Notes Chapter 10 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

Light: Mirrors and Lenses

Introduction

• Plane mirrors form erect images of same size.
• Curved mirrors form different types of images – enlarged, diminished, inverted, or erect.
• Such mirrors are called spherical mirrors.

10.1 What Are Spherical Mirrors?

• Spherical mirrors: Mirrors whose reflecting surfaces are part of a sphere.

Types of spherical mirrors:

1. 1. Concave Mirror – Reflecting surface curves inward.
   2. Convex Mirror – Reflecting surface curves outward.

Example: A shiny spoon acts like a spherical mirror.

• Inner side (concave): Image is inverted.
• Outer side (convex): Image is erect and smaller.

How they are made:

• Flat glass polished into a curve.
• Coating on outer surface → Concave mirror.
• Coating on inner surface → Convex mirror.

10.2 Characteristics of Images by Spherical Mirrors

1. Concave Mirror

• Object close: Image erect and enlarged.
• Object far: Image inverted.
• Image size: First enlarged → then smaller.
• Concave mirror can form erect, inverted, enlarged, or diminished images depending on object distance.

2. Convex Mirror

• Image is always erect and diminished (smaller).
• Image size decreases slightly as object moves away.

3. Plane Mirror (for comparison)

• Always forms erect, same-size image.
• Shows lateral inversion.

Uses of Mirrors

1.Concave Mirrors

• Reflectors in torches, car headlights, scooters.
• Dentist’s mirror – gives enlarged image of teeth.
• Used in telescopes.

2.Convex Mirrors

• Side-view mirrors of vehicles – give wide view.
• Road safety mirrors at turns and intersections.
• Surveillance mirrors in big shops.

10.3 Laws of Reflection

Important Terms

• Incident ray: Ray of light falling on the mirror.
• Reflected ray: Ray of light bouncing back from the mirror.
• Normal: Imaginary line at 90° to mirror surface at point of incidence.
• Angle of incidence (i): Angle between incident ray and normal.
• Angle of reflection (r): Angle between reflected ray and normal.

Two Laws of Reflection

1. Angle of incidence = Angle of reflection.
2. Incident ray, reflected ray, and normal lie in the same plane.

Reflection in Different Mirrors

• Plane mirror: Reflected rays remain parallel.
• Concave mirror: Reflected rays converge (come together).
• Convex mirror: Reflected rays diverge (spread out).

10.3.1 Concentration of Light by Concave Mirror

• Concave mirror can focus sunlight into a small spot.
• The spot becomes hot enough to burn paper.
• Applications:
  • Solar concentrators.
  • Solar furnaces.
  • Large-scale cooking and electricity generation.

10.4 What Is a Lens?

• Lens: Transparent glass/plastic with curved surfaces.

Types:

1. Convex Lens (Converging Lens): Thicker in middle.
2. Concave Lens (Diverging Lens): Thinner in middle.

• Difference from mirror: Lenses let light pass through.

10.5 Characteristics of Images by Lenses

1. Convex Lens

• Object close: Image erect and enlarged.
• Object far: Image inverted.
• Image may be enlarged, diminished, erect, or inverted depending on distance.

2. Concave Lens

• Image is always erect and diminished.
• Image size changes slightly with distance.

10.6 Convergence and Divergence by Lenses

• Convex lens: Converges rays → also called converging lens.
• Concave lens: Diverges rays → also called diverging lens.
• Convex lens can focus sunlight to burn paper.

10.7 Uses of Lenses

• Eyeglasses – for clear vision.
• Cameras, telescopes, microscopes – to form images.
• Human eye – has convex lens which changes shape to see near and far.

Snapshots (Quick Points)

• Concave mirror: Image can be erect/inverted, enlarged/diminished.
• Convex mirror: Image always erect and diminished.

Laws of Reflection:

1. Angle of incidence = Angle of reflection.
2. Incident ray, normal, and reflected ray lie in same plane.

• Concave mirror: Converges light.
• Convex mirror: Diverges light.
• Convex lens: Image may be erect/inverted, enlarged/diminished.
• Concave lens: Image always erect and diminished.
• Convex lens: Converges light.
• Concave lens: Diverges light.