Light: Shadows and Reflections

Chapter 11: Light: Shadows and Reflections

11.1 Sources of Light

**Definition of Luminous Objects**: Objects that emit their own light are called luminous objects.

Example: The Sun, stars, lightning, natural fire, fireflies, and electric bulbs

The Sun is the main natural source of light on Earth

**Definition of Non-Luminous Objects**: Objects that do not emit their own light are called non-luminous objects.

Example: The Moon reflects sunlight but does not produce its own light

Books, tables, chairs, and most objects around us are non-luminous

**Types of Light Sources**:

1. **Natural Light Sources**: Sun, stars, lightning, fire, fireflies, bioluminescent organisms

2. **Artificial Light Sources**:

Fire (earliest form) - using animal fat, oil, wax, gas

Electric light sources - bulbs, fluorescent tubes, LEDs

**LED (Light Emitting Diode) Lamps**:

Modern light sources that consume much less power

Are brighter and last longer than traditional lamps

Reduce electricity bills and are better for the environment

The Indian government promotes LED lamps nationwide

Must be appropriately disposed or recycled, not thrown in garbage

**Real-Life Example**: When Keshav watches fireflies in Maharashtra forests at night, they are emitting their own light (luminous). The moonlit landscape he sees on the bus journey is reflected sunlight (non-luminous Moon)

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11.2 Does Light Travel in a Straight Line?

**Key Concept**: Light travels in a straight line in a uniform medium.

**Activity 11.1: Matchbox Holes Experiment**

**Procedure**:

1. Take three matchboxes and make holes at exactly the same position in the inner tray of each

2. Arrange the three matchboxes in a straight line

3. Ensure all three holes are at the same height and aligned

4. Place a torch light on one side with its lamp at the height of the holes

5. Place a cardboard screen on the other side

6. Observe the bright spot on the screen

7. Move one matchbox slightly to the side or up/down

**Observation**: When all three holes are in a line, light passes through and creates a bright spot on the screen. When the holes are not aligned, no light spot appears on the screen.

**Conclusion**: Light travels in a straight line

**Activity 11.2: Bent Pipe Experiment**

**Procedure**:

1. Take a long hollow flexible pipe

2. Align it so you can see a candle flame through it (straight pipe)

3. Bend the pipe and try to see the candle flame again

**Observation**: You can see the candle flame through a straight pipe but NOT through a bent pipe

**Conclusion**: Light travels in a straight line and cannot bend around corners in normal circumstances

**Deeper Understanding**: Light can sometimes bend around corners (diffraction), which you will learn in higher grades

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11.3 Light Through Transparent, Translucent, and Opaque Materials

**Definitions**:

**Transparent Materials**: Materials that allow light to pass completely through them, and you can see objects clearly through them

Examples: Clear glass, clear plastic, water, air

All or almost all light passes through

**Translucent Materials**: Materials that allow some light to pass through them, but you cannot see objects clearly through them

Examples: Tracing paper, frosted glass, thin cloth, wax paper

Some light is scattered or absorbed

**Opaque Materials**: Materials that do not allow light to pass through them

Examples: Cardboard, thick cloth, wood, metal, brick

No light passes through; light is completely blocked

**Activity 11.3: Light Through Different Materials**

**Procedure**:

1. Collect objects made of different materials (glass, cardboard, paper, tracing paper, thick cloth)

2. In a dark room, turn on a torch and position it to get a spot of light on a wall or cardboard screen

3. Predict whether light will pass fully, partially, or not at all through each material

4. Place each object between the torch and the screen

5. Observe and record whether light passes through

**Observations**:

Transparent materials: Bright light spot appears on screen (glass)

Translucent materials: Dimmer light spot appears on screen (tracing paper)

Opaque materials: No light spot appears on screen (cardboard, cloth)

**Laser Beam Observation** (with teacher supervision):

Pass a laser beam through water with a drop of milk to make it visible

The laser beam inside water follows a straight path

This demonstrates light's straight-line travel even in different media

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11.4 Shadow Formation

**Definition of Shadow**: A dark patch or area where light does not reach because an opaque object blocks the path of light.

