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Concave and convex lenses are two types of spherical lenses. Thus, the lenses that are formed after binding two spherical surfaces bulging outward are called convex lenses, whereas the lenses that are formed after binding two spherical surfaces so that they are curved inward are called concave lenses. This is one of the fundamental difference between concave and convex lens.
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Read Also: Light Reflection and Refraction Revision Notes
What are Lenses?
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Lenses are spherical materials bounded by at least one spherical surface. Convex lens (or converging lens) and concave lens (or diverging lens) are the two types of lenses bounded by two spherical surfaces.
Lenses
The convex lenses are thicker in the middle compared to the edges and are used in converging light rays while the concave lenses are thicker at the edges compared to the middle and are used in diverging light rays.
How are images formed by lenses?
The images are formed by the lenses based on the principle of refraction of light. The refraction of light is the phenomenon by which a change in the path of light rays occurs when it passes from one medium to another.
Read Also: Scattering of Light
Image Formation by lenses
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The image formation by both convex and concave lenses at different positions of the object are summarised as follows:
Image Formation by Convex Lenses
The image formation by convex lenses can be demonstrated by:
| The position of the object placed | The position of the image formed | The nature of the image formed | The relative size of the image formed |
|---|---|---|---|
| At infinity | At the second focus, F2 | Real and inverted image | Highly diminished, point-sized image |
| Beyond 2F1 | Between the second focus, F2, and 2F2 | Real and inverted image | Diminished image |
| At 2F1 | At 2F2 | Real and inverted image | Same size as the object |
| Between the first focus, F1, and 2F1 | Beyond 2F2 | Real and inverted image | Enlarged image |
| At the first focus, F1 | At infinity | Real and inverted image | Infinitely larger than the size of the object or a highly enlarged image |
| Between the first focus, F1, and optical center O | The image formed will be on the same side as the object is | Virtual and erect image | Enlarged image |
The image formation by convex lenses using ray diagram depiction can be briefly explained as follows,
When the object is placed at Infinity
When the object is placed at infinity, the image formed by the convex lens will be at the second focus, F2. The nature of the image obtained will be a real image and it will be in inverted form. Also, the size of the image formed will be highly diminished, which is similar to the size of a point.
When the object is placed beyond 2F1
When the object is placed beyond 2F1, the image formed by the convex lens will be between the second focus, F2 and 2F2. The nature of the image obtained will be a real image and it will be in inverted form. Also, the size of the image formed will be of diminished form.
When the Object is placed at 2F1
When the object is placed at 2F1, the image formed by the convex lens will be at the position of 2F2. The nature of the image obtained will be a real image and it will be in inverted form. Also, the size of the image formed will be of the same size as the object.
When the object placed is in between F1 and 2F1
When the object is placed between F1 and 2F1, the image formed by the convex lens will be beyond the point, 2F2. The nature of the image obtained will be a real image and it will be in inverted form. Also, the size of the image formed will be of the enlarged form12.
Object is placed at 2F1
When the object is placed at the first focus, F1
When the object is placed at the first focus, F1, the image formed by the convex lens will be at infinity. The nature of the image obtained will be a real image and it will be in inverted form. Also, the size of the image formed will be infinitely larger than the size of the object or will be of a highly enlarged form.
When the object is placed between the first focus, F1, and the optical center O of the lens
When the object is placed between the first focus, F1, and the optical center O of the lens, the image formed by the convex lens will be on the same side as that of the object. The nature of the image obtained will be a virtual image and it will be in erect form. Also, the size of the image formed will be of the enlarged form.
Image Formation by Concave Lenses
Image formation by concave lens can be shown by:
| The position of the object placed | The position of the image formed | The nature of the image formed | The relative size of the image formed |
|---|---|---|---|
| At infinity | At the first focus, F1 | Virtual and erect image | Highly diminished, point-sized image |
| Between infinity and the optical center O | Between the first focus, F1, and the optical center O | Virtual and erect image | Diminished image |
The image formation by concave lenses using ray diagram depiction can be briefly explained as follows,
When the object is placed at infinity
When the object is placed at infinity, the image formed by the concave lens will be at the first focus, F1. The nature of the image obtained will be a virtual image and it will be in erect form. Also, the size of the image formed will be highly diminished, which is similar to the size of a point.
When the object is placed between the optical center of the lens (O) and Infinity
When the object is placed between the infinity and the optical center O of the lens, the image formed by the concave lens will be between the first focus, F1 and the optical center O. The nature of the image obtained will be a virtual image and it will be in erect form. Also, the size of the image formed will be of the diminished form.
| Important Topic Related Links | ||
|---|---|---|
| Plane Mirrors | Power of a Lens | Luminance |
| Fisheye Lens | Difference between real image and virtual image | Tyndall Effect |
Things to Remember
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- Lenses that are formed after binding two spherical surfaces bulging outward are called convex lenses.
- Lenses that are formed after binding two spherical surfaces so that they are curved inward are called concave lenses.
- Lenses are spherical materials bounded by at least one spherical surface.
- The images are formed by the lenses based on the principle of refraction of light.
Read More:
| Important Chapter Related Links | ||
|---|---|---|
| Refractive Index | Convex Mirror | Power of a Lens |
| Lens Formula and Magnification | Reflection of Light | Refraction of Light |
Sample Questions
Ques: What are lenses? (1 mark)
Ans. Lenses are spherical materials bounded by at least one spherical surface.
Ques: Which type of lens is also known as a converging one? (1 mark)
Ans. The convex lenses are thicker in the middle compared to the edges and are used in converging light rays.
Ques: In order to get a real image of the size of the object, where should the object be placed in front of a convex lens?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) Between the optical center of the lens and its principal focus.
(d) At Infinity (2 marks)
Ans. (b) At twice the focal length
Given that the object is placed in front of a convex lens to obtain a real image, which is of the same size as the object.
From the question, we get the nature of the image as a real image and the relative size of the image formed as the same size as the object.
We know that a real, same-sized image similar to the object is obtained when the object is placed at 2F1 and the image is formed at 2F2.
Therefore, to get a real image of the same size as the object, the object should be placed at 2F1, which is at twice the focal length in front of the convex lens.
Ques: What is the nature of the images that are formed by concave lenses? At which position should the object be placed so that a convex lens forms an image of a similar nature as the concave lens? (2 marks)
Ans. The concave lens forms virtual and erect images.
The convex image forms a virtual and erect image only at one position. When the object is placed between the first focus, F1, and the optical center O of the lens, the image is formed on the same side as that of the object, and the image will be natural, virtual, and erect, and the relative size enlarged.
Ques: What is Convex Lens? (1 mark)
Ans. The lenses that are formed after binding two spherical surfaces bulging outward are called convex lenses.
Ques: Determine which lens is thicker in the middle and thinner at the edges? (1 mark)
Ans. It is the convex lens which is thicker in the middle and thinner at the edges.
Ques. Which lens has negative focal length? (1 mark)
Ans. Concave lens is known to have negative focal length.
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