Contents
Physics Topics can help us understand the behavior of the natural world around us.
Image formed by plane mirror is real or virtual ?
When an object is placed in front of a plane mirror, its image is formed in the plane mirror. We can locate the image of an object in a plane mirror by using the laws of reflection of light. This can be done as follows.
Suppose a small object O (say, a pin-head) is placed in front of a plane mirror MM’ (see Figure). An image of this point object will be formed in the plane mirror. We can find out the position of image of this object in the plane mirror by drawing a ray-diagram as follows:
1. We take two diverging rays OA and OB coming from the object O. The rays OA and OB are incident rays which strike the mirror at points A and B making different angles of incidence.
2. The first incident ray OA is reflected by the mirror at point A and goes in the direction AX after reflection (making an angle of reflection r1 equal to the angle of incidence i1). So, AX is the first reflected ray.
3. The second incident ray OB is reflected by the mirror at point B and it goes in the direction BY after reflection (making an angle of reflection r2 equal to the angle of incidence i2). Thus, BY is the second reflected ray.
4. The two reflected rays AX and BY are diverging rays (going apart) which cannot meet in front of the mirror to form a real image. So, we produce the reflected rays AX and BY backwards (behind the mirror) by dotted lines. On producing backwards, the reflected rays meet at point behind the mirror (see Figure).
5. When we look into the plane mirror from an angle (as shown in Figure), the reflected rays AX and BY enter our eye. The reflected rays appear to be coming from point I behind the mirror. So, point I is the image of object O.
Since the reflected rays of light do not actually meet at the image point I but only appear to do so, we say that a virtual image of object O is formed at I. Such a virtual image cannot be obtained on a screen.
Characteristics of Image Formed by a Plane Mirror
- The image formed by a plane mirror is virtual (or unreal).
- The image formed by a plane mirror is behind the mirror.
- The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.
- The image formed in a plane mirror is of the same size as the object.
- The image formed by a plane mirror is erect.
- The image in a plane mirror is laterally inverted.
We have studied the characteristics of images formed by a plane mirror in detail in Class VII. Here we will explain the various characteristics of an image formed by a plane mirror briefly. By saying that the image formed by a plane mirror is virtual, we mean that the image formed by a plane mirror cannot be obtained on a screen. It can be seen only by looking into the plane mirror. For example, when we look into a plane mirror, we see the image of our face.
Our image in the plane mirror is virtual (or unreal). Our image seen in the plane mirror cannot be formed on a screen placed behind the plane mirror (where the image appears to be). In fact, when we look into a plane mirror (like the one on a dressing table), we find that our image in the plane mirror is formed behind the plane mirror.
By saying that the image in a plane mirror is the same distance behind the mirror as the object is in front of it, we mean that if a person is standing at a distance of 1 metre in front of a plane mirror, then his image will be formed at the same distance of 1 metre behind the plane mirror. Now, since the person is 1 metre in front of plane mirror and his image is 1 metre behind the plane mirror, therefore, the distance between the person and his image will be 1 metre + 1 metre = 2 metres. In other words, if a person is 1 metre in front of a plane mirror, then he will seem to be 2 metres away from his image.
When we say that the image in a plane mirror is of the same size as the object, we mean that the dimensions of image (such as length, breadth and height) are exactly the same as that of the object. In other words, the image is neither enlarged (neither bigger than the object) nor diminished (nor smaller than the object). For example, if a person is 1.75 metre tall, then his image in the plane mirror will also be exactly 1.75 metre tall. By saying that the image of an object in a plane mirror is erect, we mean that the image in plane mirror is the same side up as the object—the top of object is the top of image and bottom of object is the bottom of image. For example, if we look into a big plane mirror, we will see the image of our whole body. We will notice that our image in the plane mirror has head on the top and feet at the bottom just like us. We say that our image in the plane mirror is erect. It is the same side up as we are.
By saying that the image in a plane mirror is laterally inverted, we mean that the image in a plane mirror is sideways reversed with respect to the object. Actually, in an image formed by a plane mirror, the left side of object appears on the right side in the image whereas the right side of object appears on the left side in the image. This change of sides of an object and its mirror image is called lateral inversion.
For example, if we stand in front of a plane mirror (like the one on a dressing table) and lift our left hand, then our image in the plane mirror appears to lift its right hand. And if we lift our right hand, then our image in the plane mirror appears to lift its left hand. This means that the left side of our body becomes the right side in the mirror image whereas the/right side of our body becomes left side in the mirror image. We say that our image in the plane mirror is laterally inverted (or sideways reversed). The change in the sides of an object and its mirror image is due to the phenomenon of lateral inversion.