**Requirement for Shadow Formation**: Three things are necessary:

1. **Source of Light** - Can be the Sun, torch, bulb, or any light source

2. **Opaque Object** - Any object that does not allow light to pass through (blocks light)

3. **Screen** - Any surface on which the shadow falls (wall, floor, ground, paper, cloth)

**How Shadows Are Formed**:

Light travels in straight lines

When an opaque object is placed in the path of light, it blocks the light

The region behind the object where light cannot reach becomes dark

This dark region is the shadow

**Real-Life Examples**:

Your shadow on the ground when you stand in sunlight

Shadow of a tree in a park

Shadow of a person on a wall under lamplight

**Activity 11.4: Shadow Properties Experiment**

**Procedure**: Collect opaque objects of different shapes and sizes. Perform the following actions and observe the shadow on the screen:

| Action | Observation |

|--------|-------------|

| Remove the screen | Shadow disappears (no surface to show shadow) |

| Remove the object | No shadow forms (light passes unblocked) |

| Switch off the torch | No shadow forms (no light source) |

| Change the color of the object | Shadow color remains black (shadow color doesn't change) |

| Move object closer to screen | Shadow becomes sharper and smaller |

| Move object closer to light | Shadow becomes larger and sharper |

| Tilt the object | Shadow shape changes |

**Key Properties of Shadows**:

1. **Shape and Size**:

Shadow shape depends on the object's shape

Shadow size depends on the object's distance from the light source

Closer to light = larger shadow

Farther from light = smaller shadow

2. **Position**:

Shadow appears on the opposite side of the object from the light source

Position changes when you move the object

3. **Sharpness**:

Shadow edges are sharpest when object is far from light source

Shadow becomes blurred when object is close to light source

This creates a penumbra (partial shadow) and umbra (complete dark shadow)

4. **Color**:

Shadows are always dark/black regardless of object color

Changing object color does not change shadow color

The color of the object does NOT affect the shadow color

5. **For Different Material Types**:

**Opaque objects**: Form dark shadows

**Translucent objects**: Form lighter/fainter shadows (some light passes through)

**Transparent objects**: Form very faint or no visible shadows

**Shadow Puppetry (Cultural Heritage)**:

An ancient art form used for centuries in India

Flat cut-out figures (shadow puppets) are placed between light and screen

Used for entertainment and communication

**Regional styles**:

Charma Bahuli Natya (Maharashtra)

Keelu Bomme and Tholu Bommalata (Andhra Pradesh)

Togalu Gombeyaata (Karnataka)

Ravana Chhaya (Odisha)

Tholpavakoothu (Kerala)

Bommalattam (Tamil Nadu)

Puppeteers create life-like movements by moving puppets and adjusting the light source

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11.5 Reflection of Light

**Definition of Reflection**: The change in direction of light when it falls on a shiny or polished surface (like a mirror) is called reflection of light.

**Key Points About Reflection**:

Light bounces off polished or shiny surfaces

Shiny surfaces redirect light in a new direction

The direction of reflected light depends on the angle of the surface

Light always travels in straight lines, but changes direction upon reflection

**Activity 11.5: Sunlight Redirection with Mirror**

**Procedure**:

1. Find a shiny flat steel plate or a plane mirror (flat, not curved)

2. Take it outside in sunlight

3. Let sunlight fall on the shiny surface

4. Angle the mirror to redirect light onto a wall that is not in direct sunlight

5. Observe the bright spot of light on the wall

**Conclusion**: A shiny surface changes the direction of light that falls on it. The bright spot moves when you tilt the mirror, showing that light is being redirected.

**Activity 11.6: Light Beam and Mirror Reflection**

**Procedure**:

1. Take a plane mirror with stand, a torch, a comb, white paper, and black paper

2. Using black paper, close all openings in the comb except one to create a thin slit

3. Spread white paper on a table

4. Hold the comb perpendicular to the paper and shine the torch through the slit

5. Adjust the comb and torch until you see a thin beam of light along the paper

6. Place the plane mirror in the path of the light beam

7. Keep the comb steady and observe

**Observation**:

Without mirror: Light beam travels straight on the paper

With mirror: The light beam changes direction when it hits the mirror

The reflected beam travels in a new direction from the mirror

**Conclusion**: Light is reflected by the mirror. The path of light changes after falling on the mirror surface.

**Types of Mirrors** (you will study more in higher classes):

Plane mirrors (flat)

Concave mirrors (curved inward)

Convex mirrors (curved outward)

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11.6 Images Formed in a Plane Mirror

**Definition**: An image is the picture of an object that appears in a mirror when light from the object reflects off the mirror's surface.

