During the day, the Sun's light enables us to see the things. At night, our surroundings become dark and we switch on electric bulbs or lamps. So, we can say that light is a form of energy that enables us to see the things around us.


luminous objects

Objects such as the Sun, a burning candle and an electric bulb give out light of their own. Objects that emit light of their own are called luminous objects, Objects that do not emit light of their own are called non-luminous objects Book, table and spoon are a few examples of non-luminous objects.

 Moon and the planets are also non-luminous objects as they do not have light of their own. Non-luminous objects become visible to us because they reflect the light that falls on them. Moon is visible in the dark sky as it reflects the light of the Sun. Objects in a room can be seen as they reflect the light from a source.

 An object that gives out light is also called a source of light. In other words, a luminous object is also a source of light. Sources of light can be natural or artificial (man-made). Examples of natural sources of light include the Sun, stars, firefly and glow worm. Electric bulb, candle, torch and laser are artificial or man-made sources of light.


Depending upon how much light can pass through materials, they can be classified into transparent, translucent and opaque.

Transparent materials are those that allow light to pass through them completely, so that the object on the other side can be seen clearly. Clear glass, clean water, air and cellophane paper are transparent. We can see through these objects clearly.

Translucent materials are those that allow some light to pass through them, but the object on the other side can be seen partially. Frosted glass, butter paper, oily paper and muddy water are translucent. Though we can see the object on the other side, but not its details.

 Opaque materials are those that do not allow light to pass through them and the object on the other side cannot be seen at all. Wood, cement, cardboard, brick and metals are examples of opaque materials. We cannot see through these objects.


Light travels at a very fast speed. In fact, it is the fastest thing in the universe! Nothing can travel faster than light. In air, light moves about 3,00,000 km in just one second. Unlike sound, light can travel in vacuum. Sunlight takes about 8 minutes to reach the Earth. That means the light you are seeing at this moment had left the Sun 8 minutes before. Olaus Roemer, a Danish astronomer, calculated the speed of light for the first time in 1676.

Light Travels in Straight Line

 In a given medium, light travels in a straight line. This is referred to as rectilinear propagation of light. Rectilinear means 'consisting of straight lines' and propagation means 'movement through a medium'. As long as light is travelling in a particular medium, it moves in straight lines.

 Rectilinear propagation

Rectilinear propagation of light is represented by straight lines in the form of rays and beams. A ray of light is a narrow path of light represented by a thin line with an arrowhead. The arrowhead shows the direction in which light is travelling. A beam of light consists of several rays emerging from a light source. If the source of light is very small, it is called a point source. A beam of light emerging from a point source goes out in all directions. The property of light travelling in straight lines is responsible for the formation of shadows.


You must have seen your shadow forming on the ground on a sunny day. As you move, your shadow also moves. Can you run away from your shadow? Let us learn what causes a shadow to form.

 A shadow is formed when an opaque object comes in the path of light. As light travels in straight lines, the path of the light is blocked by the opaque object and a dark region called shadow is formed on the opposite side of the light source.

A shadow is defined as a dark area formed when an opaque object obstructs the path of light. A transparent object will not cast any shadow, whereas a translucent object will cast a faint shadow.

Shadows are formed because light does not bend while travelling through a given medium. For the formation of a shadow, three things are essential:
(i)A source of light   (ii)An opaque object  (iii)  A screen or surface behind the object
If any of these things is absent, the shadow will not be formed.


 In many cases, a wall or the ground acts as the screen. We cannot obtain a shadow in the dark because there is no light source. If the screen is placed too far, we may not again get a shadow. That is why the shadows of flying aeroplanes cannot be seen on the ground.

Characteristics of a Shadow

A shadow has the following characteristics :
(i)The colour of the shadow is always dark, that is, black or grey, irrespective of the colour of the object.

(ii)A shadow shows only the outline and the shape of the object; and not its details.

(iii)The size of the shadow depends on (a) the distance between the light source and the object, and (b) the distance between the object and the screen.

(iv) A shadow cannot be obtained without a screen.


solar eclipse

The Sun is a giant source of light. The Earth and the Moon are opaque and non-luminous heavenly bodies. When the sunlight is blocked by these heavenly bodies, shadows are formed.

A solar eclipse occurs when the Sun, the Moon and the Earth come in a straight line such that the shadow of the Moon falls on a part of the Earth. From that region, the Sun cannot be seen completely as it gets 'eclipsed' or hidden.

 A lunar eclipse occurs when the Sun, the Earth and the Moon come in a straight line such that the shadow of the Earth falls on the Moon. In this case, the Moon gets 'eclipsed' or hidden.


reflection of light.

When light bounces off a surface, it is said to be reflected. It is because of reflected light that we are able to 'see' the things around us. The process of bouncing back of light rays after hitting a surface is known as reflection of light.

 When the surface is smooth and shiny, such as polished metal, glass or water, the light is reflected at the same angle as it strikes the surface.This is called regular reflection. Reflection in a mirror and in a new steel spoon are examples of regular reflection.

 When the surface is rough, the light is reflected in different directions. This is called diffused reflection, Reflection by surfaces such as wall, floor and table are examples of diffused reflection.

Reflection by Mirror

 When you stand in front of a mirror, you see your image. An image is different from a shadow. Your image looks exactly like you. It shows the colour as well as the outline. If you raise your right hand, your image appears to raise its left hand. If you move your left leg, your image appears to move its right leg. This left-right reversal of an image is called lateral inversion. Thus, we say that the image formed by a plane mirror is laterally inverted.

Did you know why ambulances for carrying patients have DMAJUUMA written on them? This is because when seen in the rear view mirror or side view mirror of other vehicles, it appears as AMBULANCE, due to lateral inversion. This is done so that other vehicles give way to the ambulance on the road.