Light and Shadow- Thinking based

LIGHT AND SHADOW

Class 6 ICSE Physics

Discover how light travels, shadows form and eclipses occur!

Observe. Imagine. Investigate. Learn.

What Will You Learn?

• Natural and artificial sources of light.
• Luminous and non-luminous objects.
• Transparent, translucent and opaque materials.
• Rectilinear propagation of light.
• Formation and characteristics of shadows.
• Umbra and penumbra.
• Working of a pinhole camera.
• Solar and lunar eclipses.

1. A Little Story: The Mystery of the Changing Shadow!

One sunny morning, Aarav went into the garden with his sister Meera. He noticed that his shadow was very long.

At noon, he looked down and discovered that his shadow had become much shorter. In the evening, it grew long again.

"Meera, why does my shadow keep changing? Is it playing hide-and-seek with me?" Aarav asked.

Meera smiled. "Your shadow is not playing games! Its length and direction change because the Sun appears at different positions in the sky throughout the day."

Aarav decided to investigate. He took a torch, a toy and a white sheet of paper. When he placed the toy between the torch and the paper, a dark patch appeared.

He moved the toy closer to the torch. The shadow became larger!

Then Aarav placed a clear glass sheet between the torch and the paper. Unlike the toy, the glass did not produce a dark shadow.

Aarav had discovered three important scientific ideas: light travels in straight lines, opaque objects block light, and the position of an object can change the size of its shadow.

Science detective challenge: Can you explain all three observations using what you know about light?

2. What Is Light?

Light is a form of energy that enables us to see objects.

We see a luminous object when light from it enters our eyes. We see a non-luminous object when light from another source falls on it and is reflected into our eyes.

For example, we can see a book because light falls on its surface and is reflected towards our eyes.

Sources of Light

A source of light is an object that emits light.

Natural Sources of Light

These sources occur naturally.

• Sun
• Stars
• Lightning
• Fireflies

Artificial Sources of Light

These are sources made by humans.

• Electric bulb
• Torch
• LED lamp
• Candle flame

Luminous and Non-Luminous Objects

Luminous objects produce their own light.

Examples include the Sun, stars, a burning candle and a glowing electric bulb.

Non-luminous objects do not produce their own visible light. We see them because they reflect light from a luminous source.

Examples include the Moon, Earth, books, chairs and trees.

Think About It!

The Moon looks bright in the night sky. Does it produce its own light?

No. The Moon is non-luminous. It appears bright because its surface reflects sunlight towards the Earth.

3. Transparent, Translucent and Opaque Materials

Different materials allow different amounts of light to pass through them.

Type	How It Allows Light to Pass	Examples
Transparent	Allows most light to pass through. Objects can be seen clearly.	Clear glass, clean water
Translucent	Allows some light to pass through, but objects cannot be seen clearly.	Frosted glass, tracing paper
Opaque	Does not allow light to pass through.	Wood, metal sheet, thick book

Think and Explain

A person standing behind a clear glass window can be seen clearly. The same person standing behind a wooden door cannot be seen. Why?

Clear glass is transparent and allows most light to pass through. Wood is opaque and blocks light.

4. Light Travels in a Straight Line

In a uniform transparent medium, light travels in a straight line. This property is called rectilinear propagation of light.

Let us understand this through a simple experiment.

Activity: The Three-Cardboard Experiment

Materials required: Three cardboard sheets with small holes, a torch and a stand or support.

Procedure:

1. Make a small hole near the centre of each cardboard sheet.
2. Place the three sheets upright in a row.
3. Adjust them so that all three holes are in a straight line.
4. Shine a torch through the first hole and look from the other end.
5. Now move the middle cardboard slightly sideways.
6. Observe what happens to the light.

Observation: Light can pass through all three holes when they are aligned. When the middle hole is moved sideways, the light no longer passes through all three holes along the same straight path.

Conclusion: The experiment demonstrates that light travels in straight lines through a uniform transparent medium.

Applications of Rectilinear Propagation

• Formation of shadows.
• Working of a pinhole camera.
• Formation of solar and lunar eclipses.

5. How Is a Shadow Formed?

A shadow is a dark region formed when an opaque object blocks light from reaching a surface.

Three things are needed to form a shadow:

1. A source of light.
2. An opaque object that blocks the light.
3. A screen or surface on which the shadow falls.

Imagine shining a torch towards a white wall while holding a thick book between the torch and the wall. The book blocks the light, so a dark patch appears on the wall. This dark patch is the shadow.

Characteristics of a Shadow

• A shadow forms when an opaque object blocks light.
• It appears on the side opposite the light source.
• It does not show the object's colours or surface details.
• Its size can change when the positions of the light source, object and screen change.
• It can have sharp or blurred edges, depending on the light source and the arrangement.

Predict Before You Read!

You shine a torch towards a wall and place a toy between the torch and the wall. What happens to the shadow when you move the toy closer to the torch while keeping the torch and wall fixed?

The shadow generally becomes larger. When the toy moves closer to the torch, it blocks a wider region of the light spreading towards the wall.