Reflected Light Can Be Reflected Again
If the rays of light reflected by a plane mirror are incident on another plane mirror, then the reflected rays are reflected again. In this case, the reflected rays of the first plane mirror become incident rays for the second plane mirror. The further reflection from second plane mirror also takes place according to the laws of reflection of light. An optical instrument (or device) in which reflected light is reflected again is a periscope. We will now describe a periscope.
A periscope is a long, tubular device through which a person can see objects that are out of the direct line of sight. A periscope gives us a higher view than normal. For example, by using a periscope, we can see the objects on the other side of a high wall which cannot be seen by us directly. The periscope makes use of two plane mirrors to see over the top of things. Actually, a periscope works on the reflection of light from two plane mirrors arranged parallel to one another. A periscope consists of a long tube T having two plane mirrors M1 and M2 fitted at its two ends (as shown in Figure). The two plane mirrors are fitted in such a way that they are parallel to one another and their reflecting surfaces face each other. Each plane mirror, however, makes an angle of 45° with the side of the tube (see Figure). There are two holes in the periscope tube : one hole is in front of the top mirror M1 and the other hole is in front of the bottom mirror M2.
We will now describe the working of a periscope. Suppose there is a tree behind a high wall which we cannot see directly (see Figure). We can, however, see this tree by using a periscope. This can be done as follows : The upper hole of periscope is turned towards Figure. Diagram to show the working of a the object to be seen (here a tree) so that the top mirror M1 faces the object. We look into the periscope from the bottom hole in front of lower mirror M2. The light rays coming from the tree fall on the plane mirror M1.
Mirror M1 reflects these rays of light downwards, towards the second mirror M2 (see Figure). The mirror M2 then reflects the reflected rays of light towards the eye of the person looking into the periscope through the lower hole. Since the light rays coming from the tree enter the eye of the person (or observer), it is possible to see the image of tree through the periscope (even though the tree cannot be seen directly).
In a periscope, the top mirror reflects light and the bottom mirror reflects the ‘reflected light So, the working of a periscope demonstrates that reflected light can be reflected again. The working of a periscope also explains how reflection from two plane mirrors enables us to see objects which are not visible directly.
Some of the Uses of Periscopes are Given Below:
- A periscope is used to see over the heads of a crowd (say, as in a football match).
- A periscope is used by soldiers sitting in a trench (or bunker) to observe the enemy activities outside (over the ground).
- A periscope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea (even though the submarine itself may be submerged under water).
The fact that reflected light can be reflected again enables a person to see the hair cut at the back of his head at a hair dresser’s shop. This happens as follows: After giving the hair cut to a person, the hair dresser holds a small plane mirror behind the head of the person. The light coming from hair at the back of head is reflected by this small mirror on to a big mirror which is in the front of the person. This big mirror reflects the reflected light’ again due to which the person can see the image of the back hair of his head showing how the hair have been cut at the back side of his head.
Multiple Images
We know that a plane mirror forms only a single image of an object placed in front of it. We will now describe what happens when an object is placed between two plane mirrors which are inclined at an angle to each other. When two plane mirrors are kept inclined at an angle, they can form multiple images of an object. This is because the image of object formed in one plane mirror acts as object for the other plane mirror. It has been found that if two plane mirrors are inclined at an angle x, then the number of images formed in them is given by the formula:
No. of images formed = \(\frac{360^{\circ}}{x}\) – 1
By using this formula, we can calculate the number of images formed (or seen) when two plane mirrors are inclined at angles of 180°, 120°, 90°, 60°, 45° and 0°, respectively. As an example, let us calculate the number of images formed in two plane mirrors inclined at an angle of 90°. In this case the angle between two plane mirrors (x) is 90°. So,
No. of images found = \(\frac{360^{\circ}}{90^{\circ}}\) – 1
= 4 – 1
= 3
Thus, two plane mirrors inclined at an angle of 90° form three images of an object placed between them.
When the two plane mirrors are inclined at an angle of 90°, they are said to be at right angles to each other. So, we can also say that the two plane mirrors arranged at right angles to each other form three images of an object placed between them. Thus, if we take two plane mirrors, set them at right angles to each other (with their edges touching), and place a coin in-between these mirrors, then we will see three images of the coin in the two plane mirrors.
We can also do the calculations to find the number of images of an object formed when the two plane mirrors are inclined at angles of 180°, 120°, 60°, 45° and 0°. The results of these calculations are given below:
From the above table we can see that as the angle between the two plane mirrors decreases, the number of images formed increases. And when the angle between two plane mirrors becomes 0°, that is, when the two plane mirrors become parallel to each other, then an infinite number of images are formed.
