Light: Mirrors and Lenses – Class 8 Science Chapter 10 Notes & NCERT PDF

Probe and Ponder

The chapter opens with questions to think about:

Introduction

During the summer holidays, Meena visits a science centre with her family. In one corner she finds a row of unusual curved mirrors. In one, her face looks unusually large; her brother, standing a little farther away, looks upside down; in another mirror she sees a tiny version of herself. In Grade 7 she had learnt that a plane mirror forms an image of the same size that is erect. A guide explains: "These are not plane mirrors. These are spherical mirrors." This chapter explains how curved mirrors and lenses form images.

Key Concepts

10.1 What are spherical mirrors?

A shiny metallic spoon acts like a curved mirror:

Spherical mirrors are mirrors whose reflecting surfaces are spherical — shaped like a part of a hollow glass sphere.

A step further: spherical mirrors are not made by slicing a glass sphere. A flat piece of glass is ground and polished into a curve. A reflective coating (like a thin layer of aluminium) on the outer curved surface makes a concave mirror; on the inner curved surface it makes a convex mirror.

You can tell them apart by viewing them from the side at eye level: see whether the reflecting surface curves inwards or outwards (Activity 10.2).

10.2 Images formed by spherical mirrors

Activity 10.3 places a toy 3–4 cm in front of a concave and a convex mirror and then moves it away:

Uses of concave mirrors: reflectors of torches and headlights of cars and scooters; the dentist's mirror (gives an enlarged view of teeth when held close). Most modern reflecting telescopes use a large concave mirror as the main mirror.

Uses of convex mirrors: side-view mirrors of vehicles (erect, smaller image and a much wider view of the road behind); mirrors at road intersections and sharp bends; surveillance mirrors in big stores to deter thefts.

10.3 Laws of reflection

Rays are drawn as straight lines with arrows because light travels in a straight line.

Law 1: The angle of incidence is equal to the angle of reflection (i = r).

Law 2: The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

If light falls along the normal, both angles are 0° and the light goes straight back.

In Activity 10.5, the reflected beam disappears when the extended part of the chart paper is bent, and reappears when it is flattened — proof of the second law.

The laws of reflection hold for all mirrors — plane, concave and convex. But with many parallel beams (Activity 10.6):

A concave mirror facing the Sun can focus light to a bright spot that can burn paper (Activity 10.7). Devices that concentrate sunlight using mirrors and lenses are solar concentrators; they heat liquids to make steam for electricity, large-scale cooking or solar furnaces (which can even melt steel).

10.4 What is a lens?

A drop of water on an oiled plastic strip acts as a simple lens — letters beneath it look larger (Activity 10.8). A magnifying glass is also a lens.

A lens is a piece of transparent material, usually glass or plastic, with curved surfaces.

Activity 10.9 (object behind the lens):

Activity 10.10 (parallel beams): a thin glass plate lets light pass as it is; a convex lens converges light (a converging lens); a concave lens diverges light (a diverging lens). A convex lens can also focus sunlight to burn paper (Activity 10.11).

Uses of lenses: eyeglasses, cameras (including smartphone cameras), telescopes, microscopes. Our eye has a convex lens that can change its shape, letting us read a book or see far away.

Activities in the Chapter

Key Terms

Real-life Connections

Common Misconceptions

Keep the Curiosity Alive (exercise hints)