Introduction

Light is a form of energy. Light energy is essential for all the living beings. With the help of light we see different objects. Human beings cannot see objects without light. Plants too need light energy. In the presence of light, plants make their food. These foods are then used to fulfill the need of the living beings for their survival.

Sources of light

Any luminous object can be taken as the source of light. Reading under the light of electricity Unit -5 Sun: Sun is the prime natural source of light. Because of the sun day are brighter than night. Normally there is no other light source which can produce better light than the sun. Thus sun is the prime source of light energy.\

Current electricity

Current electricity is the next important source of light energy. In the night. We use current electricity to get light. We consider the electric devices to be simple and easy to handle. In most parts of our country electricity is used for light. Big towns and developed countries also use electricity for light.

Lamp

Candle lamp, kerosene lamp, lantern etc. can also be taken as the secondary light source. Candle are made from wax. Kerosene is used in the kerosene lamp and the lantern. These two have been the main sources of light where there is no current electricity.

Luminous and non-luminous objects

Luminous object: Any object which produces light energy by itself is called the luminous object. For example sun, electric lamp, burning fire, star, firefly, etc are the luminous objects. Light comes out from these objects, by different phenomenal processes in them. In general heat is also produced along with light energy from the luminous objects. Non-Luminous object: Objects which cannot produce light of their own are known as the non-luminous objects. The objects around us are mostly nonluminous. Tables, benches, books, mirrors, chalk, stone, soil, etc are non-luminous objects. Moon is also a non-luminous object. Moon cannot produce light from its (own) surface. Moon appears bright when sun light is reflected from its surface and falls on the earth. Similarly, a mirror also appears bright and shiny when the incident sun light is reflected from it. But both the moon and the mirror are not the luminous objects. Among the non-luminous objects, they are categorized as,

(1) Transparent object

(2) Translucent object

(3) Opaque object

1. Transparent object:

Objects in which light can easily transmit through them are known as transparent objects. Air, clear water, smooth glass plate, thin colourless plastics sheets, etc are transparent objects. Light can easily transmit through them. For example, for hundreds of kilometre, earth is surrounded by atmospheric air. Sun rays reach the earth's surface transmitting through these dense layer in very short time. The incident light rays are transmitted without significant deviation or absorption on them.

2. Translucent object:

Translucent objects permit the passage of light rays but the transmitted light rays are scattered or diffused. Because of diffusion, the objects on the other side cannot be seen clearly. For example, frosted glass 32 Science and Environment, Class - 6 allows light to pass through it but an object cannot be seen clearly through it. Oil papers, tissue papers, thin tracing papers, etc behave as the translucent objects. Hence in a translucent medium such as a ground glass, light can pass through it only with an appreciable loss. So that objects cannot be seen distinctly through it.

3. Opaque object:

Any object that does not allow light to pass through it is called an opaque object. Wood, stone, iron, etc are opaque objects. The opaque object forms its shadow as it stops the light incident on it.