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Light: Shadows and Reflections

NCERT Class 7 · Science Based on NCERT Class 7 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the fundamental properties of light, including its sources, how it travels, and what happens when it interacts with different materials. It explains the formation of shadows, the reflection of light by mirrors, and the creation of images in plane mirrors and pinhole cameras. The one big idea a student must walk away with is that light travels in a straight line, and this principle governs everything from shadow formation to how we see images in mirrors and pinhole cameras.
Key Concepts (13)
Luminous objects
Objects that emit their own light are called luminous objects.
e.g. The Sun, stars, lightning, fire, fireflies, and electric bulbs are luminous objects.
Non-luminous objects
Objects that do not emit their own light are called non-luminous objects.
e.g. The Moon, a chair, a book, and a tree are non-luminous objects. The Moon shines by reflecting the light of the Sun.
Transparent materials
Materials through which light passes almost completely are called transparent materials.
e.g. Glass, clear water, and air are transparent materials.
Translucent materials
Materials through which light passes partially are called translucent materials.
e.g. Tracing paper, frosted glass, and thin cloth are translucent materials.
Opaque materials
Materials through which light does not pass at all are called opaque materials.
e.g. Cardboard, wood, thick cloth, and a metal plate are opaque materials.
Shadow
A shadow is a dark patch formed when light is blocked by an opaque object placed in its path.
e.g. When you stand in sunlight, your body blocks the light and a dark patch (your shadow) forms on the ground.
Reflection of light
The change in the direction of light by a mirror is called the reflection of light.
e.g. When sunlight falls on a shiny steel plate, the plate changes the direction of the light, creating a bright spot on a wall.
Image
An image is the appearance of an object formed by a mirror or a pinhole camera.
e.g. When you look into a plane mirror, you see an image of your face behind the mirror.
Erect image
An upright image is called an erect image.
e.g. The image of a pen in a plane mirror is erect because the tip of the pen appears on top.
Lateral inversion
The perceived left-right reversal in the image formed by a plane mirror is called lateral inversion.
e.g. When you raise your left arm, your image raises its right arm. The word 'AMBULANCE' is written in reverse on vehicles so that it appears correctly in rear-view mirrors.
Pinhole camera
A pinhole camera is a device in which light rays from an object pass through a tiny hole (a pinhole) and form an image on a screen.
e.g. A simple pinhole camera can be made using two cardboard boxes, with a pinhole on one box and a tracing paper screen on the other.
Periscope
A periscope is a device that uses two plane mirrors placed in a Z-shaped box to see objects that are not directly visible.
e.g. Periscopes are used in submarines, tanks, and by soldiers to see outside their bunkers.
Kaleidoscope
A kaleidoscope is a device that uses three rectangular plane mirror strips joined in a triangular manner to create beautiful patterns by multiple reflections.
e.g. When you peep through a kaleidoscope and turn it, you see different patterns each time due to reflections of reflections.
How It Works
Demonstrating that light travels in a straight line (Matchbox activity)
  1. Take three matchboxes and make a hole in the inner tray of each, exactly at the same position.
  2. Arrange the three matchboxes in a straight line so that all three holes are exactly at the same height and in a line.
  3. Place a torch on one side of the matchboxes, ensuring its lamp is at the height of the holes.
  4. Place a cardboard screen on the other side and obtain a bright spot on it.
  5. Move one matchbox slightly to a side or up and down. The light spot on the screen disappears.
💡 When all three holes are not in the same straight line, the light spot cannot be obtained on the screen. This proves light travels in a straight line.
Demonstrating that light travels in a straight line (Bent pipe activity)
  1. Take a long hollow pipe of flexible material and align it straight so you can see a candle flame through it.
  2. Bend the pipe and try to see the candle flame again.
  3. You can see the flame through the straight pipe but not through the bent pipe.
💡 Light cannot bend around corners in a pipe; it only travels in a straight line.
Shadow formation
  1. A source of light (e.g., torch, Sun) emits light in straight lines.
  2. An opaque object is placed in the path of the light.
  3. The object blocks the light, preventing it from reaching a screen (wall, ground, or floor).
  4. A dark patch, called a shadow, is formed on the screen where light does not reach.
💡 Shadows are always formed on the side opposite to the light source. The colour of the object does not change the colour of the shadow.
Reflection of light by a plane mirror
  1. A thin beam of light is made to fall on a plane mirror.
  2. The mirror changes the direction of the light beam.
  3. The light beam bounces off the mirror and travels in a new straight-line path.
💡 Reflection is the change in direction of light by a mirror. The light does not pass through the mirror; it bounces off.
Image formation in a plane mirror
  1. An object is placed in front of a plane mirror.
  2. Light from the object falls on the mirror and is reflected.
  3. The reflected light appears to come from behind the mirror, forming an image.
  4. The image is of the same size as the object, erect, and laterally inverted.
  5. The image cannot be obtained on a screen.
💡 The image in a plane mirror is virtual (cannot be obtained on a screen), erect, same size, and laterally inverted. The distance of the image behind the mirror equals the distance of the object in front of the mirror.
