Physics Topics are often described using mathematical equations, making them precise and quantifiable.
What is Nature and Reflection of Light ?
Light is a form of energy. Light is needed to see things around us. We are able to see the beautiful world around us because of light. We can read a book, see pictures in a magazine, and watch television and movies due to the existence of light. And it is light which makes us see our image in a looking mirror. We detect light with our eyes.
Though we see the various objects (or things) around us with our eyes but eyes alone cannot see any object. We also need a source of light to make objects visible. For example, we cannot see any object in a dark room or in the darkness of night because there is no light in a dark room or in the night to make objects visible.
But as soon as an electric bulb (a tube-light or a torch, etc.) is switched on and light falls on the objects, we are able to see them clearly. It is only when light coming from an object enters our eyes that we are able to see that object. This light may have been emitted by the object itself or it may have been reflected by the object.
Light enables us to see objects from which it comes or from which it is reflected. For example, the sun gives out light. We can see the sun because the light coming from the sun enters our eyes. The objects like the sun, other stars, electric bulb, tube-light, torch, candle and fire, etc., which emit their own light are called luminous objects (see Figure). We can see the luminous objects due to the light emitted by them.
Though luminous objects are very small in number but they help us to see a large variety of non-luminous objects around us. The objects like a flower, a chair or a table do not have light of their own but even then we are able to see them. This can be explained as follows:
Though the objects like a flower, a chair or a table, etc., do not emit light themselves, we can see them by the light which they reflect (or scatter) by taking it from a luminous object like the sun or an electric bulb, etc. (see Figure).
So, when the sunlight or bulb light falls on a flower or chair (or any other object), some of this light is reflected towards us. And Figure. We see most of the objects around us by the light reflected from them, when this reflected light enters our eyes, then we are able to see the flower or chair (because to our eyes, this light appears to be coming from the flower or chair) (see Figure).
Those objects which do not emit light themselves but only reflect (or scatter) the light which falls on them, are called non-luminous objects (see Figure). A flower, chair, table, book, trees, other plants, human beings, fan, bed, mirror, diamond, walls, floor, and road, etc., are all non-luminous objects.
In fact, most of the objects around us are non-luminous objects. We can see the non-luminous objects because they reflect light (received from a luminous object) into our eyes. Even the moon is a non-luminous object (because it does not have its own light). We can see the moon because it reflects the sunlight falling on its surface towards us (on the earth).
From the above discussion we conclude that light is a form of energy which causes in us the sensation of sight. Light travels in straight lines. The fact that a small source of light casts a sharp shadow of an opaque object tells us that light travels in a straight line path. This is because if light could bend easily and go behind the opaque object, then no shadow could be formed. We will now discuss the nature of light.
Nature of Light
There are two theories about the nature of light : wave theory of light and particle theory of light. According to wave theory : Light consists of electromagnetic waves which do not require a material medium (like solid, liquid or gas) for their propagation.
The wavelength of visible light waves is very small (being only about 4 × 10-7 m to 8 × 10-7 m). The speed of light waves is very high (being about 3 × 108 metres per second in vacuum). According to particle theory : Light is composed of particles which travel in a straight line at very high speed. The elementary particle that defines light is the ‘photon’.
Some of the phenomena of light can be explained only if light is considered to be made up of waves whereas others can be explained only if light is thought to be made up of particles. For example, the phenomena of reflection, refraction, diffraction (bending of light around the corners of tiny objects), interference and polarization of light can only be explained if light is considered to be of wave nature.
The particle theory of light cannot explain these phenomena. On the other hand, the phenomena of casting of shadows of objects by light and photoelectric effect can be explained only if light is thought to be made of particles. Wave theory of light cannot explain these phenomena. Thus, there is evidence for the wave nature of light as well as for particle nature of light.
Physics experiments over the past hundred years or so have demonstrated that light has a dual nature (double nature) : light exhibits the properties of both waves and particles (depending on the situation it is in). The modern theory of light called ‘Quantum Theory of Light’ combines both the wave and particle models of light.
Reflection of Light
When light falls on the surface of an object, some of it is sent back. The process of sending back the light rays which fall on the surface of an object, is called reflection of light. The reflection of light is shown in Figure. When a beam of light AO falls on a mirror at point O, it is sent back by the mirror in another direction OB (see Figure). And we say that the mirror has reflected the beam of light falling on it.
We can compare the reflection of light to the bouncing back of a tennis ball on hitting a wall. For example, if we throw a tennis ball at a wall, the ball bounces back. This means that the wall sends it back. Similarly, when light falls on the surface of an object, the object sends the light back. And we say that the object reflects the light. Most of the objects reflect light which falls on them.
Some objects reflect more light whereas other objects reflect less light. The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces.
We know that an object (say, a chair) kept in a room can be seen from all the parts of the room. This is due to the fact that usually, because of its rough surface, an object reflects light (or scatters light) in all the directions. Since the reflected light reaches all the parts of the room, the object can be seen from all the parts of the room. If, however, the surface of an object is smooth (like that of a mirror), then the light falling on it is reflected in only one direction (as shown in Figure).
Silver metal is one of the best reflectors of light. For example, a polished block of silver metal reflects almost all the light falling on it and does not transmit any light through it. But the surface of silver metal is easily scratched and soon becomes rough.
So, ordinary mirrors are made by depositing a thin layer of silver metal on the back side of a plane glass sheet. The silver layer is then protected by a coat of red paint. The reflection of light in a plane mirror (or any other mirror) takes place at the silver surface in it.
Thus, a plane mirror is a thin, flat and smooth sheet of glass having a shining coating of silver metal on one side. The silver coating is protected by a red paint. These days mirrors are being made increasingly by depositing a thin coating of aluminium metal at the back of a glass sheet (instead of silver coating).
This is because aluminium is much cheaper than silver and it reflects light very well. A plane mirror is shown in Figure (a). The mirror on our dressing table in which we see our face is a plane mirror.
In our everyday life, we use plane mirrors of different shapes and sizes depending on where they are being used. But in a science laboratory, a small rectangular strip of plane mirror is usually used for performing experiments on the reflection of light. In our diagrams, a plane mirror is represented by a straight line having a number of short, oblique lines on one side [see Figure (b)].
The plane side of straight line is the front side of the mirror (where the reflection of light takes place). And the side having short, oblique lines represents the back side of the plane mirror. In Figure(b), the straight line MM (read as : MM-dash) represents a plane mirror.
We will also be using rays of light in constructing the ray-diagrams. A ray of light is the straight line along which light travels. The arrow head put on the straight line tells us the direction in which the light is travelling. A “bundle of light rays’ is called a ‘beam of light’. We will now study the reflection of light from the plane surfaces like that of a plane mirror. This is necessary to understand the reflection of light from spherical mirrors.