6. Why Does a Shadow Change During the Day?

The Sun appears to change its position in the sky because the Earth rotates on its axis. As the Sun's apparent position changes, the length and direction of shadows also change.

• Morning: The Sun is generally low in the sky, so shadows are long.
• Around noon: The Sun is usually higher in the sky, so shadows are shorter.
• Evening: The Sun is low again, so shadows become long.

The exact shadow length and direction depend on the location, season and time of observation.

Observation Challenge

Stand in a safe, open area and observe your shadow in the morning and afternoon.

Ask yourself:

• Is the shadow longer in the morning?
• Does its direction change?
• What happens to its length as the Sun appears higher in the sky?

Record your observations and explain the changes using the apparent position of the Sun.

7. Umbra and Penumbra

When an extended source of light is blocked by an opaque object, two regions may form behind the object: the umbra and the penumbra.

Umbra: The darkest central region of a shadow, where light from the source is completely blocked.

Penumbra: The partially shaded region, where light from part of the source is blocked but light from another part can still reach the region.

Umbra	Penumbra
The darkest region of a shadow.	A partially shaded region.
Light from the entire source is blocked.	Light from only part of the source is blocked.
Appears darker.	Appears lighter than the umbra.

Think Like a Scientist

Why does a shadow sometimes have a dark centre and a lighter region around it?

An extended source of light produces different degrees of shading. In the umbra, light from the entire source is blocked. In the penumbra, light from only part of the source is blocked.

8. Pinhole Camera: An Amazing Application of Light

A pinhole camera is a simple device that forms an image through a tiny hole. It can be made using a light-tight cardboard box, a tiny hole on one side and a translucent screen on the opposite side.

How Does a Pinhole Camera Work?

1. Light from an object travels in straight lines.
2. Only a narrow bundle of light rays passes through the tiny hole from each point of the object.
3. Light from the top of the object reaches the lower part of the screen.
4. Light from the bottom reaches the upper part of the screen.
5. An inverted image is formed on the screen.

Remember These Facts

• The image formed by a simple pinhole camera is real.
• The image is inverted.
• The image can be obtained on a screen.
• The size of the image depends on the arrangement of the object, pinhole and screen.

Think and Explain

Why does the image generally become larger when the distance between the pinhole and the screen is increased?

When the screen is moved farther from the pinhole, the light rays spread over a larger area of the screen, producing a larger image. The image may also become less bright because the light spreads over a larger area.

9. Eclipses: When Shadows Meet Space!

An eclipse occurs when the Sun, Earth and Moon are aligned in such a way that one celestial body blocks sunlight or enters the shadow of another body.

Solar Eclipse

A solar eclipse occurs when the Moon comes between the Sun and the Earth and blocks some or all of the Sun's light from reaching a region of the Earth.

Remember the Alignment

Sun → Moon → Earth

During a solar eclipse, the Moon casts its shadow on the Earth.

• Total solar eclipse: An observer within the Moon's umbra sees the Sun completely covered.
• Partial solar eclipse: An observer within the Moon's penumbra sees only part of the Sun covered.
• Annular solar eclipse: The Moon appears too small to cover the Sun completely, leaving a bright ring visible around it.

A solar eclipse occurs at new Moon when the alignment is suitable.

Lunar Eclipse

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, and the Moon passes into the Earth's shadow.

Remember the Alignment

Sun → Earth → Moon

During a lunar eclipse, the Earth's shadow falls on the Moon.

A lunar eclipse occurs at full Moon when the alignment is suitable.

Solar Eclipse	Lunar Eclipse
The Moon comes between the Sun and Earth.	The Earth comes between the Sun and Moon.
Occurs at new Moon when the alignment is suitable.	Occurs at full Moon when the alignment is suitable.
The Moon's shadow falls on the Earth.	The Moon passes through the Earth's shadow.

Important Safety Rule

Never look directly at the Sun during a solar eclipse without proper eye protection. Use certified solar-viewing glasses that comply with the relevant safety standard or a safe indirect viewing method recommended by a qualified source.

Ordinary sunglasses are not safe for viewing the Sun. Never look at the Sun through a camera, telescope or binoculars without a properly designed solar filter fitted correctly to the front of the optical device.

10. Thinking-Based Questions: Become a Science Detective!

Read each question carefully. Think about the scientific principle before revealing the answer.

Q1. A student shines a torch at a wooden board. A dark patch appears on the wall behind it. Why?

Wood is opaque and blocks light. The region on the wall that receives no direct light from the torch appears as a shadow.

Q2. Can a shadow be formed in a completely dark room without any source of light?

No. A shadow requires light that can be blocked by an object. Without light, no shadow can be formed.

Q3. Why can we see a tree but usually cannot see the clean air surrounding it?

Light reflected by the tree enters our eyes, allowing us to see it. Clean air usually allows light to pass through and does not scatter enough visible light for us to see it clearly.

Q4. Why is a person's shadow often more distinct in direct sunlight than in a shaded area?

Direct sunlight creates a strong contrast between the illuminated ground and the region blocked by the person. In shaded areas, light is often more diffuse, so the shadow may be less distinct.

Q5. A clear glass sheet is placed between a torch and a screen. Will it always form a dark shadow like a wooden board?

