Contents
- 1 How is the Image Formed by a Spherical Mirror?
- 1.1 1. Images Formed By Concave Mirror
- 1.2 (i) Formation of Image by a Concave Mirror When the Object is Far Off (or Very Distant)
- 1.3 (ii) Formation of Image by a Concave Mirror When the Object is at a Distance Greater Than Twice the Focal Length.
- 1.4 (iii) Formation of Image by Concave Mirror When the Object is at a Distance Between Focal Length and Twice the Focal Length
- 1.5 (iv) Formation of Image by a Concave Mirror When the Object is Close to Concave Mirror (at a Distance Less Than its Focal Length)
- 1.6 Uses of Concave Mirrors
- 1.7 2. Image Formed By A Convex Mirror
- 1.8 Uses of Convex Mirrors
Physics Topics can also be used to explain the behavior of complex systems, such as the stock market or the dynamics of traffic flow.
How is the Image Formed by a Spherical Mirror?
When we look into a spherical mirror (concave mirror or convex mirror), the picture which we see is called an image. Spherical mirrors form images of the objects placed in front of them. These images are formed when light rays coming from the object fall on the mirror, get reflected and converge or diverge.
Different kinds of spherical mirrors form different types of images. We will now study the images formed by concave mirrors and convex mirrors separately, one by one.
1. Images Formed By Concave Mirror
The nature and size of the image formed by a concave mirror depends on the distance of the object from the concave mirror. For very distant objects, a concave mirror produces a real and inverted image which is much smaller than the object.
As the distance of object is reduced gradually by bringing it more and more towards the concave mirror, the image remains real and inverted but its size goes on increasing. And when an object is brought very close to a concave mirror, then a virtual and erect image is formed which is larger than the object.
Thus, depending on the distance of the object, concave mirror forms mainly two different kinds of images:
- real and inverted, and
- virtual and erect.
In order to study the formation of images by a concave mirror, we will place the object (in the form of a lighted candle) at the following four distances :
- Object placed at far off distance (which is many times the focal length of concave mirror).
- Object placed at a distance greater than twice the focal length of concave mirror.
- Object placed at a distance between focal length and twice the focal length of concave mirror.
- Object placed close to concave mirror (at a distance less than its focal length).
We will now describe the formation of images by concave mirror in all these cases, one by one.
(i) Formation of Image by a Concave Mirror When the Object is Far Off (or Very Distant)
When an object is at a considerable distance from a concave mirror, we say that the object is far off (or very distant). An object becomes far off (or very distant) when the distance of object from the concave mirror is ‘many times’ its focal length.
For example, if the focal length of a concave mirror is 10 cm, then an object placed at a distance of say, 100 cm (or more) can be said to be a far off object for this concave mirror. We will now perform an activity to study the characteristics of the image formed by a concave mirror when the object is far off (or very distant) from the concave mirror.
Activity 3
Take a concave mirror of focal length 10 cm, fix it on a stand and keep it on a table. Place a lighted candle (as object) at a distance of 100 cm (or more) in front of the concave mirror to make it a far off object (or distant object) (see Figure).
Make a screen by pasting a piece of white paper on a cardboard sheet. Place the screen in front of the concave mirror in such a way that it does not obstruct the path of light rays coming from the candle flame and falling on the mirror. Move the screen towards or away from the concave mirror until you get a sharp image of the candle flame on the screen mirror (see Figure).
Since the image of candle flame can be obtained on a screen kept in front of concave mirror, therefore, the image is real. If we look at the image of candle flame, we will find that it is upside down. Thus, the image is inverted. And if we compare the size of flame in candle and its image, we will see that the image is very small. So, the image is much smaller than the object (or highly diminished).
From the above activity we conclude that : When an object is placed at a far off distance from a concave mirror, the image formed by concave mirror is : (i) real, (ii) inverted, and (iii) much smaller than the object (or highly diminished). For example, if an object is placed at a distance of 100 cm (or more) from a concave mirror of focal length 10 cm, then the image formed by concave mirror will be : (i) real, (ii) inverted, and (iii) much smaller than the object (or highly diminished).
