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Physics Topics are also essential for space exploration, allowing scientists to study phenomena such as gravitational waves and cosmic rays.
Formation of Image By Lenses at the Four Distances and Its Uses
When we look through a lens, the picture which we see is called an image. Lenses form images of the objects placed in front of them. These images are formed when light rays coming from the object pass through the lens and converge or diverge. Different kinds of lenses form different kinds of images. We will now study the images formed by convex lens and concave lens separately, one by one.
1. Images Formed By Convex Lens
The nature and size of the image formed by a convex lens depends on the distance of the object from the convex lens. For very distant objects, a convex lens 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 convex lens, the image remains real and inverted, but its size goes on increasing.
And when an object is brought very close to a convex lens, then a virtual and erect image is formed which is larger than the object. Thus, depending on the distance of object, convex lens forms mainly two different kinds of images : (i) real and inverted, and (ii) virtual and erect.
In order to study the formation of images by a convex lens, 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 convex lens).
- Object placed at a distance greater than twice the focal length of convex lens.
- Object placed at a distance between focal length and twice the focal length of convex lens.
- Object placed close to convex lens (at a distance less than its focal length).
We will now describe the formation of images by convex lens in all these cases, one by one. Please note that the formation of images in a convex lens is very similar to the formation of images in a concave mirror (which we have already studied). This is because like a concave mirror, a convex lens also converges rays of light.
(i) Formation of Image by a Convex Lens When the Object is Far Off (or Very Distant)
Activity 9
Take a convex lens 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 about 100 cm (or more) so as to make it a far off object (see Figure). Now, place a screen at some distance behind the convex lens. Move the screen towards or away from the convex lens until you get a sharp image of the candle flame on the screen (see Figure).
Since the image of candle flame can be obtained on a screen kept behind the convex lens, therefore, the image is real. If we look at the image of the flame, we will observe that the image is upside down. So, the image is inverted. And if we compare the size of flame in the candle and its image, we will find that the image is much smaller. Thus, the image is much smaller than the object (or the image is highly diminished).
From the above activity we conclude that: When an object is placed at a far off distance from a convex lens, the image formed by convex lens 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 convex lens of focal length 10 cm, then the image formed by convex lens will be : (i) real, (ii) inverted, and (iii) much smaller than the object (or highly diminished).
Activity 10
The sun is a source of light which is very far off from us. We can use the sun as object and obtain its image on a screen by using a convex lens (just as we did in the case of concave mirror). This can be done as follows : Take a convex lens. Hold the convex lens in one hand facing the sun so that sun’s rays fall on it (as shown in Figure).
Hold a sheet of paper in the other hand and keep it some distance behind the convex lens (to act as a screen). Adjust the distance between the convex lens and sheet of paper till a sharp and bright spot of light is obtained on the sheet of paper. This bright spot of light on paper is the image of the sun formed by convex lens (see Figure).
Hold the convex lens 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 image of sun was formed due to which a hole is formed in the paper at that spot.
The burning of paper at the spot of sun’s image in the above activity can be explained as follows : The parallel sun rays fall on the convex lens (see Figure). The sun rays contain light rays as well as heat rays. The convex lens converges parallel sun rays to a small point on paper (to form the image of 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 convex lens is real because it can be received on paper (which is a kind of screen).
Do not look at the sun (or any other bright light) through a convex lens. It is very dangerous to do so. This is because looking at the sun (or other bright light) through a convex lens can damage our eyes by focussing too much light energy into the eyes.
(ii) Formation of Image by a Convex Lens When the Object is at a Distance Greater Than Twice the Focal Length
Activity 11
Take a convex lens 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 convex lens (see Figure). Now, place a screen at some distance behind the convex lens. Move the screen towards or away from the convex lens until you get a sharp image of the candle flame on the screen (see Figure).
Since the image of candle flame can be obtained on the screen, so the image is real. If we look at the image of flame we will find that the image is upside down. This means that the image is inverted. And if we compare the size of flame in the candle and its image, we will find that the size of flame in the image is smaller. Thus, the image is smaller in size than the object. We say that the image is diminished.
From the above discussion we conclude that : When an object is placed at a distance greater than twice the focal length of convex lens, the image formed by convex lens 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 convex lens of 10 cm focal length, then the image formed by convex lens will be : (i) real, (ii) inverted, and (iii) smaller than the object (or diminished).
(iii) Formation of Image by Convex Lens When the Object is at a Distance Between Focal Length and Twice the Focal Length.
Activity 12
Take a convex lens 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 convex lens (because 15 cm is more than focal length but less than twice the focal length) (see Figure). Now, place a screen at some distance behind the convex lens. Move the screen towards or away from the convex lens until you get a sharp image of the candle flame on the screen (see Figure).
Since the image of candle flame can be obtained on a screen, so the image is real. If we look at the image of flame, we find that it is upside down. This means that the image is inverted. And if we compare the size of flame in the candle and its image, we see that the size of flame in image is larger (or bigger) than that in candle. Thus, the image is larger than the object. We say that the image is enlarged or magnified.
From the above activity we conclude that : When an object is placed at a distance between focal length and twice the focal length of a convex lens, then the image formed by convex lens is : (i) real, (ii) inverted, and (iii) larger than the object (enlarged or magnified).
