Thinking based questions on Refraction of light

ICSE Physics

Refraction of Light

Why does a pencil appear bent in water? Why does a swimming pool look shallower than it really is?

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Observe • Question • Investigate • Discover

Welcome to SpeedUpScience, where Physics becomes an adventure!

1. A Fascinating World of Light

Imagine placing a straight pencil in a glass of water. The pencil appears bent at the water surface, even though it is perfectly straight.

Have you ever wondered why?

The answer lies in a beautiful phenomenon called refraction of light.

Refraction helps explain many everyday observations, from the apparent position of objects underwater to the working of lenses and optical instruments.

2. Learning Objectives

After studying this lesson, you will be able to:

• Define refraction of light.
• Explain why light changes direction when it passes from one transparent medium to another.
• State the laws of refraction.
• Explain the bending of light towards or away from the normal.
• Understand the meaning of refractive index.
• Explain refraction through a rectangular glass block.
• Describe the role of a prism in dispersion of white light.
• Apply the concepts of refraction to everyday situations.

3. What Is Refraction of Light?

Refraction of light is the change in direction of the path of light when it passes obliquely from one transparent medium to another because its speed changes.

For example, light travels at different speeds in air, water and glass.

Remember This!

Speed of light in vacuum is approximately:

3 × 10⁸ m/s

Light travels more slowly in water and glass than in air.

Why Does Light Bend?

When light enters a different transparent medium at an angle, its speed changes. This change in speed causes the light ray to change direction.

If light enters the new medium normally, that is, perpendicular to the boundary, its speed changes but its direction does not.

Important distinction: Light does not necessarily bend every time it enters another medium. A ray entering normally continues along the same straight path, although its speed changes.

4. The Normal, Incident Ray and Refracted Ray

To understand refraction, we must know three important terms.

Incident Ray

The ray of light that strikes the boundary between two transparent media.

Normal

An imaginary line drawn perpendicular to the boundary at the point where the incident ray strikes it.

Refracted Ray

The ray that travels into the second medium after crossing the boundary.

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

The angle of refraction is the angle between the refracted ray and the normal.

Quick Thinking

Are the angles of incidence and refraction measured from the surface or from the normal?

Reveal the answer

Both angles are measured from the normal, not from the surface.

5. Rules of Refraction

Case 1: From a Rarer Medium to a Denser Medium

When light travels obliquely from a rarer optical medium, such as air, into a denser optical medium, such as glass, it bends towards the normal.

Example: A ray travelling from air into glass bends towards the normal when it enters at an angle.

Case 2: From a Denser Medium to a Rarer Medium

When light travels obliquely from a denser optical medium into a rarer optical medium, it bends away from the normal.

Example: A ray travelling from glass into air bends away from the normal when it emerges obliquely.

Case 3: Light Travels Along the Normal

When a ray enters the second medium along the normal, it passes without changing its direction.

Memory tip:

Rarer to denser: towards the normal.

Denser to rarer: away from the normal.

Along the normal: no change in direction.

Note: The terms optically rarer and optically denser refer to how a medium affects the speed of light, not simply to its physical density.

6. Laws of Refraction

There are two laws of refraction.

First Law

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

Second Law: Snell's Law

For a given pair of media and a given wavelength of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

sin i / sin r = constant

For light entering a second medium from air, this ratio is approximately the refractive index of that medium relative to air.

7. Understanding Refractive Index

The refractive index indicates how much the speed of light is reduced in a medium compared with its speed in vacuum.

n = c / v

Here:

• n = absolute refractive index of the medium.
• c = speed of light in vacuum.
• v = speed of light in the medium.

Worked Example

The speed of light in a transparent medium is 2 × 10⁸ m/s. Calculate its absolute refractive index.

Given:

c = 3 × 10⁸ m/s

v = 2 × 10⁸ m/s

Using the formula:

n = c / v

n = (3 × 10⁸) / (2 × 10⁸)

Answer: n = 1.5

Science Insight

A higher refractive index generally means light travels more slowly in that medium. When light enters a medium with a higher refractive index obliquely, it bends towards the normal.

8. Refraction Through a Rectangular Glass Block

When a ray of light enters a rectangular glass block from air, it bends towards the normal.

When the ray emerges from the opposite parallel face into air, it bends away from the normal.

Because the two faces are parallel, the emergent ray is parallel to the incident ray, provided the ray passes through the block and emerges into the same surrounding medium.

However, the emergent ray is shifted sideways compared with the original path. This is called lateral displacement.

What Do We Observe?

• The ray bends towards the normal on entering the glass.
• The ray bends away from the normal on leaving the glass.
• The emergent ray is parallel to the incident ray.
• The emergent ray is laterally displaced.