Activity 4
The sun is a source of light which is very far off from us. We can use the sun as a far off object and obtain its image on a sheet of paper by using a concave mirror.’This can be done as follows : Take a concave mirror and hold it facing the sun so that sun’s rays fall on it (as shown in Figure).
Hold a sheet of paper in the other hand. Try to get the light rays reflected by concave mirror on the sheet of paper. Adjust the distance of paper from concave mirror until you get a sharp and bright spot of light on paper. This bright spot of light on paper is the image of the sun formed by the concave mirror (see Figure).
Hold the concave mirror and the sheet of paper in this position for a few minutes. We will find that after some time, the paper burns at the spot where the sun’s image was formed due to which a hole is formed in the paper at that spot.
The burning of paper at the spot of suns image in the above activity can be explained as follows : The parallel sun rays fall on the concave mirror (see Figure). The sun rays contain light rays-as well as heat rays. The concave mirror converges the sun’s parallel rays to a small point on paper (to form an image of the sun).
When a lot of sun’s heat rays are concentrated on a small point of paper, the paper gets heated too much at that point and burns to form a hole. Please note that the image of sun formed by concave mirror is real because it can be obtained on a sheet of paper (which is a kind of screen). Do not look directly towards the sun while performing the above activity. Its bright light may damage your eyes. You can, however, look at the image of the sun formed on paper.
(ii) Formation of Image by a Concave Mirror When the Object is at a Distance Greater Than Twice the Focal Length.
Suppose the focal length of a concave mirror is 10 cm. Twice (or double) of this focal length will be 10 cm × 2 = 20 cm. Now, by saying that the object is at a distance greater than twice the focal length of this concave mirror, we mean that the object is at a distance greater than 20 cm.
For example, the object may be at a distance of say, 25 cm, 30 cm, etc. We will now perform an activity to study the charateristics of the image formed by a concave mirror when the object is placed at a distance greater than twice the focal length.
Activity 5
Take a concave mirror of focal length 10 cm, fix it on a stand and keep it on a table. Place a lighted candle (as object) at a distance greater than twice the focal length, say at 25 cm from the concave mirror (see Figure). Now, place a screen in front of the concave mirror in such a way that it does not obstruct the path of light rays coming from the candle flame and falling on the mirror.
Move the screen towards or away from the concave mirror until you get a sharp image of the candle flame on the screen (see Figure). Since the image of candle flame has been formed on a screen, therefore, it is a real image. If we look at the image, it is found to be upside down.
So, the image is inverted. And if we compare the size of flame in candle and its image, we can see that the image is smaller in size. Thus, the image is smaller than the object (or diminished).
From the above activity we conclude that : When an object is placed at a distance greater than twice the focal length of a concave mirror, the image formed by concave mirror is : (i) real, (ii) inverted, and (iii) smaller than the object (or diminished).
For example, if an object is placed at a distance of 25 cm from a concave mirror of 10 cm focal length, then the image formed by concave mirror will be : (i) real, (ii) inverted, and (iii) smaller than the object (or diminished).
(iii) Formation of Image by Concave Mirror When the Object is at a Distance Between Focal Length and Twice the Focal Length
Suppose the focal length of a concave mirror is 10 cm. Twice (or double) of this focal length will be 10 cm × 2 = 20 cm. Now, by saying that an object is placed at a distance between focal length and twice the focal length, we mean that the object is placed between a distance of 10 cm and 20 cm from concave mirror.
For example, the object may be at a distance of say, 15 cm from the concave mirror (which is more than focal length but less than twice the focal length). We will now study the characteristics of the image formed by a concave mirror when the object is placed at a distance between focal length and twice the focal length.