No. Clear glass allows most light to pass through, so it usually does not produce a dark shadow. Reflections, absorption, dirt or imperfections may produce faint shadows or patterns.

Q6. Why are eclipses not seen at every new Moon and every full Moon?

The Moon's orbit is tilted relative to the Earth's orbital plane. Therefore, the Sun, Earth and Moon do not align suitably for an eclipse at every new Moon or full Moon.

Q7. A student moves a toy closer to a torch while keeping the screen fixed. What generally happens to the shadow?

The shadow generally becomes larger because the toy blocks a wider region of the light spreading towards the screen.

Q8. Can objects cast shadows in moonlight?

Yes. Moonlight is reflected sunlight. An opaque object can block moonlight and produce a faint shadow when the surroundings are suitable.

11. Quick Revision: Remember These Facts

• Light is a form of energy that enables us to see objects.
• The Sun is a natural luminous source.
• The Moon is non-luminous because it reflects sunlight.
• Transparent materials allow most light to pass through.
• Translucent materials allow some light to pass through.
• Opaque objects block light.
• Rectilinear propagation means light travels in a straight line in a uniform transparent medium.
• A shadow forms when an opaque object blocks light.
• Umbra is the darkest region of a shadow.
• Penumbra is a partially shaded region.
• A simple pinhole camera forms a real, inverted image.
• During a solar eclipse, the Moon's shadow falls on the Earth.
• During a lunar eclipse, the Moon passes through the Earth's shadow.

12. Interactive MCQ Challenge

Are you ready to become a Light and Shadow champion?

Choose one answer for each question. Submit your responses to receive your score, grade, explanations and personalised feedback.

1. Which of the following is a natural source of light?

A. Moon B. Sun C. Mirror D. Book

2. The Earth and Moon are generally classified as:

A. Luminous objects B. Transparent objects C. Non-luminous objects D. Artificial sources of light

3. Which material is translucent?

A. Clear glass B. Wooden board C. Frosted glass D. Metal plate

4. Rectilinear propagation means that light travels:

A. In a straight line in a uniform transparent medium B. Only in circles C. Only through opaque objects D. Without any direction

5. Which of the following is essential for producing a shadow?

A. A luminous source only B. An opaque object that blocks light C. A transparent object only D. A mirror only

6. The darkest part of a shadow is called:

A. Penumbra B. Reflection C. Umbra D. Refraction

7. A simple pinhole camera generally forms an image that is:

A. Erect and virtual B. Inverted and real C. Always upright D. Invisible under all conditions

8. Which alignment is correct for a solar eclipse?

A. Sun → Earth → Moon B. Earth → Sun → Moon C. Moon → Earth → Sun D. Sun → Moon → Earth

9. A lunar eclipse occurs when:

A. The Moon comes between the Sun and Earth B. The Moon passes through the Earth's shadow C. The Sun passes between the Earth and Moon D. The Moon produces its own light

10. Why are eclipses not seen every month?

A. The Sun stops producing light B. The Moon stops revolving around Earth C. The Moon's orbit is tilted relative to Earth's orbital plane D. Shadows cannot form in space

Your Science Report Card

13. Answer Key for Self-Assessment

Attempt all ten questions before checking your answers.

Click here to reveal the complete answer key

1. B — Sun
2. C — Non-luminous objects
3. C — Frosted glass
4. A — Light travels in a straight line in a uniform transparent medium
5. B — An opaque object that blocks light
6. C — Umbra
7. B — Inverted and real
8. D — Sun → Moon → Earth
9. B — The Moon passes through the Earth's shadow
10. C — The Moon's orbit is tilted relative to Earth's orbital plane

14. Your Mission: Become a Shadow Scientist!

Put your scientific knowledge into practice with these simple activities.

Mission 1: Shadow Detective

Materials: Torch, toy and white wall.

Shine the torch towards the wall and place the toy between them. Move the toy closer to the torch and then closer to the wall.

Your challenge: Observe how the shadow changes in size.

Mission 2: Transparent or Opaque?

Materials: Clear glass, tracing paper, a book and a metal plate.

With adult supervision, shine a torch through each material and observe how much light passes through.

Your challenge: Classify each material as transparent, translucent or opaque.

Mission 3: Pinhole Explorer

With your teacher's guidance, make a simple pinhole camera using a cardboard box, a tiny hole and a translucent screen.

Your challenge: Observe the image of a bright object without looking directly at the Sun.

Mission 4: Shadow Diary

Observe the shadow of the same object in the morning, around noon and in the afternoon.

Record the time, shadow length and direction in a notebook.

Your challenge: Explain the changes using the apparent position of the Sun.

15. Values Through Science

Science teaches us to observe carefully, ask questions, test our ideas and respect evidence.

When an experiment gives an unexpected result, a good scientist does not give up. Instead, the scientist investigates the reason.

Remember: Curiosity leads to questions, observation provides evidence, and reasoning helps us understand the world.

Keep Exploring!

Observe the light. Investigate the shadow. Discover the science.

SpeedUpScience

Making Physics Simple, Interesting and Meaningful!