Image formation in a pinhole camera
  1. Light from an object (e.g., a candle flame) passes through a tiny hole (pinhole) in a cardboard.
  2. The light rays travel in straight lines through the pinhole.
  3. An inverted (upside-down) image of the object is formed on the screen placed behind the pinhole.
💡 The image formed by a pinhole camera is always inverted (upside down). It is a real image that can be obtained on a screen.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'shadow' with 'image'
✗ A shadow is the same as the image of an object.
→ A shadow is a dark patch formed when light is blocked, while an image is a reproduction of the object formed by reflection or through a pinhole. Shadows do not show details like colour or shape of the object, while images do.
✓ A shadow is a dark region formed when an opaque object blocks light. An image is a reproduction of the object formed by a mirror or pinhole camera, showing details like shape and colour.
⚠ Thinking the Moon is a luminous object
✗ The Moon is a luminous object because it shines at night.
→ The Moon does not emit its own light. It only reflects the light of the Sun. It is a non-luminous object.
✓ The Moon is a non-luminous object. It shines by reflecting the light of the Sun.
⚠ Believing shadows have colour
✗ If you change the colour of the object, the colour of the shadow also changes.
→ Shadows are formed by the absence of light. They are always dark (black or grey) regardless of the colour of the object. Changing the colour of the object does not change the colour of the shadow.
✓ Changing the colour of an opaque object does not change the colour of its shadow. Shadows are always dark because they are formed where light is blocked.
⚠ Confusing 'erect' with 'not inverted' in a plane mirror
✗ The image in a plane mirror is erect, meaning it is not upside down and not laterally inverted.
→ Erect means upright (not upside down), but a plane mirror image is also laterally inverted (left-right reversed). Students often forget lateral inversion.
✓ The image formed by a plane mirror is erect (upright) and laterally inverted (left-right reversed).
⚠ Thinking a pinhole camera image is erect
✗ The image formed by a pinhole camera is erect, like in a plane mirror.
→ A pinhole camera always forms an inverted (upside-down) image because light travels in straight lines and crosses at the pinhole.
✓ The image formed by a pinhole camera is inverted (upside down).
⚠ Assuming all transparent objects form no shadow
✗ Transparent objects never form shadows.
→ Even some transparent objects can create faint shadows because they may not allow 100% of light to pass through, or they may refract light slightly.
✓ Transparent objects allow most light to pass through, but some transparent objects can create faint shadows.
Compare & Contrast
Nature and formation
Shadow
Image
A shadow is a dark patch formed when light is blocked by an opaque object. An image is a reproduction of an object formed by reflection (mirror) or by light passing through a pinhole (pinhole camera). Shadows are always dark and show no details; images show details like shape and colour.
Orientation and nature
Image in a plane mirror
Image in a pinhole camera
The image in a plane mirror is erect (upright), laterally inverted, and cannot be obtained on a screen (virtual). The image in a pinhole camera is inverted (upside down) and can be obtained on a screen (real).
Exam Focus
Marks pattern: 2 MCQs (1 mark each) on luminous/non-luminous objects and properties of shadows/images, 1 SA (3 marks) on properties of plane mirror image or pinhole camera, 1 LA (5 marks) on shadow formation or reflection of light
Most asked: Luminous and non-luminous objects · Properties of shadows (formation, shape, size, colour) · Properties of image formed by a plane mirror (same size, erect, laterally inverted, cannot be obtained on screen) · Image formation in a pinhole camera (inverted image) · Transparent, translucent, and opaque materials
Don't skip: Explain the properties of an image formed by a plane mirror (5 marks); Explain how a shadow is formed and list the conditions required (3 marks); Differentiate between a shadow and an image (3 marks); Describe the working of a pinhole camera and the nature of the image formed (5 marks)
One-Line Revision (read aloud before the exam)
  1. Luminous objects emit their own light; non-luminous objects do not.
  2. The Sun is the main natural source of light on Earth.
  3. The Moon is a non-luminous object that reflects the light of the Sun.
  4. Light travels in a straight line.
  5. Transparent materials allow light to pass almost completely through them.
  6. Translucent materials allow light to pass partially through them.
  7. Opaque materials do not allow light to pass through them at all.
  8. A shadow is formed when an opaque object blocks light from falling on a screen.
  9. Three things are needed to form a shadow: a source of light, an opaque object, and a screen.
  10. The colour of an object does not change the colour of its shadow.
  11. Reflection of light is the change in direction of light by a mirror.
  12. The image formed by a plane mirror is of the same size as the object, erect, and laterally inverted.
  13. The image formed by a plane mirror cannot be obtained on a screen.
  14. A pinhole camera forms an inverted (upside-down) image on a screen.
  15. A periscope uses two plane mirrors to see objects that are not directly visible.
  16. A kaleidoscope uses three mirrors to create beautiful patterns by multiple reflections.
  17. Lateral inversion means left appears right and right appears left in a plane mirror image.
  18. Fireflies are luminous objects that use light to communicate.
  19. LED lamps consume less power, are brighter, and last longer than traditional lamps.
  20. Light from the Sun takes about 8 minutes and 20 seconds to reach Earth.