Activity 6
Take a concave mirror of focal length 10 cm, fix it on a stand and keep it on a table. Place a lighted candle (as object) at a distance between focal length and twice the focal length, say at 15 cm from the concave mirror (see Figure).
Now, place a screen in front of the concave mirror in such a way that it does not obstruct the path of light rays coming from the candle flame and falling on the mirror. Move the screen towards or away from the concave mirror until you get a sharp image of the candle flame on the screen (see Figure).
Since the image of candle flame can be received on the screen, therefore, the image is real. If we look at the image, we will observe that the image is upside down. So, the image is inverted. And if we compare the size of the flame in candle and its image, the image appears to be larger. Thus, the image is larger than the object (enlarged or magnified).
From the above acitivity we conclude that : When an object is placed at a distance between focal length and twice the focal length of a concave mirror, then the image formed by concave mirror is : (i) real, (ii) inverted, and (iii) larger than the object (or magnified).
For example, when an object is placed at a distance of 15 cm from a concave mirror of focal length 10 cm, then the image formed by concave mirror is : (0 real, (ii) inverted, and (iii) larger than the object (or magnified).
The inner surface (or front side) of a shining steel spoon acts like a concave mirror. So, if we look into the inner side of a shining steel spoon, we will see a real and inverted image of our face.
(iv) Formation of Image by a Concave Mirror When the Object is Close to Concave Mirror (at a Distance Less Than its Focal Length)
When we say that the object is close to concave mirror, we mean that the object is at a distance less than the focal length of concave mirror. Suppose the focal length of a concave mirror is 10 cm.
Now, if we place an object in front of this concave mirror at a distance less than 10 cm (say, at 8 cm or 7 cm, etc.) then the object is said to be close to the concave mirror. We will now perform an activity to study the characteristics of the image formed when an object is placed close to a concave mirror (at a distance less than its focal length).
Activity 7
Take a concave mirror of focal length 10 cm, fix it on a stand and keep it on a table. Place a lighted candle (as object) close to the concave mirror at a distance less than its focal length, say at 8 cm, in front of concave mirror (see Figure). Now, place a screen in front of concave mirror in such a way that it does not obstruct the path of light rays coming from the candle flame and falling on the concave mirror.
Move the screen backwards and forwards, and try to obtain the image of candle flame on the screen. We will find that in this case we cannot obtain the image of candle flame on the screen (see Figure). Thus, when the candle is close to the concave mirror, it is not possible to obtain the image of its flame on the screen. Now, look into the concave mirror directly. We will see a large image of the candle and its flame in the concave mirror (as shown in Figure).
Since the image can be seen only by looking into the concave mirror and cannot be obtained on the screen, therefore, the image is virtual. If we look at the image in the concave mirror, we find it to be same side up as the candle, so the image is erect.
And if we compare the size of candle and its image, we find that the image is larger than the candle. Thus, the image is larger than the object (enlarged or magnified).
From the above activity we conclude that : When an object is placed close to a concave mirror (at a distance less than its focal length), the image formed by concave mirror is : (i) virtual, (ii) erect, and (iii) larger than the object (enlarged or magnified).
For example, when an object is placed at a distance of 8 cm from a concave mirror of focal length 10 cm, then the image formed by concave mirror is : (i) virtual, (ii) erect, and (iii) larger than the object (enlarged or magnified).
We know that the inner side (or front side) of a shining steel spoon acts like a concave mirror. If we look at our face in the inner side of a shining steel spoon by holding it very close to our face, we will see an erect and magnified image of our face in it.
Figure also shows the use of a concave mirror as a shaving mirror (or make-up mirror). To use a concave mirror as a shaving mirror (or make-up mirror), a person holds his face close to the concave mirror. In this position, a virtual, erect and magnified image of the face is seen in the concave mirror.
From all the discussion which we have had so far, we conclude that depending upon the distance at which an object is placed, the image formed by a concave mirror can be ‘real and inverted’ or ‘virtual and erect’. The image formed by a concave mirror can also be smaller or larger in size than the object.
