Notes For All Chapters – General Science Class 8
1. Introduction
- Light is a form of energy that helps us see objects.
- When light hits a surface and bounces back, it is called reflection.
- The laws of reflection help us understand how light behaves.
2. Reflection of Light
- Reflection is when light bounces off a surface instead of passing through it.
- Example: When you look in a mirror, you see your reflection.
Types of Reflection
| Type of Reflection | Description | Example |
|---|---|---|
| Regular Reflection | Happens on a smooth and shiny surface. The light rays remain parallel after reflection, forming a clear image. | Plane mirror, still water |
| Irregular Reflection | Happens on a rough surface. The light rays scatter in different directions, forming a blurred or no image. | Wall, rough paper, wavy water |
3. Laws of Reflection
First Law of Reflection
- The angle of incidence is equal to the angle of reflection.
- (∠i = ∠r)
Second Law of Reflection
- The incident ray, reflected ray, and normal all lie in the same plane.
Terms Related to Reflection
- Incident Ray → The ray that falls on the mirror.
- Reflected Ray → The ray that bounces back from the mirror.
- Normal → A line perpendicular to the mirror at the point of incidence.
- Angle of Incidence (∠i) → The angle between the incident ray and normal.
- Angle of Reflection (∠r) → The angle between the reflected ray and normal.
4. Reflection in Plane Mirrors
- A plane mirror is a flat mirror that reflects light.
- The image formed by a plane mirror has these properties:
| Property | Description |
|---|---|
| Virtual | The image cannot be obtained on a screen. |
| Erect (Upright) | The image is not inverted. |
| Same Size | The image is the same size as the object. |
| Laterally Inverted | The left side of the object appears as the right side in the mirror. |
5. Applications of Reflection
(A) Periscope
- A periscope is a device used to see objects that are not directly in front.
- It uses two plane mirrors placed at 45° angles.
Uses:
- Submarines to see above water.
- Tanks and soldiers for safety in war.
(B) Kaleidoscope
- A kaleidoscope forms beautiful patterns using multiple reflections.
- It has three mirrors placed in a triangle with small colored objects.
Uses:
- In toy-making and design work.
(C) Solar Cooker
- Plane mirrors reflect sunlight to focus heat on food.
- More heat is concentrated, making cooking faster.
Uses:
- Eco-friendly cooking method.
6. Experiment to Show Light Needs a Medium to Travel
- Place a plane mirror on a table and shine a light ray on it.
- Observe the incident and reflected rays.
- Conclusion: Light always follows the laws of reflection.
7. Multiple Reflections
- When light reflects from one mirror to another, it is called multiple reflection.
- Example: Two mirrors at 90° form multiple images.
- Used in barber shops and dressing rooms for many reflections.
8. Uses of Reflection in Everyday Life
| Application | How Reflection is Used |
|---|---|
| Mirrors | Used for dressing, cars, and decoration. |
| Periscopes | Used in submarines and tanks. |
| Kaleidoscopes | Used for making patterns. |
| Solar Cookers | Used for cooking food using sunlight. |
9. Solved Problems
Q1: If the angle between a mirror and an incident ray is 40°, what is the angle of incidence?
- Angle of incidence (i) = 90° – 40° = 50°
- Since angle of incidence = angle of reflection, the angle of reflection is also 50°.
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