Contents
Physics Topics can be challenging to grasp, but the rewards for understanding them are immense.
Nature and Characteristics of Image Formed by a Plane Mirror
When we look into a mirror held in our hand, we see our face. What we see in the mirror is actually a ‘reflection’ of our face and it is called ‘image’ of our face. Thus, when we look into a mirror, we see the image of our face in it. In this case, our face is the ‘object’ and what we see in the mirror is its ‘image’. Actually, when light from our face falls on the mirror, the mirror reflects this light.
The reflected light seems to come out of the mirror and we see an image of our face in the mirror. The image of our face seen in the mirror is formed where light rays, after reflection from the mirror, appear to originate from. The image of our face appears to be situated behind the mirror. There are two types of images that can be formed with light: real images and virtual images. These are discussed below.
1. Real Images
The image which can be obtained on a screen is called a real image. In a cinema hall, we see the images of actors and actresses on the screen. So, the image formed on a cinema screen is an example of real images (see Figure).
The image formed in a pinhole camera is also real image (because it can be received on a screen). A real image is formed when light rays coming from an object actually meet at a point after reflection from the mirror. A real image can be formed on a screen because light rays actually pass through it. Real images can be formed by a curved mirror known as concave mirror. We will study this in detail in higher classes.
2. Virtual Images
The image which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror. The image of our body formed by a plane mirror cannot be received on a screen, it can be seen only by looking into the mirror. So, our image in a plane mirror is an example of virtual image (see Figure).
If we put a screen behind the plane mirror, we will not see anything on it. There are no light rays behind the mirror. The image is not really there at all. The virtual image is just an illusion. So, virtual images are also called unreal images.
A virtual image is formed when light rays coming from an object appear to meet after reflection from the mirror. It is not possible to obtain a virtual image on the screen because light rays actually do not pass through it. A plane mirror (ordinary looking mirror) always forms virtual images.
Highly polished metal objects also form virtual images. Sometimes we see the reflections of trees, buildings and other objects in the still water of a lake or pond. These reflections of objects in water are virtual images. A type of curved mirror known as convex mirror also forms only virtual images. And a concave mirror can form a virtual image only when the object is very close to it.
The virtual image of ourselves which we see when we look into a plane mirror is the same side up as we are (it is called an erect image); the image is of the same size as we are; and the image is at the same distance behind the mirror as we are in front of it. The main difference between us and our image in the mirror is that it has been “reversed sideways.” For example, our right arm appears to be the left arm of our mirror image (see Figure). Let us discuss this in a little more detail.
If we stand in front of a big plane mirror, we see the image of our body in it. Though our image appears to be just as we are, but there is a difference. This is because if we lift our right hand, then our image lifts its left hand. And if we lift our left hand, then the image appears to lift its right hand.
This means that the right side of our body becomes the left side in the image; whereas the left side of our body becomes right side in the image. It appears as if our image has been ‘reversed sideways’ with respect to our body. This effect of reversing the sides of an object and its image is called lateral inversion. And we say that the image formed in a plane mirror is laterally inverted.
In other words, the image formed in a plane mirror is ‘sideways reversed’ with respect to the object. It is due to lateral inversion that the image of our right hand appears to be our left hand. So, when we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand. And it is also due to lateral inversion that the parting in our hair on the right appears to be on the left when seen in a mirror.
We will now study the characteristics of images formed by a plane mirror with the help of a ray diagram. Before we do that please remember that though an object gives out a large number of light rays but we will use only two incident rays coming from the object to show the formation of image.
This is done just to keep the ray-diagram simple. Another point to be noted is that the real rays of light are represented by full lines (solid lines) whereas virtual rays of light are represented by dotted lines (broken lines). The real rays of light can exist only in front of a mirror. The virtual rays of light are those which we show behind a mirror.
The virtual rays do not exist at all (because light cannot reach behind the mirror). They just appear to be coming from behind the mirror. A yet another point to be noted is that an image is formed in a plane mirror where at least two light rays coming from an object appear to meet after reflection from the plane mirror.
To Study the Characteristics of Image formed by a Plane Mirror
We will now describe the various characteristics (or properties) of the images formed in plane mirrors by taking an example. Look at Figure in which a bulb (with shade) is hanging from the ceiling in front of a plane mirror.
The light rays coming from the bulb (called incident rays) travel in straight lines to the mirror (see Figure). The mirror reflects these rays of light. The reflected light rays enter into our eyes. To us, the light rays seem to be coming from a point behind the mirror (as shown by dotted lines).