Think Like a Scientist

What would happen to the direction of the ray if it entered the glass block along the normal?

Reveal the answer

It would continue along the same straight line without changing direction at the first surface. It would also emerge along that line, assuming parallel faces and the same surrounding medium.

9. Story Time: The Mystery of the Missing Coin

Meet Ananya, the Curious Scientist!

One evening, Ananya placed a coin at the bottom of an empty bowl. She stepped back until the edge of the bowl just hid the coin from her eyes.

Her father carefully poured water into the bowl without moving the coin.

Suddenly, Ananya could see the coin!

"How did the coin move?" she asked.

Her father smiled. "The coin did not move. The light coming from it changed direction as it passed from water into air."

Ananya remembered her Physics lesson. She realised that the light rays were refracted at the water surface, making the coin appear higher than its actual position.

The next day, she placed a pencil in a glass of water and observed that it appeared bent at the surface.

She wrote in her science notebook:

"Sometimes our eyes see an apparent position, not the actual position of an object. Science helps us understand the difference."

Story-Based Questions

1. Why did Ananya see the coin after water was poured into the bowl?
2. Did the coin actually move?
3. Why does a pencil appear bent when partly immersed in water?

Check the answers

1. Light from the coin was refracted at the water-air surface and could reach Ananya's eyes.

2. No. The coin remained at the bottom of the bowl.

3. Light from the submerged part changes direction as it passes from water into air, making that part appear displaced.

10. A Prism and the Colours of White Light

When white light passes through a glass prism, it splits into its constituent colours. This phenomenon is called dispersion of light.

The band of colours obtained is called the spectrum of white light.

The seven commonly named colours are:

Violet

Indigo

Blue

Green

Yellow

Orange

Red

Remember VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange and Red.

Newton's Discovery

Isaac Newton investigated white light using a glass prism and showed that white light can be separated into different colours.

Important: A prism does not create the colours present in white light. It separates the colours already present in the light because they refract by different amounts.

11. Refraction in Everyday Life

1. A Pencil in Water

A pencil partly immersed in water appears bent because light changes direction as it passes from water into air.

2. A Swimming Pool Appears Shallow

Light rays coming from the bottom of a pool refract as they pass from water into air. The bottom therefore appears nearer to the surface than it really is.

3. Lenses

Lenses use refraction to converge or diverge light rays. They are used in spectacles, cameras, microscopes and telescopes.

4. A Rainbow

Water droplets refract and disperse sunlight. Internal reflection also occurs inside the droplets, helping produce a rainbow.

12. A Quick Connection: Reflection and Mirrors

Your textbook also recaps reflection and spherical mirrors. These concepts are different from refraction but are important parts of the study of light.

Feature	Reflection	Refraction
What happens?	Light returns from a surface.	Light changes direction when entering another medium obliquely.
Medium	Light remains in the original medium after reflection.	Light travels into a different medium.
Example	Image in a plane mirror.	Pencil appearing bent in water.

Plane and Spherical Mirrors

• A concave mirror can produce different types of images depending on the object's position.
• A convex mirror always forms a virtual, erect and diminished image of a real object.
• Convex mirrors are used as rear-view mirrors because they provide a wider field of view.
• Concave mirrors can be used in torches and vehicle headlights to produce a directed beam of light.

13. Become a Young Light Investigator!

Activity A: The Pencil Experiment

Materials: A transparent glass, water and a pencil.

1. Fill the glass with water.
2. Place the pencil partly inside the water.
3. Observe the pencil from the side.
4. Notice how the pencil appears bent at the water surface.
5. Explain the observation using refraction.

Learning outcome: Light changes direction when it passes between water and air.

Activity B: The Coin Experiment

Materials: A bowl, a coin and water.

1. Place the coin in the bottom of an empty bowl.
2. Move your head until the edge of the bowl just hides the coin.
3. Keep your head still while another person carefully adds water.
4. Observe whether the coin becomes visible.
5. Discuss how refraction changes the path of light reaching your eyes.

Perform the activity with adult supervision and take care around glass containers.

14. Thinking Beyond the Textbook

Question 1: The Underwater Treasure

A diver sees a coin lying at the bottom of a swimming pool. Does the coin appear at its actual depth?

Think about the direction of light travelling from water into air.

Question 2: The Straight Ray

A ray of light enters a glass block along the normal. Will it bend towards the normal?

Remember the direction in which the ray is travelling.

Question 3: The Glass Block

Why is the emergent ray parallel to the incident ray when light passes through a rectangular glass block surrounded by air?

Think about the parallel faces of the block.

Question 4: The Prism Puzzle

Does a glass prism produce new colours from nothing, or does it separate the colours already present in white light?