**Activity 11.7: Image Properties in Plane Mirror**

**Procedure**:

1. Take a plane mirror and a pen or pencil

2. Place the pen in front of the mirror

3. Observe the image in the mirror

4. Move the pen to different positions in front of the mirror

5. Compare the size of the image at each position

6. Observe if the image is always upright

7. Try placing a screen behind and in front of the mirror to catch the image

**Observations and Properties of Images Formed by Plane Mirror**:

1. **Size**:

The image is the same size as the object

Image size does not change when object position changes

The image appears to be behind the mirror

2. **Orientation (Erect or Inverted)**:

The image is always **erect** (upright, not inverted)

An upright image is called an erect image

The top of the object appears at the top in the image

The tip of the pen always appears on top in the image

3. **Real vs Virtual Image**:

**Cannot be obtained on a screen** (cannot be projected)

The image appears to be behind the mirror

Even if you place a screen behind the mirror, the image will not appear on it

This is called a **virtual image**

**Activity 11.8: Distance and Lateral Inversion**

**Procedure**:

1. Stand in front of a plane mirror at some distance

2. Observe how far your image appears from the mirror

3. Step closer to the mirror

4. Observe if the image also comes closer to the mirror

5. Stand at several different distances and compare

6. Raise your left arm and note which arm your image raises

7. Touch your right ear and note which ear your image touches

**Observations**:

1. **Distance from Mirror**:

When you stand close to the mirror, your image also appears close

When you stand far from the mirror, your image appears far

The distance of the image from the mirror equals your distance from the mirror

If you are 1 meter from mirror, image is 1 meter behind the mirror

2. **Lateral Inversion**:

**Definition**: A perceived left-right reversal in the mirror image

When you raise your **left arm**, your image appears to raise its **right arm**

When you touch your **right ear**, your image appears to touch its **left ear**

Your left appears right in the image, and your right appears left

This is NOT a mirror error; it's how mirrors work

**Real-Life Application - Ambulance Writing**:

The word "AMBULANCE" is written backwards on the front of ambulances

It reads as "AMBULANCE" (normally) when viewed in the rear-view mirror of a vehicle ahead

This happens due to lateral inversion in the mirror

Drivers see the correct word in their rear-view mirror and know to move aside

**Summary of Plane Mirror Image Properties**:

Same size as object ✓

Erect (upright) ✓

Same distance from mirror as object ✓

Virtual (cannot be obtained on screen) ✓

Laterally inverted (left-right reversed) ✓

**Historical Note on Mirrors**:

When mirrors were invented is not known

Early mirrors were made by polishing stone or metal

Glass mirrors replaced metal mirrors, and metal mirror-making art was gradually lost

**Aranmula Kannadi**: A unique metal surface mirror still made in Kerala for centuries, keeping the ancient tradition alive

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11.7 Pinhole Camera

**Definition of Pinhole Camera**: A device in which light rays from an object pass through a tiny hole (a pinhole) and form an image on a screen.

**How It Works**:

Light from an object passes through a small hole

Light rays travel in straight lines through the hole

The rays hit a screen behind the hole

An image forms on the screen

The image is **inverted (upside down)**

**Activity 11.9: Simple Pinhole Camera**

**Procedure** (Use candle under adult supervision):

1. Take a piece of cardboard and a lighted candle

2. Make a small hole in the cardboard

3. In a dimly lit room, position the cardboard at a short distance from a screen (wall or paper)

4. Place the lighted candle in front of the hole

5. Adjust the positions until you see an image on the screen

**Observations**:

Light from the candle flame passes through the hole

An image of the flame appears on the screen

**Surprising observation**: The image is **upside down (inverted)**

The flame image is in the opposite direction from the actual flame

**Reason for Inverted Image**:

Light rays travel in straight lines

Rays from the top of the object pass through the hole and hit the bottom of the screen

Rays from the bottom of the object pass through the hole and hit the top of the screen