Thus, if an object is placed between two parallel plane mirrors facing each other, then theoretically, an infinite number of images (very, very large number of images) should be formed. In reality, only a limited number of images are seen. This is because some light is absorbed by the mirrors at each successive reflection due to which many images are so faint that they cannot be seen by us.
From this discussion we conclude that: We can see a large number of images of ourselves if we stand before two plane mirrors hanging on opposite walls. This is because our image formed in one mirror acts as object for the other mirror, and this process goes on and on, resulting in a large number of images. We can, however, not see all the images because they become fainter and fainter with the increasing number of reflections of light.
The fact that multiple images (or a number of images) of an object are formed by plane mirrors which are kept inclined at an angle to one another is used in kaleidoscope to make numerous beautiful patterns which are liked by children as well as by adults. Let us discuss the kaleidoscope now.
Kaleidoscope
The kaleidoscope is an instrument or toy containing inclined plane mirrors which produce multiple reflections of coloured glass pieces (or coloured plastic pieces) and create beautiful patterns. The kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 60° to one another forming a hollow prism, and fitted into a cardboard tube. One end of the cardboard tube is closed by an opaque disc (cardboard disc) having a small hole at its centre. The other end of cardboard tube is closed with two circular discs of glass : the inner disc being of transparent glass (clear glass) and the outer disc of ground glass (translucent glass). A number of small pieces of different coloured glass (or plastic) and having different shapes are kept between the two glass discs (which can move around freely in the space between the two glass discs).
When we hold the kaleidoscope tube towards light and look inside it through the small hole, we see beautiful patterns of coloured glass (see Figure). Actually, the coloured glass pieces act as objects and the inclined plane mirrors form multiple images of these glass pieces by repeated reflections, which look like beautiful patterns (or designs).
If we turn the kaleidoscope tube slightly, the glass pieces will rearrange and produce a new pattern. A kaleidoscope produces hundreds of ever- changing coloured patterns (or designs). An interesting feature of a kaleidoscope is that we can never see the same pattern again. Every time a new pattern is formed. Kaleidoscopes are used by designers of wall papers and fabrics, as well as by artists to get ideas for new patterns.
Activity 2
We can make a kaleidoscope ourselves as follows: Take three rectangular strips of plane mirror each about 15 cm long and 4 cm wide. Join the three plane mirror strips lengthwise by using adhesive tape so as to form a hollow prism (see Figure). The reflecting surfaces (shiny surfaces) of the three mirror strips are kept facing one another.
Fix the hollow prism formed by joining three mirror strips in a cardboard tube which is somewhat longer than the mirror strips. Close one end of the cardboard tube with a cardboard disc having a small hole at the centre (see Figure). At the other end of cardboard tube, fix a transparent glass disc touching the hollow prism of plane mirror strips. Place several small pieces of coloured glass (like broken pieces of different coloured glass bangles) on the transparent glass disc and then fix a ground glass disc to close the end of cardboard tube.
Enough space should be left between the transparent glass disc and ground glass disc to allow the coloured glass pieces (placed between them) to move around freely when the cardboard tube is rotated. The kaleidoscope is now ready. When we peep in through the hole on the front side and rotate the cardboard tube gently, we will be able to see a variety of beautiful patterns (or designs) formed by the multiple reflections of coloured glass pieces.
Sunlight—White or Coloured
The sunlight is referred to as white light. The white sunlight actually consists of seven colours (mixed together). The fact that white sunlight consists of lights of seven different colours can be shown by using a glass prism as follows.
Activity 3
Take a glass prism and place it on a table in a darkened room. Place a white cardboard screen at some distance behind the prism. Allow a thin beam of sunlight (coming through a tiny hole in the window) to fall on the prism. (see Figure). We will see that the beam of white sunlight splits on entering the glass prism and forms a broad patch of seven colours (called spectrum) on the white screen placed on the other side of prism (see Figure).
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. The formation of spectrum (band of seven colours) shows that white sunlight is made up of seven colours. The seven colours of the spectrum of white light are : Violet, Indigo, Blue, Green, Yellow, Orange and Red. It is, however, usually not possible to distinguish all the seven colours of the spectrum easily due to overlapping of various colours.
Rainbow in the sky is a natural phenomenon showing the dispersion of sunlight. Rainbow is produced by the dispersion of sunlight by tiny rain drops suspended in the atmosphere (which act as tiny prisms made of water). The formation of rainbow also tells us that sunlight consists of seven colours.