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Notes Chapter 10 Science Curiosity Class 8 CBSE Board

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Light: Mirrors and Lenses


Introduction

  • Light helps us see things around us.
  • Mirrors and lenses change how we see objects by reflecting or bending light.
  • This chapter explains how plane mirrors, spherical mirrors (concave and convex), and lenses (convex and concave) work.

1. Types of Mirrors

Mirrors reflect light to form images. There are three main types of mirrors:

Plane Mirror: Flat surface, forms an image same size as the object, erect (upright), and laterally inverted (left appears right).

Spherical Mirrors: Curved mirrors shaped like a part of a sphere.

Concave Mirror: Reflecting surface curves inward (like the inside of a spoon).

Convex Mirror: Reflecting surface curves outward (like the outside of a spoon).


How Spherical Mirrors Are Made

Not cut from a hollow sphere but made by grinding and polishing flat glass into a curved shape.

Concave Mirror: Reflective coating on the outer curved surface.

Convex Mirror: Reflective coating on the inner curved surface.


2. Characteristics of Images in Spherical Mirrors

1. Concave Mirror:

  • Close to the mirror: Image is erect and larger (enlarged).
  • Farther away: Image becomes inverted, first enlarged, then smaller.

2. Convex Mirror:

  • Always forms an erect, smaller (diminished) image.
  • Image size slightly decreases as the object moves farther.

3. Plane Mirror:

  • Always forms an erect, same-sized image with lateral inversion.

Uses of Spherical Mirrors

1. Concave Mirrors:

  • Torch and car headlights (reflects light to focus it).
  • Dental mirrors (enlarges view of teeth).
  • Telescopes (large concave mirrors for reflecting light).

2. Convex Mirrors:

  • Side-view mirrors in vehicles (shows a wider area, objects appear smaller and closer).
  • Road safety mirrors at intersections (prevents collisions).
  • Surveillance mirrors in stores (monitors large areas).

3. Laws of Reflection

Law 1: The angle of incidence (i) equals the angle of reflection (r).

Law 2: The incident ray, reflected ray, and normal (a line perpendicular to the mirror at the point of incidence) lie in the same plane.

These laws apply to all mirrors: plane, concave, and convex.


4. Lenses

A lens is a transparent material (glass or plastic) with curved surfaces that bends light.

Types of Lenses:

1. Convex Lens: Thicker in the middle, thinner at edges (converging lens).

2. Concave Lens: Thinner in the middle, thicker at edges (diverging lens).

Uses of Lenses

1. Convex Lenses:

  • Eyeglasses (correct vision).
  • Cameras, telescopes, microscopes (focus light).
  • Human eye (a natural convex lens that changes shape to focus).

2. Concave Lenses:

  • Used in glasses to correct nearsightedness.
  • Combined with convex lenses in optical devices.

5. Solar Concentrators

Devices that use mirrors or lenses to focus sunlight into a small area.

Used for heating liquids to produce steam for electricity, cooking, or melting steel in solar furnaces.


6. Key Points (Snapshots)

1. Concave Mirror:

  • Image can be enlarged, diminished, or same size; erect or inverted based on object distance.

2. Convex Mirror:

  • Always forms erect, diminished images.

3. Laws of Reflection:

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

4. Convex Lens:

  • Image can be enlarged, diminished, or same size; erect or inverted based on object distance.

5. Concave Lens:

  • Always forms erect, diminished images.

6. Light Behavior:

  • Concave mirrors and convex lenses converge light.
  • Convex mirrors and concave lenses diverge light.

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