Light

Published by: BhumiRaj Timalsina

Published Date: 24 Jan 2022

Light in Grade-7, Reference Note

Introduction

Light is a form of energy, which gives the sensation of vision. It makes things visible. Light is very important in the life of living creatures. We cannot see things without light. We use torch light, bulb light during night time to see the things clearly. We cannot perform our activities in dark. Light is not only used to see the things but it is also used by all living creature to perform their life activities like plants use light for the photosynthesis process, etc.
The speed of light is very fast. No object can travel as fast as light can. The speed of light in a vacuum is about 3 lakh kilometre per second. The light coming from the sun takes only 8 minutes and 20 seconds to reach the earth's surface. The distance between the sun and the earth is about 15 core kilometres. Light travels in a straight line.

Sources of light

Those objects, which emits light, are called sources of light. There are various sources of light like sun, star, torch light, bulb light, fire, candle etc. The sun is the major source of light. Every object does not produce light, only a few objects emit their own source of light. Those objects which produce their own source of light are called luminous objects and those objects which do not produce their own light are called non- luminous objects. Some examples of luminous objects are sun, star, firefly etc. and non- luminous objects are earth, the moon, rock etc.

Ray and Beam of light

A ray of light is a column of light. It is a very narrow path of light represented by a line with an arrowhead showing the direction of propagation of light. The collection of a ray of light, which travel along a straight line, is called beam of light. A ray is a single part of the beam of light.

Reflection of light

When the ray of light coming from the source falls on a smooth surface, it returned back to the same medium after striking the surface. This phenomenon of light is known as the reflection of light. When we put the torchlight in front of the mirror, the mirror reflects the torch light. When we see in the clear water, we see our face in water due to the reflection.
Non- luminous objects do not emit their own light but absorb the light coming from the luminous objects. They reflect the light as a result, we can see the non- luminous objects in dark.
Not all the objects reflect the light. Only some of them reflect the light. Objects which are black in colour does not reflect the light instead they absorb all the light that falls on them. The reflection of light depends on the reflecting surface of the body. The smooth surface reflects a parallel beam of light in one direction and rough surface reflects a parallel beam of light in various direction. Depending upon the reflecting surface, there are two types of reflection which are as follows,

Regular reflection: If a parallel beam of light is reflected as a parallel beam when it strikes on a smooth and polished surface, then it is called regular reflection. Reflection of light from the plane mirror is the example of parallel reflection.

Irregular reflection: If a parallel beam of light is reflected as in different direction when it strikes a rough surface, then it is called irregular reflection. Reflection of light from the surface of the water (wavy water) is the example of irregular reflection.

Source: www.ducksters.com Fig: Regular and Irregular Reflection of Light

Activity 2
Bring a plane mirror and a torch light. Then placed the mirror in the suitable position. Now, show the torch light in the mirror in a dark room. In this activity, the ray of light coming from the torch light that falls on the smooth surface is the incident and the ray of light that gets reflected from the plane mirror is reflected ray. What do you learn from this activity? Draw a neat diagram showing this phenomenon and label the incident rays and reflected rays.

There are various terms which are used in reflection. Some of them are given below,

Point of incidence: The point at which the ray of light coming from the source hits the surface or mirror is called the point of incidence.

Incident ray: The ray of light coming from the source of light, which hits the surface at the point of incidence, is called incident ray. Incident ray is shown in the given figure.

Reflected ray: The ray of light, which is reflected back to the same medium after incident ray strike the surface is called reflected ray. Reflected ray is shown in the given figure.

Source: www1.curriculum.edu.au Fig: Reflection of Light

Normal: It is the line drawn perpendicular to the surface from the point where the incident ray meets the surface. Normal is shown in the given figure.

Angle of incidence: The angle made by the incident ray and the normal is called the angle of incidence. The angle of incidence is shown in the given figure.

Angle of reflection: The angle made by the reflected ray and the normal is called the angle of reflection. The angle of reflection is shown in the given figure.

Laws of reflection

The reflection of light follows the given two laws,

  • The angle of incidence is equal to the angle of reflection.
  • The incident ray reflected ray and the normal lie at the same plane.

Reflection of light from the plane mirror.

A plane mirror is a smooth plane surface which reflects the entire light reflect on it. Reflection from the plane mirror is the type of regular reflection. When the parallel beam of light strikes on a plane mirror then the reflected beam of light are also parallel to each other. The angle of reflection is equal to the angle of incidence when the rays of light fall on the plane. While drawing the reflection of light on a plane mirror, the reflecting surface of the mirror is represented by the straight line and the coated side of the mirror by shaded lines. Polished surfaces of metals, glass, nickel, motionless water surface, etc. act as a plane mirror. The image formed by the plane mirror is erect and virtual.
 

Activity 3
Bring a protractor, a lightbox and a small plane mirror. Then place the plane mirror in suitable position. Put the protractor of the plane mirror (contact). Do this activity in the dark room or you can place the arrangement inside the paper box. Then show the light from the lightbox in any one of the angles of the protractor. What do you observe? What do you conclude from this activity?

Source: www.webinnate.co.uk Fig: Periscope

 

Periscope

It is an instrument used for the observation or used by the observer to see surroundings or things beyond his or her line of sight or which are not in direct line of sight. It is mainly used in land and sea welfare, submarine navigation, etc. It is based on the principle of laws of reflection.

Structure

It consists of two plane mirror M1 and M2 and a wooden tube which bends twice at right angles. There are two openings X and Y in the wooden tube. Two mirrors are placed near the opening at the bends of tube making an angle of 45 degrees with the side of the tube. They are placed parallel to each other and facing their reflecting surface to each other.

Working

At first, the ray of light enter through the opening X which strikes on the mirror M1 at 45 degrees and gets reflected from the mirror which travels through the tube and again strike on the mirror M2 at 45 degrees. Then, the ray gets reflected parallel to their original path or opening Y through which it reaches the viewer's point.

Uses

  • It is used in submarine navigation .
  • It is also used in medicine to look into the human body and is used in cystoscopes and endoscopes.
  • It is used to observe the activities of the enemy when direct observation is dangerous.

Kaleidoscope

A kaleidoscope is an optical instrument consisting of three rectangular mirror, broken glass pieces, and coloured objects or stones. It is based on the principle of laws of reflection. The three rectangular glasses are arranged or fitted at 60 degrees to each other in the form of a long prism and facing their reflecting surface to each other. It is wrapped with black paper or black cello tape so that the light does not enter. Both the ends are covered by greased white tissue paper. Broken pieces of glasses or coloured objects or stones are kept at one end of the kaleidoscope and another end for observation. When we held the kaleidoscope pointing towards the sky and rotate it, we can see the beautiful patterns of colour. It is used as a toy.

Menu