Light - Reflection and Refraction

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CHAPTER Nine Light - Reflection and Refraction

We see a variety of objects in the world around us. However, we are unable to see anything in a dark room. On lighting up the room, things become visible. What makes things visible? During the day, the sunlight helps us to see objects. An object reflects light that falls on it. This reflected light, when received by our eyes, enables us to see things. We are able to see through a transparent medium as light is transmitted through it. There are a number of common wonderful phenomena associated with light such as image formation by mirrors, the twinkling of stars, the beautiful colours of a rainbow, bending of light by a medium and so on. A study of the properties of light helps us to explore them.

By observing the common optical phenomena around us, we may conclude that light seems to travel in straight lines. The fact that a small source of light casts a sharp shadow of an opaque object points to this straight-line path of light, usually indicated as a ray of light.

If an opaque object on the path of light becomes very small, light has a tendency to bend around it and not walk in a straight line - an effect known as the diffraction of light. Then the straight-line treatment of optics using rays fails. To explain phenomena such as diffraction, light is thought of as a wave, the details of which you will study in higher classes. Again, at the beginning of the twentieth century, it became known that the wave theory of light often becomes inadequate for treatment of the interaction of light with matter, and light often behaves somewhat like a stream of particles. This confusion about the true nature of light continued for some years till a modern quantum theory of light emerged in which light is neither a 'wave' nor a 'particle' - the new theory reconciles the particle properties of light with the wave nature.

In this Chapter, we shall study the phenomena of reflection and refraction of light using the straight-line propagation of light. These basic concepts will help us in the study of some of the optical phenomena in nature. We shall try to understand in this Chapter the reflection of light by spherical mirrors and refraction of light and their application in real life situations.

Nine point one. REFLECTION OF LIGHT

A highly polished surface, such as a mirror, reflects most of the light falling on it. You are already familiar with the laws of reflection of light.

Let us recall these laws -

(i) The angle of incidence is equal to the angle of reflection, and

(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces. You are familiar with the formation of image by a plane mirror. What are the properties of the image? Image formed by a plane mirror is always virtual and erect. The size of the image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it. Further, the image is laterally inverted. How would the images be when the reflecting surfaces are curved? Let us explore.

Activity Nine point one

Take a large shining spoon. Try to view your face in its curved surface.

Do you get the image? Is it smaller or larger?

Move the spoon slowly away from your face. Observe the image. How does it change?

Reverse the spoon and repeat the Activity. How does the image look like now?

Compare the characteristics of the image on the two surfaces.

The curved surface of a shining spoon could be considered as a curved mirror. The most commonly used type of curved mirror is the spherical mirror. The reflecting surface of such mirrors can be considered to form a part of the surface of a sphere. Such mirrors, whose reflecting surfaces are spherical, are called spherical mirrors. We shall now study about spherical mirrors in some detail.

Nine point two. SPHERICAL MIRRORS

Nine point two point one Image Formation by Spherical Mirrors

Activity Nine point three

Nine point two point two Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

Uses of concave mirrors

(b) Image formation by a Convex Mirror

Activity Nine point five

Activity Nine point six

Uses of convex mirrors

Nine point two point three Sign Convention for Reflection by Spherical Mirrors

Nine point two point four Mirror Formula and Magnification

Magnification

Example Nine point one

Solution

Example Nine point two

Solution

Nine point three REFRACTION OF LIGHT

Activity Nine point seven

Activity Nine point eight

Activity Nine point nine

Nine point three point one Refraction through a Rectangular Glass Slab

Activity Nine point ten

The following are the laws of refraction of light.

Nine point three point two The Refractive Index

Nine point three point three Refraction by Spherical Lenses

Nine point three point four Image Formation by Lenses

Nine point three point five Image Formation in Lenses Using Ray Diagrams

Nine point three point six Sign Convention for Spherical Lenses

Nine point three point seven Lens Formula and Magnification

Magnification

Example nine point three

Solution

Example nine point four

Solution

Nine point three point eight Power of a Lens

What you have learnt

EXERCISE S

SE

Overview

The document provides an in-depth look at how light interacts with various surfaces, particularly through reflection and refraction using spherical mirrors. It includes explanations backed by activities to visualize optical phenomena.

Key Points

1Light travels in straight lines and can reflect off surfaces
2Spherical mirrors can be concave or convex, each affecting image formation differently
3The laws of reflection state that the angle of incidence equals the angle of reflection
4Activities are included to demonstrate image formation and the concept of focal length in mirrors

Details

Category: Physical Sciences

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