This causes the image to be inverted

**Activity 11.10: Sliding Pinhole Camera (Adjustable)**

**Materials**: Two cardboard boxes (one sliding into the other), torch/sunlight

**Construction**:

1. Take two cardboard boxes of different sizes (one smaller, one larger)

2. The smaller box should slide into the larger box with minimal gap

3. Cut open one side of each box

4. On the **opposite face of the larger box**, make a small hole in the middle (about 5mm diameter)

5. On the **opposite face of the smaller box**, cut out a square (about 5-6 cm side)

6. Cover this square opening with **thin translucent paper** (like tracing paper) to form a screen

7. Slide the smaller box inside the larger box so that the tracing paper screen is **inside** (facing the larger box)

**Using the Pinhole Camera**:

1. Hold the camera with the pinhole facing the object you want to view

2. Look through the open side of the smaller box

3. Cover your head and the camera with a dark cloth

4. Look at a distant object (tree, building, landscape) in bright sunlight

5. Move the smaller box forward or backward (telescoping motion)

6. Adjust until a clear image appears on the tracing paper screen

**Observations**:

The image is visible on the translucent paper screen

The image shows the **colors** of the objects

The image is **upside down (inverted)**

The image is much smaller than the actual object

The farther the object, the smaller the image

By moving the smaller box, you can focus the image

**Diagram to Draw: Sliding Pinhole Camera**

Draw two boxes: smaller inside larger

Label the **pinhole** on the large box's back

Label the **translucent paper screen** inside the smaller box

Show light rays from an object passing through the hole

Show the inverted image on the screen

Show the **open side** where you look through

**Connection to Real Cameras**:

This pinhole camera principle is similar to how cameras work

In cameras, a lens is used instead of a pinhole

The lens focuses light better than a pinhole

This gives sharper and brighter images

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11.8 Making Some Useful Items

**Introduction**: Using the principles that light travels in a straight line and is reflected by mirrors, we can make useful optical devices.

11.8.1 Periscope

**Definition**: A periscope is an optical instrument that allows you to see objects that are not visible directly from your position.

**Principle**: Uses reflection of light from two plane mirrors to change the direction of light twice, allowing you to see around obstacles.

**How to Make a Simple Periscope**:

1. Take a rectangular box and arrange it in a Z-shape

2. Place one **plane mirror** in the upper section at a 45° angle

3. Place another **plane mirror** in the lower section at a 45° angle

4. The mirrors should be parallel to each other

5. Cut viewing holes at the top and bottom

6. Cut an opening at the side for the object to be viewed

**How It Works**:

Light from the object enters from the side opening

Light hits the **lower mirror** and reflects upward

The reflected light hits the **upper mirror**

The light reflects again and reaches your eye at the top

Light changes direction twice due to the two mirrors

You see the object even though it's not in your direct line of sight

**Real-Life Applications**:

**Submarines**: Sailors use periscopes to see above water without surfacing

**Tanks**: Military personnel use them to observe the battlefield while staying protected

**Bunkers**: Soldiers use them to see outside the bunker entrance safely

**Practical use**: You can use it to look ahead when standing behind taller friends in a crowd

**Diagram to Draw: Periscope**

Draw a Z-shaped box (tall and narrow)

Show two plane mirrors at 45° angles inside

Label the **upper mirror** and **lower mirror**

Draw an opening at the **side** for viewing

Draw viewing holes at **top** and **bottom**

Show light rays entering from the side

Show light reflecting off the lower mirror upward

Show light reflecting off the upper mirror to the eye

Use arrows to show the path of light

11.8.2 Kaleidoscope

**Definition**: A kaleidoscope is an optical instrument that uses multiple mirrors and colored objects to create symmetrical, colorful patterns.

**Principle**: Light is reflected multiple times between plane mirrors arranged at specific angles, creating repeated patterns of objects placed inside.

