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:
- Concave Mirror – Reflecting surface curves inward.
- 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
- Angle of incidence = Angle of reflection.
- 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:
- Convex Lens (Converging Lens): Thicker in middle.
- 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:
- Angle of incidence = Angle of reflection.
- 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.
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