Contents
Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
The Phenomenon in which Light Travels in Straight Lines
Light is a form of energy. Light is needed to see things around us. Light enables us to see objects from which it comes or from which it is reflected. We detect light with our eyes.
If we shine a torch on a dark night, we will see that the beam of light produced by torch travels straight into darkness (see Figure). This observation shows that light travels in straight lines.
Similarly, the beams of light from the headlights of cars, scooters, buses, trucks, aeroplanes and engines of trains also go straight in the darkness of night Figure. The beam of light produced by showing that light travels in a straight line path.
The straight beams of light coming out from the searchlights of airport tower (or from the lighthouse in the sea) also tell us that light travels in a straight line. When the air in a room is dusty, we can see a straight beam of sunlight entering the room through a narrow opening or hole. This observation also shows that light travels in straight line path.
If we look at a lighted candle through a straight pipe with one eye (keeping the other eye closed), we can see the light of candle flame. On the other hand, if we look at the lighted candle through a bent pipe in a similar way, we are not able to see the light of candle flame.
This means that the light of candle flame can travel through the straight hole of a straight pipe but not through the bent hole of a bent pipe. The observation that we can see the light of candle flame through a straight pipe (but not through a bent pipe) shows that light travels in a straight line path. If, however, light could bend and travel in curved lines, then we could see the light of candle flame even through a bent pipe. But this does not happen.
From the above discussion we conclude that light travels in a straight line path. The path of light changes when it falls on a mirror (or any other polished and shining surface). Thus, a mirror changes the direction of light that falls on it. This change in direction of light by a mirror is called reflection of light. After reflection also, the light continues to travel in a straight line path. We will now discuss the reflection of light from a plane mirror in somewhat detail.
Reflection of Light
When light falls on the surface of an object, the object sends the light back. The process of sending back the rays of light which fall on the surface of an object is called reflection of light. The ray of light which falls on an object (say, a mirror) is called incident ray. And the ray of light which is sent back by an object (like a mirror) is called reflected ray. The reflection of light from a plane mirror has been shown in Figure.
In Figure , three parallel rays of light called incident rays, are falling on a plane mirror at an angle to its surface. The plane mirror reflects the three parallel rays of light and sends them in another direction. The three reflected rays of light are also parallel to one another and they make the same angle with the mirror surface as the incident light rays. We can perform an activity to show the reflection of light with a plane mirror as follows.
Activity 1
Take a torch. It is a source of light. Take a black chart paper and make three slits in it (A long and narrow cut is called a slit). Cover the glass of torch with this black chart paper having three slits in it. The three slits have been made in black chart paper for obtaining three rays of light (or three narrow beams of light) when the torch is switched on. Spread a sheet of another chart paper on a drawing board. Fix a rectangular plane mirror strip vertically on this chart paper (keeping the longer side of mirror strip on the chart paper) (see above Figure).
Now, switch on the torch. This will produce three rays of light. Place the torch in such a way that its light rays are seen along the chart paper fixed on the drawing board. Adjust the position of torch in such a way that the three rays of light coming from the three slits fall on the plane mirror at an angle to its surface (see Figure).
We will see that the plane mirror changes the direction of the three rays of light falling on it and makes them go in another direction (making same angle with its surface as the original rays of light (see above Figure). In other words, the plane mirror reflects the three rays of light falling on it. This activity shows how light gets reflected from a plane mirror.
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. In fact, any polished and shining surface can act as a mirror (by reflecting most of the light). For example, a shining stainless steel plate and a shining stainless steel spoon can act as mirrors.
This is why we can see the image of our face in a shining stainless steel plate and spoon. The smooth surface of still water can also act as a mirror. We can usually see the images (or reflections) of trees and buildings, etc., in the still water of lakes and ponds (see Figure). This is because the still surface of water is very smooth and it behaves like a mirror in forming the images of objects.
Most of the objects around us have unpolished and dull surfaces due to which they reflect only a small amount of light falling on them (the remaining light is absorbed by them). For example, a table or chair having an unpolished and dull surface reflects only a small amount of light falling on it.
In fact, we can see most of the objects which are around us by the light reflected from their surfaces. We can see the objects around us because they reflect light received from a luminous object (such as the sun, electric bulb, tube light or torch, etc.) into our eyes.
