Light 1 PDF - Chapter 5

# Chapter 5: Light ## Learning Objectives - Light and its sources - Rectilinear propagation of light and its applications - Pinhole camera - Formation of shadows ## Light Light is a form of energy. Light is needed to see things. We cannot see an object in the dark, we need a source of light to...

# Chapter 5: Light ## Learning Objectives - Light and its sources - Rectilinear propagation of light and its applications - Pinhole camera - Formation of shadows ## Light Light is a form of energy. Light is needed to see things. We cannot see an object in the dark, we need a source of light to make the object visible. During the day, sunlight allows us to see objects. At night, it is the light of a torch, an electric bulb or a tube light that makes us see objects. So, as soon as the light from a torch (or an electric bulb) falls on the object at night, we are able to see it clearly. Thus, light enables us to see objects from which it comes (or from which it is reflected). We will now discuss the various sources of light. ## Sources of Light (An object which gives out light is called a source of light.) The various sources of light around us are: - The Sun - Stars - An electric bulb - Tube light - Kerosene oil lamp - Candle - Torch - Fireflies (Glow-worm) The most important source of light for us is the Sun. Even though the Sun is about 150 million kilometres away from us, its light makes the daytime bright on the Earth. ## Natural and Man-Made Sources of Light All the sources of light can be divided into two groups: natural sources of light and man-made sources of light. - Those sources of light that occur in nature as such are called natural sources of light. - The Sun is the best natural source of light. - Nearly all the natural light we receive on the Earth comes from the Sun. - The stars, meteors (shooting stars) and fireflies are the other natural sources of light. - Those sources of light that have been made by man are called man-made sources of light (or artificial sources of light). - The electric bulb, tube light, kerosene oil lamp, candle and torch are all man-made sources of light. ## Luminous and Non-Luminous Objects - An object which gives out its own light is called a luminous object. - The Sun, stars, a burning candle, lighted electric bulb, a lighted torch, a glowing tube light, a piece of red hot iron, the flame of a gas burner and a firefly are all luminous objects. - Since luminous objects give out their own light, they can be seen even in the dark. A luminous object can be seen because the light it gives out enters our eyes. - For example, we can see the Sun because the light given out by the Sun enters our eyes. - An object which does not give out its own light is called a non-luminous object. - The table, chair, book, book rack, flower pot, plants, trees, clothes, bag, carpet, mirror, camera, the Moon, the Earth and other planets, etc., are all non-luminous objects. - Non-luminous objects can be seen only when light coming from a luminous object falls on them. - This light is reflected by non-luminous objects in all directions. - When this reflected light enters our eyes, we can see the non-luminous objects. - For example, the Moon is a non-luminous object that does not have its own light. - We can see the Moon because it reflects sunlight into our eyes. - Actually, when sunlight falls on the surface of the Moon, some of this sunlight is reflected by the Moon towards the Earth. - And to us, it appears as if the light is being given out by the Moon itself. But in reality, it is the sunlight that is reflected from the surface of the Moon. ## Transparent, Translucent and Opaque Materials When light falls on a material (or object), then three things can happen: all the light can pass through the material, only some of the light can pass through the material, or no light can pass through the material. - So, depending on their behaviour towards light, all the materials can be classified as transparent, translucent or opaque. - Let us discuss these three types of materials in detail. ### Transparent materials - Those materials which allow all the light to pass through them are called transparent materials. - The materials like glass, polythene, air, water and groundnut oil are transparent materials. - They allow almost all the light (which falls on them) to pass through them. - We can see clearly through the transparent materials. - For example, if we look at a burning candle through a sheet of clear glass, we can see the candle and its flame clearly. - This is because glass is a transparent object. ### Translucent materials - Those materials which allow only some of the light to pass through them are called translucent materials. - In other words, light passes only partially through translucent materials. - The materials like ground-glass (frosted glass), butter paper (tracing paper), oiled paper, tissue paper, a sheet of white cotton, muddy water and clouds are translucent materials. - They allow only a part of the light falling on their surface to pass through them. - We cannot see clearly through a translucent material. - For example, if we look at a burning candle through a piece of ground-glass (or butter paper), we can only see the dim light of the candle. - We can neither see the candle itself nor the candle flame clearly through the ground-glass. ### Opaque Materials - Those materials which do not allow any light to pass through them are called opaque materials. - Light cannot pass through opaque materials at all. - The materials like cardboard, book, wooden door, metal sheet, brick wall, and stone, etc., are all opaque materials. - These materials do not allow any light to pass through them. - We cannot see anything through an opaque object. - For example, if we look at a burning candle through a piece of cardboard held in front of our eyes, we cannot see anything at all. - This is because the cardboard (being opaque) does not allow the light of candle flame to pass through it and reach our eyes. - The opaque materials do not allow light to pass through them because they absorb or reflect all the light falling on them. ## Rectilinear Propagation of Light If we shine a torch on a dark night, we will see that the beam of light produced by torch travels straight into darkness. This observation shows that light travels in straight lines. The beams of search lights at the airports (or from light houses in the sea) show that light travels in straight lines. And the beam of light coming from the projection room of a cinema hall and falling on the screen also shows that light travels in straight lines. The property of light travelling in straight lines is called rectilinear propagation of light. We will now describe two simple activities which will show us that light travels in straight lines.