Question 5: The Swimming Pool

Why should you not judge the actual depth of a swimming pool only by looking at its bottom?

Reveal the answers

1. No. The coin generally appears closer to the water surface than its actual position.

2. No. It continues in the same direction, although its speed changes in glass.

3. The ray bends towards the normal on entering and away from the normal on leaving. Because the two faces are parallel and the surrounding medium is the same, the emergent ray is parallel to the incident ray but laterally displaced.

4. A prism separates the colours already present in white light.

5. Refraction makes the bottom appear shallower than it really is.

15. Values We Learn Through Science

Curiosity

Asking why a pencil appears bent or a coin becomes visible encourages scientific thinking.

Observation

Careful observation helps us notice patterns and understand the world around us.

Patience

Scientific experiments require patience, attention and repeated observation.

Honesty

A good scientist records what is actually observed instead of changing results to match expectations.

Safety

Use glassware carefully and never look directly at the Sun or powerful light sources during experiments.

16. Interactive MCQ Challenge

Test your understanding with 20 questions covering refraction, refractive index, glass blocks, dispersion and applications of light.

Each correct answer carries 1 mark. Maximum score: 20.

1. What is refraction of light?

The bouncing back of light from a surface The change in direction of light when it passes obliquely between transparent media The production of light by an object The formation of a shadow

2. The speed of light in vacuum is approximately:

3 × 10^6 m/s 3 × 10^7 m/s 3 × 10^8 m/s 3 × 10^9 m/s

3. When light travels obliquely from air into glass, it generally bends:

Towards the normal Away from the normal Along the surface Back into the air

4. A ray of light travels obliquely from glass into air. It bends:

Towards the normal Towards the surface only Without changing speed Away from the normal

5. A ray enters glass normally. What happens to its direction?

It bends towards the normal It continues along the same straight path It bends away from the normal It is reflected completely

6. The normal is drawn:

Parallel to the boundary Along the incident ray Perpendicular to the boundary Along the refracted ray

7. The angle of incidence is measured between:

The incident ray and the normal The incident ray and the surface The refracted ray and the surface The two surfaces of a block

8. According to the first law of refraction, the incident ray, refracted ray and normal lie:

In different planes On different surfaces In perpendicular planes In the same plane

9. Which formula gives the absolute refractive index?

n = v/c n = c/v n = c × v n = c + v

10. Light travels through a medium at 2 × 10⁸ m/s. What is its absolute refractive index?

0.5 1.0 1.5 2.5

11. Light passes through a rectangular glass block. The emergent ray is generally:

Parallel to the incident ray but laterally displaced Perpendicular to the incident ray Always reflected backwards Completely absorbed

12. The sideways shift of a ray passing through a rectangular glass block is called:

Dispersion Reflection Diffusion Lateral displacement

13. The splitting of white light into its constituent colours is called:

Reflection Dispersion Absorption Diffraction

14. A glass prism separates white light because:

It creates light from nothing It absorbs all colours except red Different colours refract by different amounts It reflects all light without refraction

15. A swimming pool appears shallower than it really is because of:

Refraction Sound reflection Magnetic attraction Evaporation

16. A pencil partly immersed in water appears bent because:

The pencil actually bends Water produces light Light stops at the water surface Light is refracted at the water-air surface

17. Which mirror is commonly used as a rear-view mirror in vehicles?

Plane mirror only Convex mirror Concave mirror only A glass prism

18. The image formed by a convex mirror is always:

Real and inverted Real and enlarged Virtual, erect and diminished Inverted and diminished

19. The angle of refraction is measured between:

The refracted ray and the normal The refracted ray and the surface The incident ray and the surface The normal and the surface

20. A ray enters glass normally from air. Which statement is correct?

It bends away from the normal It bends towards the normal Its speed remains unchanged Its direction remains unchanged, but its speed decreases

Your Physics Report Card

17. Quick Revision

• Refraction occurs because the speed of light changes when it enters another medium.
• Light bends towards the normal when it enters an optically denser medium obliquely.
• Light bends away from the normal when it enters an optically rarer medium obliquely.
• A ray travelling along the normal does not change direction.
• The incident ray, refracted ray and normal lie in the same plane.
• Refractive index: n = c/v for the absolute refractive index.
• A rectangular glass block produces lateral displacement.
• A prism separates white light into its constituent colours.
• Dispersion is the splitting of white light into colours.
• A convex mirror gives a wide field of view and forms a virtual, erect and diminished image.

Your Mission as a Young Scientist!

The next time a pencil appears bent in water or a swimming pool looks shallow, remember that you are observing the fascinating behaviour of light.

Observe carefully. Ask questions. Think scientifically.

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