For example, we can see a table lying in a room because it reflects light falling on it (from the sun or bulb, etc.) in all the directions. And when this reflected light coming from the table enters our eyes, we are able to see the table. To us it appears as if the light is coming from the table itself. Thus, the objects are visible to us because light reflected by them reaches our eyes. The paper of the page of this book which we are reading is also reflecting light. This is why we are able to see it.
The polished, shiny and smooth surfaces (like that of mirrors, metal objects like stainless steel and still water) reflect light in a regular way, so they form images of the objects kept in front of them. On the other hand, the unpolished, dull and rough surfaces (like that of paper, table and wall) reflect light in an irregular way due to which they do not form images of objects kept in front of them.
We can see the image of our face in a mirror (or in any other shining object) because the smooth and shining surface of mirror produces regular reflection of light. We cannot see the image of our face in a sheet of paper because the rough and dull surface of paper produces irregular reflection of light.
Image of an Object
When we look into a mirror, 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 (see Figure). 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 (or picture) 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 image (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.
A convex lens can also form real images. We will study the formation of real images by concave mirror and convex lens later on in this Chapter. Please note that a plane mirror, a convex mirror and a concave lens can never form real images.
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. Similarly, a concave lens also forms only virtual images. A concave mirror and a convex lens can form a virtual image only when the object is very close to them. We will study the formation of virtual images by various types of mirrors and lenses later on in this Chapter.
To Study the Characteristics of Image Formed by a Plane Mirror
Place a lighted candle in front of a plane mirror (see Figure). The candle is the object in this case. When we look into the plane mirror, it appears as if a similar candle is placed behind the mirror (see Figure). The candle which appears to be behind the mirror is the image of the candle formed by the plane mirror (see Figure).
This image is formed as follows : Light rays from the candle travel in straight lines to the mirror. The mirror reflects (sends back) these rays of light. Some of the reflected light is entering into our eyes. To us, the light seems to be coming from behind the mirror. So, we can see the image of candle in that position.
The image of candle seen in the plane mirror is virtual. Our brain thinks that there is an image behind the mirror but the candle is not really there. The image of candle seen in the plane mirror is just an illusion. We will now describe the various characteristics of the images formed in a plane mirror by taking the example of the image of candle (formed in Figure).
1. When we look into the mirror, the image of candle appears to be behind the mirror (see Figure). In general we can say that: The image formed in a plane mirror is behind the mirror.
2. Place a vertical screen behind the plane mirror (where the image of candle appears to be situated). We will find that the image of candle (or its flame) cannot be obtained on the screen. Even if the screen is placed in front of the plane mirror, the image of candle (or its flame) cannot be obtained on the screen.
The image of candle in Figure can be seen only by looking into the mirror. Since the image of candle formed in the plane mirror cannot be obtained on a screen, it means that the image of candle in the plane mirror is a virtual image. In general we can say that : The image formed in a plane mirror is virtual. The image formed in a plane mirror cannot be received on a screen.
3. If we look at Figure, we find that the length and breadth of the image of the candle and its flame is the same as that of the original candle and its flame. We say that the image of candle in the plane mirror is of the same size as the original candle. The image is neither smaller nor bigger than the candle itself.
In general we can say that: The image in a plane mirror is of the same size as the object. It is neither smaller nor bigger than the object. If we look into a plane mirror, we find that the image of our face is exactly of the same size as our face (irrespective of the fact whether the mirror is small or big). This is because a plane mirror always forms an image which is exactly equal in size to the size of the object. The size of image formed in a plane mirror does not depend on the size of the mirror.
4. If we look at Figure, we find that the candle has a flame at the top and the image of candle also has a flame at the top. So, the top of candle remains the top in image. In the same way, the bottom of candle remains the botton in image. This means that the top and bottom of the image of candle in the plane mirror are the same as the top and bottom of original candle (which is the object).
Such an image is called an erect image (or upright image). Thus, the image of candle in the plane mirror is erect (or upright). 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. When we see our full body image in a big plane mirror, we find that just like us, our image also has head at the top and feet at the bottom. So, our image in the plane mirror is said to be erect (or upright).
5. Take a graph paper marked in centimetre squares. Fix the graph paper on a drawing hoard. Draw a thick line in the middle of graph paper. Place a thin rectangular strip of plane mirror vertically on this line (see Figure). Now place an iron nail on the graph paper at a distance of 3 centimetres from the mirror (by counting 3 squares of graph paper starting from mirror) (see Figure).