Uses of Concave Mirrors
1. Concave mirrors are used as shaving mirrors to see a large image of the face. When the face is held close to a concave mirror, then an enlarged image of the face is seen in the concave mirror (This helps a man in making a smooth shave). For the same reason, concave mirrors are also used as make-up mirrors by women for putting on make-up (such as painting eye-lashes) [see Figure (a)].
2. Concave mirrors are used by dentists to see the large images of teeth of patients. When a tooth is close to a concave mirror, then an enlarged image of the tooth is seen in the concave mirror. Due to this, it becomes easier to locate the defect in the tooth. The concave mirrors used by dentists are very small in size. They are fitted in a frame with a long handle [see Figure (b )].
2. Image Formed By A Convex Mirror
In the case of a concave mirror we have studied that the nature of image formed depends on the distance of the object from concave mirror. But this is not so in the case of a convex mirror. Whether an object is placed at a large distance or it is close to convex mirror, a convex mirror always forms an image which is virtual, erect and smaller than the object.
Changing the position of object in front of a convex mirror only changes the size of image. But in every case, the image remains smaller than the object. Please note that like a plane mirror, a convex mirror always forms a virtual and erect image of an object.
The difference is that in a plane mirror the image is of the same size as the object but in a convex mirror the image is always smaller than the object. We will now perform an activity to study the characteristics of the image formed by a convex mirror when the object is placed at any distance in front of it.
Activity 8
Take a convex mirror of 10 cm focal length (or any other focal length), fix it on a stand and keep it on a table. Place a lighted candle (as object) at any distance (say, 15 cm) from the convex mirror (see Figure). Now, place a screen ¡n front of the convex mirror in such a way that it does not obstruct the path of light rays coming from the candle flame and falling on the convex mirror.
Move the screen towards or away from the convex mirror and try to obtain the image of candle flame on the screen. We will find that in this case, we cannot obtain the image of candle flame on the screen (see Figure). Now, look into the convex mirror directly. We will see a small image of the candle and its flame in the convex mirror (as shown in Figure).
Since the image of candle and its flame can be seen only by looking into the convex mirror, and cannot be obtained on a screen, it is a virtual image. If we look at the image in the convex mirror, we will find that it is same side up as the candle; So, the image is erect. And if we compare the size of the candle and its image, the image appears to be smaller.
Thus, the image is smaller in size than the object (or diminished). Even if we change the distance of candle (object) from the convex mirror we will find that ¡n every case the image of candle formed by the convex mirror remains virtual, erect and smaller in size than the candle.
From this activity we conclude that : Whatever be the distance of object from a convex mirror, the image formed by a convex mirror is always : (i) virtual, (ii) erect, and (iii) smaller than the object (or diminished).
We can never get a real and inverted image with a convex mirror. We can also not obtain an image larger than the object (or magnified image) with a convex mirror.
The outer surface (or back side) of a shining steel spoon acts like a convex mirror. So, if we look into the outer side (or back side) of a shining steel spoon, we will see a virtual, erect and smaller image of our face! The shining top surface of a bicycle bell is also a kind of convex mirror. So, if we look into a bicycle bell, we will find that our image is virutal, erect and smaller in size.
Uses of Convex Mirrors
1. Convex mirrors are used as rear view mirrors or side view mirrors in vehicles (such as cars, scooters, buses and trucks, etc.) to see the traffic at the rear side (back side) on the road (see Figure).
Convex mirrors are used as rear view mirrors (or side view mirrors) because they form erect and very small images of the things due to which we can see a large number of things spread over a large area on the road behind us (even in a small convex mirror). A convex mirror gives a wider field of view than a plane mirror of the same size. So, it is better to use a convex mirror as a rear view mirror than a plane mirror.
2. Big convex mirrors are used as ‘shop security mirrors’. By installing a big convex mirror in the shop, the shop owner can keep an eye on the customers to look for thieves and shoplifters.