MCQs — 10 Questions with Answers

Q1. Which of the following is a non-luminous object?

  • A. The Sun
  • B. A lightning bolt
  • C. The Moon ✓
  • D. An electric bulb

Answer: C — The Moon does not emit its own light; it only reflects sunlight, making it non-luminous.

Q2. What is the main conclusion from Activity 11.2 (the bent pipe experiment)?

  • A. Light bends around corners easily
  • B. Light travels in a straight line ✓
  • C. Light needs a mirror to travel
  • D. Light moves very slowly through pipes

Answer: B — When the pipe is bent, you cannot see the candle flame, proving light travels in a straight line and cannot bend around corners.

Q3. Which material is transparent?

  • A. Cardboard
  • B. Clear glass ✓
  • C. Thick cloth
  • D. Wooden board

Answer: B — Clear glass allows light to pass through completely, making it transparent; all other options block or partially block light.

Q4. What three things must be present to observe a shadow?

  • A. Light, object, and mirror
  • B. Light, opaque object, and screen ✓
  • C. Light, transparent object, and water
  • D. Light, color, and transparent material

Answer: B — Shadows form only when light is blocked by an opaque object, and the blocked region appears on a screen or surface.

Q5. You are watching a Togalu Gombeyaata (shadow puppet show) in Karnataka. If you move the puppet closer to the light source, what happens to the shadow?

  • A. The shadow becomes smaller
  • B. The shadow becomes larger ✓
  • C. The shadow disappears
  • D. The shadow changes color

Answer: B — When the puppet moves closer to the light, it blocks a larger portion of the light rays, creating a bigger shadow on the screen.

Q6. Rajeev holds a piece of tracing paper between a torch and a wall. What will he observe?

  • A. Complete darkness on the wall
  • B. A bright spot with clear details of the paper
  • C. A dim glow or faint shadow on the wall ✓
  • D. No shadow at all on the wall

Answer: C — Tracing paper is translucent, so it lets some light pass through, creating a dim, fuzzy shadow rather than complete darkness or brightness.

Q7. Your friend claims that a red opaque ball will cast a red shadow. Is this correct?

  • A. Yes, because the object is red
  • B. No, shadows are always dark regardless of object color ✓
  • C. Yes, because red objects reflect red light
  • D. No, red objects do not create shadows

Answer: B — Shadows are formed where light is blocked, so they are always dark (black) irrespective of the color of the opaque object.

Q8. In ancient India, before electricity, what was the earliest artificial light source used by humans?

  • A. Candles made of wax
  • B. Oil lamps using animal fat and oil ✓
  • C. Gas lamps
  • D. Electric bulbs

Answer: B — Historically, humans learned to create fire and later used oil lamps with animal fat and oil as early artificial light sources before wax candles.

Q9. Priya performs the matchbox activity but accidentally displaces one matchbox slightly to the side. What will happen?

  • A. Light will still pass through and create a spot on the screen
  • B. Light will bend around the misaligned hole
  • C. No light spot will appear on the screen ✓
  • D. The shadow will become larger

Answer: C — When holes are not aligned in a straight line, light cannot pass through all three holes simultaneously because light travels in a straight line only.

Q10. Why are LED lamps preferred over traditional bulbs in Indian homes today?

  • A. They are cheaper to purchase initially
  • B. They consume less electricity, are brighter, and last longer ✓
  • C. They produce more heat than traditional bulbs
  • D. They are easier to manufacture

Answer: B — LED lamps are energy-efficient, reducing electricity bills and benefiting the environment, which is why the Indian government promotes their use.

Flashcards

Define a luminous object with one example.

A luminous object emits its own light; the Sun is a natural example and an electric bulb is an artificial example.

Why can you not see a candle flame through a bent pipe?

Light travels in a straight line, so it cannot bend around corners and must follow the straight path of the pipe.

What three things are necessary to form a shadow?

A light source, an opaque object that blocks light, and a screen (or surface) where the shadow appears are all three requirements.

What happens to shadow size when you move the object closer to the light source?

