. What angle does the reflected ray make with the normal? 30° 60° 90° 120°

10. Which is the correct definition of the normal? A line parallel to the reflecting surface. The ray that strikes the mirror. A perpendicular line drawn to the reflecting surface at the point of incidence. The reflected ray itself.

14. Quick Revision Quiz

Test your understanding of the chapter.

1. The bouncing back of light into the same medium is called: Refraction Reflection Dispersion Absorption

2. The angle of reflection is measured between: The incident ray and mirror. The reflected ray and the normal. The normal and mirror surface. The incident ray and reflected ray only.

3. The angle of incidence is equal to: The angle of reflection. The angle of the mirror. The length of the normal. The speed of light.

4. Which surface generally produces regular reflection? Rough wall Tree trunk Plane mirror Rough road

5. The approximate speed of light in vacuum is: 340 m/s 3 × 10⁸ m/s 100 km/h 30 m/s

6. The normal is perpendicular to the reflecting surface at the: Centre of the mirror Point of incidence End of the reflected ray Source of light

7. Diffused reflection usually occurs on: Smooth mirrors Polished metal Rough surfaces Perfectly smooth glass only

8. The incident ray, reflected ray and normal lie in: Different media The same plane Different planes A circular path

15. Moral Values and Life Skills

1. Observation

Science begins with careful observation. Notice how light behaves when it strikes different surfaces.

2. Curiosity

Ask questions about everyday phenomena such as mirrors, shadows, glare and reflections in water.

3. Road Safety

Avoid looking directly at bright reflected sunlight. Drivers should slow down and drive carefully when glare reduces visibility.

4. Respect for Evidence

During experiments, record the readings you actually observe. Do not change results simply to match an expected answer.

5. Cooperation

Work together during classroom experiments, share materials responsibly and listen to your classmates' ideas.

6. Responsible Use of Technology

Use mirrors, optical equipment and light sources carefully. Never direct a powerful beam of light into another person's eyes.

16. Curiosity Corner: Did You Know?

Sunlight's Journey

Sunlight takes about 8 minutes 20 seconds to travel from the Sun to Earth.

Why Is the Sky Visible Everywhere?

Light is scattered by molecules and tiny particles in the atmosphere, helping illuminate the sky.

Why Can You See a Wall?

Light reflected diffusely from the wall reaches your eyes. A surface need not be shiny to be visible.

Why Does a Wet Road Look Shiny?

Water can make parts of a road smoother, increasing directional reflection and causing glare.

17. Revision Questions

1. Define reflection of light.
2. What is the approximate speed of light in vacuum?
3. Define incident ray and reflected ray.
4. What is the normal to a reflecting surface?
5. Define the angle of incidence and the angle of reflection.
6. State the two laws of reflection.
7. Differentiate between regular and irregular reflection.
8. Why does a plane mirror form a clear image?
9. Why can irregular reflection help us see objects around us?
10. Why can a wet road cause glare at night?

1. Reflection of light is the bouncing back of light into the same medium after striking a surface.
2. Approximately 3 × 10⁸ m/s in vacuum.
3. The incident ray strikes the reflecting surface. The reflected ray bounces back from the surface.
4. The normal is an imaginary line perpendicular to the reflecting surface at the point of incidence.
5. The angle of incidence is between the incident ray and the normal. The angle of reflection is between the reflected ray and the normal.
6. First law: The incident ray, reflected ray and normal lie in the same plane. Second law: The angle of incidence equals the angle of reflection.
7. Regular reflection occurs on smooth surfaces and reflected parallel rays remain parallel. Irregular reflection occurs on rough surfaces, scattering parallel incident rays into different directions.
8. A plane mirror has a smooth reflecting surface that reflects light in an orderly manner and forms a clear image.
9. Diffuse reflection sends light in many directions, allowing some reflected light to reach our eyes from different positions.
10. Water can make the road surface smoother in some areas. Reflected light may enter drivers' eyes and reduce visibility.

18. Key Takeaways

• Reflection is the bouncing back of light into the same medium.
• The speed of light in vacuum is approximately 3 × 10⁸ m/s.
• The normal is perpendicular to the reflecting surface.
• The angle of incidence equals the angle of reflection.
• Regular reflection occurs on smooth surfaces.
• Irregular reflection occurs on rough surfaces.
• Both types of reflection obey the laws of reflection.
• Reflected light enables us to see objects around us.

Light helps us explore the world. Observe carefully, question thoughtfully and discover the science hidden in everyday life!

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