Note the position of image of this nail on the image of graph paper as seen in the mirror. We will find that the position of image of nail is at a distance of 3 centimetres behind the plane mirror (as indicated by counting 3 squares of graph paper from the mirror up to the image) (see Figure).
In this case, the nail (which is the object here) has been placed at a distance of 3 centimetres from the plane mirror (in the front) and its image is formed at a distance of 3 centimetres behind the mirror. This means that the image of nail is at the same distance behind the mirror as the nail is in front of the mirror. In general we can say that : The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.
We can verify this by moving in front of a plane mirror. If we move towards the mirror, our image also moves towards the mirror; and if we move away from the mirror, our image also moves away from the mirror. In every case, the distance of our image from mirror is equal to our own distance from the mirror.
Thus, the distance of image from the mirror is equal to the distance of object from the mirror. But the object and image are on the opposite sides of the mirror. If a person is standing at a distance of 4 m from a plane mirror then his image will also be formed at a distance of 4 m behind the mirror. And the total distance between the person and his image will be 4 m + 4 m = 8 m. This means that the person will be at a distance of 8 m from his image in the plane mirror.
6. 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 (see Figure).
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. We can now define lateral inversion as follows:
When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image; and the left side of object appears to become the right side of image. This change of sides of an ‘object’ and its ‘mirror image’ is called lateral inversion. The phenomenon of lateral inversion will become clear from the following example.
Suppose we have a placard having the word RED written on it [see Figure (a)], When we hold this placard in front of a plane mirror, the image of word RED appears to be like ⱭƎЯ [see Figure (b)]. Please note that the object (placard) has the letter R on its left side but image has this letter in reversed form Я on its right side.
Similarly, the image of letter E appears to be reversed like Ǝ. And the letter D on the right side of the object (placard) is on the left side of the image in the reversed form as Ɑ. Thus, all the letters written on the placard are reversed from left to right. We say that the image is laterally inverted. This is an example of lateral inversion.
The word AMBULANCE on the hospital vans is written in the form of its mirror image as 
(see Figure). This is because when we are driving in our car and see the hospital van coming from behind in our rear-view mirror, then we will get the laterally inverted image of and read it as AMBULANCE.
Since an ambulance carries seriously ill patients, we can make way for it to pass through so that it can reach the hospital quickly. The letters of English alphabet in which the image formed in a plane mirror appears exactly like the letters themselves are : A, H, I, M, O, T, U, V, W, X and Y. 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. In general we can say that : The image formed in a plane mirror is laterally inverted (or sideways reversed) with respect to the object.
We can now write down all the characteristics of the images formed by a plane mirror as follows :
- The image formed in a plane mirror is behind the mirror.
- The image formed in a plane mirror is virtual (or unreal). It cannot be obtained on a screen.
- 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 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 laterally inverted (sideways reversed) with respect to the object.
We will now solve some problems based on the formation of images by a plane mirror.
Example Problem 1.
David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be:
(a) 3 m
(b) 5 m
(c) 6 m
(d) 8 m
Solution:
(i) In this problem, David is the object. Initially, the distance between the mirror and David’s image is 4 m. Since the distance of object from plane mirror is equal to distance of image from plane mirror, therefore, the distance of David (which is object), from the plane mirror is also 4m (see Figure).
(ii) David moves 1 m towards the mirror. This means that the distance of David from mirror will be 4 – 1 = 3 m. Since the image is formed at the same distance behind the mirror as the object is in front of it, therefore, the distance of image from mirror in this case will also be 3 m (see Figure)
Now, Distance of David from mirror = 3 m (in front)
And, Distance of image from mirror = 3 m (at the back)
So, Distance between David and his image = 3 m + 3 m
= 6 m
Thus, the correct option is: (c) 6 m.
Example Problem 2.
The rear view mirror of a car is..a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror, the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be:
(a) 1 m/s
(b) 2 m/s
(c) 4 m/s
(d) 8 m/s
Solution:
In a plane mirror, the object and its image always remain at the same distance from the mirror. So, when the car reverses at a speed of 2 m/s, then the image will also appear to move towards the mirror at the same speed of 2 m/s. So, the speed at which the image of truck appears to approach the car driver will be 2 m/s + 2 m/s = 4 m/s. Thus, the correct option will be : (c) 4 m/s.