For example, when an object is placed at a distance of 15 cm from a convex lens of focal length 10 cm, then the image formed by convex lens is : (i) real, (ii) inverted, and (iii) larger than the object (enlarged or magnified).
(iv) Formation of Image by a Convex Lens When the Object is Close to Convex Lens (at a distance Less than its Focal Length)
Activity 13
Take a convex lens 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 convex lens at a distance less than its focal length, say at 8 cm in front of convex lens (see Figure). Now, place a screen at some distance behind the convex lens.
Move the screen backward and forward, 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 (or object) is close to convex lens, it is not possible to obtain its image on the screen. Now, look through the convex lens by keeping your eye on the back side of the convex lens. We will see a large image of the candle and its flame through the convex lens (as shown in Figure).
Since the image can be seen only by looking through the convex lens and cannot be obtained on the screen, therefore, the image ¡s virtual. If we look at the image, 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. We say that the image is enlarged or magnified.
From the above activity we conclude that: When an object is placed close to a convex lens (at a distance less than its focal length), the image formed by convex lens 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 convex lens of focal length 10 cm, then the image formed by convex lens will be : (i) virtual, (ii) erect, and (iii) larger than the object (enlarged or magnified).
Magnifying Glass
The word ‘magnify means ‘to make something look bigger (or larger) than it actually is. We have just studied that a convex lens gives a magnified image of a close object. Used in this way, a convex lens is commonly known as a magnifying glass.
Thus, a magnifying glass is a convex lens which is used to make small things look bigger (by producing their magnified images) (see Figure). To use a convex lens as a magnifying glass, the object to be viewed should be placed close to the convex lens so that a magnified and erect image of the small object can be seen on looking through the convex lens.
In other words, to use a convex lens as a magnifying glass, the object should be at a distance less than the focal length of convex lens. For example, if the focal length of a convex lens is 10 centimetres, then the object to be magnified with it should be at a distance of less than 10 centimetres. It may be at a distance of say, 8 cm or 5 cm, etc. If we look at the page of a book through this convex lens (by keeping the page at less than 10 centimetres), we will see that the words printed on the page of the book appear much bigger in size than they actually are (see Figure).
This is because the convex lens produces a magnified and erect image of the printed words of the book. In this case the small printed words of the book are the objects and the magnified words which we see through the convex lens, are the images. The convex lens which is used as a magnifying glass is usually fitted in a metal frame having a handle so that it can be used easily.
A magnifying glass is used to read very small print of a book conveniently. A magnifying glass is also used in a science laboratory to observe the various body parts of small animals such as a cockroach or an earthworm clearly. A watch maker uses a magnifying glass to see the small parts inside a watch clearly. A magnifying glass is also used by palmists to see the details of the lines of our hand.
Uses of Convex Lenses
- Convex lenses are used as magnifying glasses.
- Convex lenses are used in making spectacles, cameras, microscopes, telescopes and binoculars, etc.
2. Image Formed By A Concave Lens
In the case of a convex lens we have studied that the nature of image formed depends on the distance of the object from the convex lens. But this is not so in the case of a concave lens. Whether an object is placed at a large distance or it is close to the concave lens, a concave lens always forms an image which is virtual, erect and smaller than the object.
Changing the position of object in front of a concave lens only changes the size of image. But in every case, the image formed by concave lens remains smaller than the object. Since a concave lens always forms a smaller image than the object, therefore, objects always look smaller when seen through a concave lens. In other words, a concave lens always makes things look smaller (than they really are).
Please note that the formation of image in a concave lens is very similar to the formation of image in a convex mirror (which we have already studied). This is because like a convex mirror, a concave lens also diverges rays of light. We will now perform an activity to study the characteristics of the image formed by a concave lens when the object is placed at any distance in front of it.
Activity 14
Take a concave lens 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 concave lens (see Figure). Now, place a screen at some distance behind the concave lens.
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 kept behind the concave lens (see Figure).
Even if we place the screen on the front side of lens, the image of candle flame cannot be obtained on it. Now, look through the concave lens by keeping your eye behind the lens. We will see a small image of the candle and its flame through the concave lens (as shown in Figure).
Since the image can be seen only by looking through the concave lens and cannot be obtained on a screen, therefore, the image is virtual. If we look at the image, we find it to be the 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 smaller than the candle. We say that the image is diminished.
From the above activity we conclude that: Whatever be the distance of object from a concave lens, the image formed by a concave lens 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 concave lens. We can also not obtain an image larger than the object (or magnified image) with a concave lens.
While discussing the formation of images by a convex lens, we have learnt that we can obtain a real image of the sun on a piece of paper by focussing sun rays with a convex lens. And this image spot can burn a hole in paper. If, however, we hold a concave lens in the path of sun rays, no image of sun is formed on a piece of paper. This is because concave lens is a diverging lens. It cannot converge sun’s rays to form a real image on paper.
Uses of Concave Lenses
- Concave lenses are used in making spectacles.
- Concave lenses are used in making certain kinds of telescopes.
- Concave lenses are used in the ‘peep holes’ in the doors of hotel rooms (to see the image of the person standing outside).