So, we see an image of the bulb in that position.The image of bulb seen in the mirror is a virtual image. Our brain thinks that there is an image behind the mirror, but the bulb is not really there. We will now discuss the various characteristics of the image formed in a plane mirror by taking the example of the image of the bulb formed in Figure.
1. The image of bulb in Figure can be seen only by looking into the mirror. The image of light bulb formed in the plane mirror cannot be obtained on a screen. This means that the image of bulb in the plane mirror is a virtual image. In general we can say that: The image formed in a plane mirror is virtual. Thus, the image formed in a plane mirror cannot be received on a screen placed behind the plane mirror.
2. If we look at Figure, we find that the distance of image of bulb from the mirror is equal to the distance of bulb from the mirror. This means that the image of bulb is the same distance behind the mirror as the bulb itself is in front of the mirror.
For example, if the bulb is at a distance of 50 centimetres in front of the mirror, then its image will be at a distance of 50 centimetres behind the mirror. In general we can say that: The image formed in a plane mirror is the same distance behind the mirror as the object is in front of the mirror. But the object and the image are on the opposite sides of the mirror.
3. If we look at Figure, we find that the length and breadth of the image of the bulb and its shade is the same as that of the original bulb and its shade. We say that the image of bulb in the plane mirror is of the same size as the original bulb. The image is neither smaller nor bigger than the bulb. In general we can say that: The image in a plane mirror is of the same size as the object. It is neither diminished nor enlarged.
4. If we look at Figure, we find that the top of bulb remains top in image and the bottom of bulb remains bottom in the image. This means that the top and bottom of the image of bulb is the same as the top and bottom of the original bulb. Such an image is called an erect image.
Thus, the image of the bulb in the mirror is erect. In general we can say that: The image formed in a plane mirror is erect (or upright). It is the same side up as the object. For example, when we see our full image in a big mirror, we find that just like us, our image in the mirror has also head at the top and feet at the bottom. So, our image in the plane mirror is said to be erect (or upright).
5. If we look at Figure once again, we will see that the shade of bulb has a stamp on it on the right side. But the image has this stamp on the left side. This means that the right side of the bulb becomes the left side of its image in the plane mirror. Thus, there is a change of sides of the bulb and its image in the plane mirror.
This change of sides is called lateral inversion. So, the image of bulb in the plane mirror is laterally inverted. In general we can say that: The image in a plane mirror is laterally inverted (or sideways reversed) with respect to the object. We are now in a position to give all the characteristics of the image formed in a plane mirror.
Characteristics of Image Formed by a Plane Mirror
- The image formed in a plane mirror is virtual (or unreal). It cannot be obtained on a screen.
- The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of it.
- The image in a plane mirror is of the same size as the object. It is neither enlarged nor diminished.
- The image in a plane mirror is erect. It is the same side up as the object.
- The image in a plane mirror is laterally inverted (sideways reversed) with respect to the object.
Uses of Plane Mirrors
- Plane mirrors are used at home to view ourselves.
- Plane mirrors are used in making periscopes.
- Plane mirrors are fixed on the walls of certain shops (like jewellery shops) to make the shops look bigger.
Periscope
The periscope is a device which gives us a higher view than normal. By using a periscope, we can see the objects on the other side of a high wall, without having to climb over the wall. A periscope can also be used to see over the heads of crowd (say, in a foot-ball match).
A soldier sitting in a trench can observe the enemy activities above the ground by using a periscope. And by using a periscope, a person sitting in a submarine can see ships over the surface of the sea, even though the submarine itself may be submerged under water. A periscope is made by using two plane mirrors. We will now describe the construction and working of a simple periscope.
A periscope consists of a long tube T having two plane mirrors M1 and M2 fitted at its two ends (see Figure). The two plane mirrors are arranged 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. A periscope works on the reflection of light from two plane mirrors arranged parallel to one another. The working of a periscope will become clear from the following example: In Figure, there is a tree behind a high wall which we cannot see directly.
We can, however, see this tree by using a periscope as follows: The upper hole of periscope is turned towards the object to be seen (here a tree) so that mirror M1 faces the object. And 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 towards the mirror M2 (see Figure). The mirror M2 then reflects the light towards the eye of the person looking into periscope through the lower hole. Since the light rays coming from the tree enter the eye, it is possible to see the image of the tree in the lower mirror M2 (even though the tree cannot be seen directly).