Light
1. What is Light?
Light is a form of energy that enables us to see objects.
We see objects when light from them enters our eyes.
Luminous and Non-Luminous Objects
| Type | Definition | Examples |
|---|---|---|
| Luminous | Objects that produce their own light | Sun, stars, bulb, candle, firefly |
| Non-luminous | Objects that do not produce their own light | Moon, chair, tree, book, table |
Common Mistake: Calling the Moon a luminous object. The Moon reflects the Sun's light; it does not produce its own light.
Exam Focus (2 marks): 'Classify the following as luminous or non-luminous: Sun, Moon, candle, star, mirror.'
Luminous: Sun, candle, star.
Non-luminous: Moon (reflects sunlight), mirror (reflects light).
2. Rectilinear Propagation of Light
Light travels in a straight line. This is called rectilinear propagation.
Evidence:
- When you shine a torch, the beam appears straight.
- Shadows form because light cannot bend around obstacles.
- Light passing through a small hole produces an inverted image (pinhole camera).
Pinhole Camera
A simple device that demonstrates rectilinear propagation.
Working: Light from the object passes through the small hole and forms an inverted image on the screen. This happens because light travels in straight lines.
Worked Example: Why is the image formed by a pinhole camera inverted?
Light from the top of the object travels in a straight line through the hole to the bottom of the screen. Light from the bottom goes to the top. This crossing of rays produces an inverted image.
3. Shadows
A shadow is a dark region formed when light is blocked by an opaque object.
Conditions for a Shadow
- A source of light.
- An opaque object.
- A screen (surface where the shadow falls).
Types of Objects
| Type | Allows light to pass | Examples |
|---|---|---|
| Transparent | Completely | Glass, water, air |
| Translucent | Partially | Frosted glass, butter paper |
| Opaque | Not at all | Wood, metal, stone |
Shadow Formation
- Size of shadow depends on the distance between the light source, object, and screen.
- Closer to light source = larger shadow.
- Closer to screen = smaller shadow.
Common Mistake: Thinking a shadow has details of the object. Shadows only show the outline, not details.
4. Eclipses
An eclipse occurs when one celestial body blocks light from reaching another.
Solar Eclipse
The Moon comes between the Sun and the Earth, blocking the Sun's light.
- Occurs on a new moon day.
- The Moon's shadow falls on the Earth.
Lunar Eclipse
The Earth comes between the Sun and the Moon, blocking sunlight from reaching the Moon.
- Occurs on a full moon day.
- The Earth's shadow falls on the Moon.
Exam Focus (3 marks): 'Differentiate between a solar eclipse and a lunar eclipse.'
| Feature | Solar Eclipse | Lunar Eclipse |
|---|---|---|
| Alignment | Sun-Moon-Earth | Sun-Earth-Moon |
| When it occurs | New moon day | Full moon day |
| What is blocked | Sun's light to Earth | Sun's light to Moon |
| Frequency | Rarer | More common |
5. Reflection of Light
When light bounces back from a surface, it is called reflection.
Laws of Reflection
- The angle of incidence (angle i) equals the angle of reflection (angle r).
- The incident ray, the reflected ray, and the normal (perpendicular) all lie in the same plane.
Types of Reflection
| Type | Surface | Example |
|---|---|---|
| Regular reflection | Smooth surface | Mirror, calm water |
| Diffuse reflection | Rough surface | Paper, wall, cloth |
Common Mistake: Thinking diffuse reflection does not follow the laws of reflection. It does! The surface is rough, so the normals are in different directions, causing light to scatter.
6. Plane Mirror Image
A plane mirror produces an image with these properties:
- The image is upright (erect).
- The image is the same size as the object.
- The image is laterally inverted (left-right swapped).
- The image is virtual (cannot be projected on a screen).
- The image is as far behind the mirror as the object is in front.
Worked Example: If you stand 50 cm in front of a plane mirror, where is your image?
The image is 50 cm behind the mirror (appears to be at 100 cm from you).
Lateral inversion: Your left hand appears as the right hand in the mirror.
7. Self-Test
- Differentiate between luminous and non-luminous objects with examples.
- What is rectilinear propagation of light? Give one evidence.
- Explain the formation of a shadow.
- Differentiate between solar and lunar eclipses.
- State the laws of reflection.
- A girl stands 1.5 m in front of a plane mirror. How far away is her image? Describe its nature.
- Why is a pinhole camera image inverted?
- Classify as transparent, translucent, or opaque: (a) Air (b) Frosted glass (c) Iron sheet.
8. Answers to Self-Test
- Luminous produces its own light (Sun, bulb); non-luminous does not (Moon, book).
- Light travels in straight lines. Evidence: shadow formation.
- An opaque object blocks light, creating a dark region (shadow) behind it.
- Solar: Moon blocks Sun (new moon). Lunar: Earth blocks Sun from Moon (full moon).
- (1) Angle i = Angle r. (2) Incident ray, reflected ray, and normal lie in the same plane.
- Image is 1.5 m behind the mirror (3 m from the girl). Image is erect, virtual, same size, laterally inverted.
- Light from the top of the object reaches the bottom of the screen and vice versa due to straight-line travel.
- (a) Transparent (b) Translucent (c) Opaque.
