Light, Shadows and Reflections — Class 6 Physical Science
'Light allows us to see the world — and understanding its behaviour unlocks amazing phenomena like shadows, reflections, and the pinhole camera.'
1. Introduction
Light is a form of energy that makes things visible. Without light, we cannot see anything. Light travels in straight lines — this property explains shadows, the pinhole camera, and reflections.
Why This Chapter Matters
- Understand how we see things
- Learn about light's properties
- Foundation for optics and physics
- Practical: pinhole camera, understanding eclipses later
2. Luminous and Non-Luminous Objects
Luminous Objects
Objects that give out their own light.
| Luminous Object | Source of Light |
|---|---|
| Sun | Nuclear fusion |
| Stars | Nuclear fusion |
| Candle flame | Burning wax |
| Electric bulb | Electricity heats filament |
| Firefly (insect) | Chemical reaction (bioluminescence) |
| Tube light | Gas discharge |
Non-Luminous Objects
Objects that do NOT give out their own light — we see them because light reflects off them and reaches our eyes.
| Non-Luminous Object | How We See It |
|---|---|
| Moon | Reflects sunlight |
| Book | Reflects room light |
| Tree | Reflects sunlight |
| Chair | Reflects light |
| Your face | Reflects light |
'We see non-luminous objects because light from a luminous source bounces off them and enters our eyes.'
Comparison
| Feature | Luminous | Non-Luminous |
|---|---|---|
| Own light | YES | NO |
| Examples | Sun, bulb, candle | Moon, table, book |
| Visible in dark? | Yes | No (need a light source) |
3. Transparent, Translucent, and Opaque
Based on how much light passes through, materials are classified as:
| Type | Light Passes | Can See Through? | Examples |
|---|---|---|---|
| Transparent | Completely | Clearly | Glass, water, air, clear plastic |
| Translucent | Partially | Blurred/blurry | Butter paper, frosted glass, tissue paper |
| Opaque | Not at all | No | Wood, metal, stone, cardboard |
How They Affect Shadows
| Material | Shadow Formed? | Type |
|---|---|---|
| Transparent | No (light passes through) | No shadow |
| Translucent | Yes | Faint/light shadow |
| Opaque | Yes | Dark, clear shadow |
4. Shadows
A shadow is a dark region formed when an opaque object blocks light.
Conditions for a Shadow
- A source of light (sun, torch, bulb)
- An opaque object (blocks light)
- A screen (wall, ground, paper) where shadow falls
Properties of Shadows
| Property | Description |
|---|---|
| Shape | Similar to the object's outline (not always identical) |
| Colour | Always BLACK (regardless of object's colour) |
| Size | Depends on distance from light source |
| Position | Opposite to the light source |
Shadow Size Variation
| Object Distance from Light | Shadow Size |
|---|---|
| Close to light | LARGE |
| Far from light | SMALL |
Example: Your shadow is longer in the morning/evening (sun low) and shorter at noon (sun overhead).
Shadow vs Image
| Feature | Shadow | Image |
|---|---|---|
| Colour | Always black | Shows object's colours |
| Details | Only outline | Shows details |
| Formation | Blocked light | Reflected/refracted light |
| Type | Real (exists on screen) | Can be real or virtual |
5. Pinhole Camera
A pinhole camera is a simple device that forms an image using the principle that light travels in straight lines.
Construction
- Take a box (or tin can) — paint the inside black
- Make a tiny hole (pinhole) on one side
- Cover the opposite side with translucent paper/screen
How It Works
Object → |·| → Image (on screen)
Pinhole
Light from the top of the object passes through the pinhole and hits the bottom of the screen. Light from the bottom hits the top. This makes the image upside down (inverted).
Properties of Pinhole Image
- Inverted (upside down)
- Real (can be seen on a screen)
- Same colour as the object
- Size: Depends on distance between object and pinhole
Pinhole Camera vs Shadow
| Pinhole Camera Image | Shadow |
|---|---|
| Inverted | Same orientation |
| Shows colours | Always black |
| Needs dark box | Just needs screen |
| Image is formed | Dark region is formed |
6. Reflection
Reflection is the bouncing back of light when it hits a smooth, shiny surface.
