Contents
Physics Topics are often described using mathematical equations, making them precise and quantifiable.
How the Reflection of Light From the Mirrors Differs From One Object to Other
If we throw a tennis ball at a wall, then the ball bounces back. This means that when the tennis ball strikes the wall, the wall sends it back. Similarly, when light falls on the surface of an object, the object sends the light 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 from a mirror has been shown in Figure. When a ray 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 ray of light falling on it. We can demonstrate the reflection of light by a mirror as follows :
Fix a mirror on the wall in a room and make the room completely dark by putting on curtains, etc. Take a torch and throw a beam of light on the mirror obliquely along the path AO (see Figure). This beam of torch light will be reflected by the mirror along the path OB. And if a screen (like a sheet of cardboard) is held by a person at position B, then a patch of reflected light will be formed on the screen.
It is not only a mirror which reflects 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.
For example, a shining silvered mirror (ordinary mirror) reflects back almost all the light which falls on it. On the other hand, a table or chair having dull surfaces reflect back only a small amount of light which falls on them. And a black surface reflects almost no light.
In this chapter we will be using the term ‘ray’ (of light), so we should know what it means. A narrow beam of light is called a ray. Since light travels in a straight line, so in our diagrams, we will represent a ray of light by a straight line with an arrow head on it. The direction of this arrow shows the direction in which the light is travelling at that time.
Thus, in diagrams, lines with arrows (called rays) show which way the light is going. The narrow beam of light ‘before reflection’ is called ‘incident ray’, and ‘after reflection’ is called ‘reflected ray’. In other words, the light hitting the mirror is called the incident ray whereas the light leaving the mirror is called the reflected ray.
Mirrors
We need a shiny surface for the reflection of light. This is because a shiny surface reduces the absorption of light and causes the maximum reflection of light. In science, an object which reflects light well is called a mirror. For example, a highly polished and shiny metal object reflects light well and acts as a mirror. In fact, silver metal is one of the best reflectors of light.
So, silver metal is used to make the ‘glass mirrors’ which we use in our everyday life. In everyday language, a looking glass is called mirror. We see mirrors in our homes on the dressing table and in the bath-room. We use these mirrors for looking at our face while combing the hair or putting on make-up. The mirrors which we use everyday are plane mirrors. A plane mirror is a flat mirror. We can now define a plane mirror as follows:
A plane mirror is a thin, flat and smooth sheet of glass having a shiny coating of silver metal (or aluminium metal) on one side. The silver coating is protected by a layer of red paint. A plane mirror is shown in Figure (a).
The glass sheet of plane mirror offers a smooth surface. This smooth surface produces regular reflection and helps in forming a clear image. The silver metal coating (or silvering) makes the plane mirror shiny. The shiny surface of plane mirror gives us the maximum reflection of light.
This helps in forming a bright image. The red paint behind the silvered surface of plane mirror reduces the transmission of light through the mirror and also protects the delicate silver coating of the mirror.
The reflection of light in a plane mirror takes place at the silvered surface inside it. This silvered surface is visible from front side of mirror (through the glass sheet). Please note that a mirror works because it reflects light. A mirror reflects most of the light falling on it. Moreover, a mirror reflects rays of light in definite directions.
We use plane mirrors of different sizes in our everyday life depending on where they are being used. But in a science laboratory, a small rectangular strip of plane mirror is used for performing experiments on the reflection of light. In our diagrams, we represent a plane mirror by drawing a straight line having a large number of short, oblique lines on one side [see Figure (F)].
The plane side of the 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. So, in Figure (F), the line MM’ (read as: MM-dash) represents a plane mirror.
How the Reflection of Light From A Mirror Differs From the Reflection of Light From Other Objects
The way light is reflected from a surface depends on whether the surface is rough or smooth. Most of the objects have rough surfaces. Due to their rough surfaces, most of the objects reflect light in all directions. This is called irregular reflection (or diffuse reflection). For example, a piece of white paper has a rough surface (as seen through a microscope), so it reflects the light falling on it in all directions (see Figure). In fact, a piece of paper scatters light in all directions causing irregular reflection. It is due to the irregular reflection of light produced by the rough surface of a piece of white paper that we cannot see the image of our face on looking into the piece of paper.
A mirror has a smooth and shiny surface. A mirror reflects all the light falling on it in the same direction (without scattering it) (see Figure). In fact, a mirror reflects a ray of light at the same angle at which it strikes the mirror. This is called regular reflection.
Regular reflection produces reflected light in the same arrangement as the incident light ray. Regular reflection of light can form an image. For example, the regular reflection of light by the smooth surface of a plane mirror produces an image of our face when we look into the mirror.
In fact, all the smooth and shiny surfaces reflect light in a regular way forming images. Regular reflection occurs only at very smooth surfaces. Some of the common objects having very smooth surfaces are mirrors, polished metal objects and still water surface (as in a pond or lake). Light rays coming from an object and striking a smooth surface are reflected in a regular way.
When this reflected light rays enter our eyes, we see an image (or picture) of the object in the smooth surface. For example, if we place a candle in front of a plane mirror (looking mirror), then the smooth and shining mirror surface produces a regular reflection of light coming from the candle due to which we can see an image of the candle in the mirror (see Figure).
Similarly, the smooth surface of “still water” in a lake causes regular reflection of light and produces images of various objects like trees, buildings, etc., in the water. Actually, the smooth surface of still water in a lake (or pond) acts like a mirror.