**Basic Structure**:

Three or more plane mirrors arranged inside a tube

Mirrors are typically placed at 60° or 45° angles to each other

Colored beads, glass pieces, or other translucent objects are placed at one end

An eyehole at the other end for viewing

Light enters from the end with objects

**How It Works**:

1. Colored objects are placed at one end of the mirror tube

2. Light enters and illuminates these objects

3. Light reflects between the mirrors multiple times

4. Each reflection creates a repeated image of the objects

5. Due to the mirror angles, you see **symmetrical patterns**

6. When you rotate the tube, the objects move and create new patterns

**What You See**:

Beautiful symmetrical patterns (usually 6, 8, or 12-fold symmetry depending on mirror angles)

Colors are repeated in a geometric arrangement

Patterns appear to be perfectly symmetric

Rotating the tube changes the pattern continuously

**How to Make a Simple Kaleidoscope**:

1. Take three rectangular mirrors (about 10 cm × 15 cm)

2. Join them together at their long edges to form a triangular prism

3. Make sure the mirrors form a 60° angle

4. Seal one end with a translucent paper (ground glass or frosted film) with colored objects behind it

5. Seal the other end with a cardboard with an eyehole

6. Apply black paper on the outside to prevent outside light from entering

**Applications**:

**Educational**: Teaches symmetry and reflection of light

**Entertainment**: Popular toy for children and adults

**Art**: Used as inspiration for design and pattern creation

**Design**: Fashion and textile designers use kaleidoscope patterns

**Diagram to Draw: Kaleidoscope**

Draw a triangular prism shape (front view)

Show three plane mirrors forming the sides

Show colored beads/objects at the base

Label the **eyehole**

Show light rays entering from the object end

Show multiple reflections between the mirrors

Indicate the **symmetrical pattern** seen through the eyehole

Show several copies of the object arranged symmetrically

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Key Terms Summary

**Luminous Objects**: Objects that emit their own light (Sun, stars, bulbs, fireflies)

**Non-Luminous Objects**: Objects that do not emit light but reflect light from other sources (Moon, books, tables)

**Transparent Materials**: Allow light to pass completely through; you can see objects clearly (glass, water, air)

**Translucent Materials**: Allow some light to pass through; objects are not clearly visible (tracing paper, frosted glass)

**Opaque Materials**: Do not allow light to pass through (cardboard, cloth, metal, wood)

**Shadow**: A dark area formed when an opaque object blocks light from reaching a surface

**Reflection of Light**: The change in direction of light when it bounces off a mirror or shiny surface

**Plane Mirror**: A flat mirror that forms virtual, erect images of the same size as the object

**Image**: The picture of an object formed by a mirror or optical device

**Erect Image**: An upright image that is not inverted

**Virtual Image**: An image that cannot be obtained on a screen (appears to be behind the mirror)

**Lateral Inversion**: Left-right reversal of image in a plane mirror

**Pinhole Camera**: A device where light passes through a small hole and forms an inverted image on a screen

**Periscope**: An optical instrument using two mirrors to see objects that are not directly visible

**Kaleidoscope**: An optical instrument using multiple mirrors and colored objects to create symmetrical patterns

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Important Exam Points

1. **Light Travel**: Light travels in straight lines in a uniform medium

2. **Three Requirements for Shadows**:

Light source

Opaque object

Screen

3. **Shadow Properties Don't Change**:

Shadow color is always black/dark

Shadow color does NOT depend on object color

Shadow always forms when three requirements are met

4. **Plane Mirror Properties** (memorize all five):

Same size as object

Erect (upright)

Virtual (cannot project on screen)

Distance from mirror = object distance from mirror

Laterally inverted

5. **Pinhole Camera**: Forms inverted (upside-down) images

6. **Material Classifications**:

Transparent: full light passage

Translucent: partial light passage

Opaque: no light passage

7. **Real-Life Applications**:

LED lamps: energy-efficient lighting

Shadow puppetry: cultural heritage and communication

Ambulance writing: practical use of lateral inversion

Periscope: submarines, tanks, military use

Kaleidoscope: symmetry and design

8. **Reflection Principle**: Light bounces off shiny/polished surfaces at specific angles (law of reflection—angle of incidence = angle of reflection, which you'll learn in detail in higher classes)

9. **Images in Mirrors**: Only plane mirrors are discussed in Class 7; curved mirrors (concave and convex) are for higher grades

10. **Difference Between Shadow and Image**:

**Shadow**: Dark area formed when light is blocked by opaque object

**Image**: Picture of object formed by reflection in mirror or refraction through lens/pinhole