Laws of Reflection
- The angle of incidence = Angle of reflection
- The incident ray, reflected ray, and normal all lie in the same plane
(For Class 6, we focus on the concept of reflection itself, not detailed laws.)
Types of Reflection
| Type | Surface | Example |
|---|---|---|
| Regular reflection | Smooth, shiny | Mirror, still water |
| Diffuse reflection | Rough, uneven | Paper, wall, cloth |
In diffuse reflection, light bounces in all directions — this is why we can see a book from any angle.
7. Mirror Images
A plane mirror (flat mirror) shows a mirror image.
Properties of Mirror Image
- Virtual: Cannot be projected on a screen (behind the mirror)
- Laterally inverted: Left appears right, right appears left
- Same size: As the object
- Same distance: Behind the mirror as the object is in front
Lateral Inversion
When you raise your RIGHT hand in front of a mirror, the image raises its LEFT hand.
Object: Left hand --- Right hand
Mirror: Right hand --- Left hand (swapped!)
Examples of Lateral Inversion
- AMBULANCE is written backwards on the front so that drivers see it correctly in their rear-view mirror
- The word 'MIRROR' seen in a mirror looks like 'ЯОЯЯIM'
Uses of Reflection
- Mirrors (at home, in vehicles)
- Periscope (used in submarines, tanks)
- Kaleidoscope (colourful patterns)
- Solar cookers (reflect sunlight to focus heat)
8. Common Mistakes — Fix Them Now
| # | Mistake | Correction |
|---|---|---|
| 1 | Saying the moon is a luminous object | Moon is NON-luminous — it reflects sunlight |
| 2 | Thinking a shadow shows the object's colour | Shadows are ALWAYS black, regardless of object colour |
| 3 | Believing a pinhole image is the same as the object | Pinhole image is INVERTED (upside down) |
| 4 | Confusing translucent with transparent | Translucent: partial light (blurry). Transparent: full light (clear). |
| 5 | Thinking reflection only happens in mirrors | Reflection happens from ALL surfaces — smooth ones give clear images, rough ones give scattered light |
9. Exam Focus
Marks Blueprint
| Question Type | Marks | Topic |
|---|---|---|
| MCQ | 1 | Luminous/non-luminous / transparent/opaque |
| Short answer | 2 | Shadow formation / pinhole camera |
| Short answer | 2 | Reflection / mirror image |
| Long answer | 3 | How does a pinhole camera work? |
| Activity | 3 | Demonstrate shadow formation |
Quick Self-Test (5 Questions)
Q1: Why can we see non-luminous objects?
<details><summary>Answer</summary>Light from a luminous source reflects off the object and enters our eyes.</details>Q2: What type of material is used to make a window?
<details><summary>Answer</summary>Transparent material (glass) — light passes through completely.</details>Q3: Why is a shadow always black?
<details><summary>Answer</summary>A shadow is the absence of light in that region — where no light reaches, everything appears dark/black.</details>Q4: What is the image formed by a pinhole camera like?
<details><summary>Answer</summary>Inverted (upside down), real, same colours as the object.</details>Q5: What is lateral inversion?
<details><summary>Answer</summary>Swapping of left and right in a mirror image. Your right hand appears as left hand in the mirror.</details>10. Chapter Summary
- Luminous: gives its own light (sun, bulb); Non-luminous: reflects light (moon, book)
- Transparent: clear (glass); Translucent: blurry (butter paper); Opaque: blocks light (wood)
- Shadow: dark region formed when opaque object blocks light; always black
- Pinhole camera: light travels straight → inverted image
- Reflection: light bounces off surfaces
- Mirror image: virtual, same size, laterally inverted
'Light is the reason we can see — and its straight-line path explains shadows, reflections, and the magic of the pinhole camera.'