The shadow becomes larger because the object blocks more light rays from spreading onto the screen.

Distinguish between transparent and translucent materials.

Transparent materials let light pass completely through, while translucent materials allow only some light to pass through partially.

Does the color of an opaque object affect the color of its shadow?

No, shadows are always dark (black) regardless of the object's color because shadow is the absence of light.

What is the difference between a shadow and a reflection?

A shadow is a dark patch where light is blocked by an opaque object, while a reflection is light bouncing off a shiny surface.

Why is the Moon considered a non-luminous object?

The Moon does not produce its own light; it only reflects sunlight that falls on its surface.

What does opaque mean in the context of light and materials?

Opaque means the material does not allow light to pass through it at all, blocking light completely.

How does the matchbox activity prove that light travels in a straight line?

When the holes in the three matchboxes are aligned in a straight line, light passes through; when misaligned, light cannot reach the screen.

Important Board Questions

What is a luminous object? Give two examples. [1 mark]

Define as object that emits own light. Give two from: Sun, stars, fire, fireflies, electric bulb, candle, lightning.

Explain why you cannot see a candle through a bent pipe but can see it through a straight pipe. [2 marks]

State that light travels in straight line only. Bent pipe does not allow straight path, so light rays cannot reach your eyes.

Describe three actions mentioned in Table 11.2 and their effects on shadow formation. Explain what each observation tells us about shadows. [3 marks]

Choose three from: removing screen (no shadow seen), removing object (no shadow forms), switching off light (no shadow), moving object closer to light (shadow larger), moving object closer to screen (shadow smaller). Explain each cause-effect clearly.

Define transparent, translucent, and opaque materials. For each, give one example and explain what happens to light when it passes through. Draw and label a diagram showing light passing through all three types of materials. [5 marks]

Transparent: all light passes (glass, water). Translucent: partial light passes (tracing paper, frosted glass). Opaque: no light passes (wood, metal, cardboard). Diagram must show three materials with light rays, and label light sources, materials, and transmitted light clearly.

Assertion-Reason Questions for Light: Shadows and Reflections

Q1. Assertion (A):

The Sun is the main source of natural light on Earth.

Reason (R):

The Sun emits its own light and is a luminous object.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the Sun is the main source of natural light. The Sun's emission of its own light (luminous property) is the reason it serves as our primary natural light source.

Q2. Assertion (A):

The Moon appears bright in the night sky because it is a non-luminous object.

Reason (R):

Non-luminous objects reflect light from luminous objects like the Sun.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true—the Moon is indeed non-luminous and it does reflect sunlight, making it appear bright. However, the reason doesn't directly explain why it appears bright in the night sky; the reason explains HOW non-luminous objects become visible (through reflection), not why the Moon appears bright. The reason is correct but doesn't fully explain the assertion.

Q3. Assertion (A):

Light travels in a straight line as demonstrated in Activity 11.1.

Reason (R):

The torch, matchbox holes, and screen must be perfectly aligned for light to reach the screen.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The reason correctly explains the assertion—if light traveled in curves or bent randomly, alignment would not be necessary. The straight-line requirement proves straight-line travel, making this a correct cause-effect relationship.

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Case-Based Questions (CBQ) for Light: Shadows and Reflections

Case Study:

Priya conducted Activity 11.3 in her school laboratory. She collected various materials and tested them with a torch and cardboard screen in a dark room. Her observations are recorded below: When glass was placed between the torch and screen, a bright clear spot appeared on the screen. When tracing paper was used, a dimmer and slightly blurry spot appeared. When thick cloth was used, only a very dim glow was observed. When cardboard was placed, no light spot was visible at all, but a dark shadow was clearly visible on the screen.

Q1. Based on Priya's observations, which material would be classified as transparent?

  • A. Thick cloth
  • B. Glass ✓
  • C. Cardboard
  • D. Tracing paper

Answer: B

Q2. Why did thick cloth show only a very dim glow instead of a clear spot like glass?

  • A. Glass is brighter than cloth
  • B. Cloth absorbs most of the light while glass allows most light to pass through ✓
  • C. Cloth reflects light better than glass
  • D. The torch was moved farther from the cloth

Answer: B

Q3. What can be concluded about the materials tested in terms of light transmission?

  • A. Glass, tracing paper, cloth, and cardboard allow different amounts of light to pass through ✓
  • B. All materials allow light to pass in the same way
  • C. Cardboard allows the most light to pass through
  • D. Light cannot pass through any of these materials

Answer: A

Q4. If Priya wanted to make a lampshade that is bright but also provides privacy (not see-through), which material from her test would be BEST suited?

  • A. Glass
  • B. Cardboard
  • C. Tracing paper ✓
  • D. Thick cloth

Answer: C

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