Formation of Image by a Convex Lens
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When the Object is Placed Between F1 and O
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Formation of Image by a Convex Lens
The image is -
- Formed on the same side of the lens
When the Object is Placed Between the Optical Center (O) and First Focus (F1)
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Here we consider two rays starting from the top of the object placed at F
1 and optical center. The ray parallel to the principal axis after refraction passes through the focus (F
2). The ray passing through the optical center goes through the lens undeviated. These refracted rays appear to meet only when produced backwards. Thus, when an object is placed between F
1 and O of a convex lens, a virtual, erect and magnified image of the object is formed on the same side of the lens as the object.
When the Object is placed at 2F1
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The image is -
Here one of the rays starting from the top of the object placed at 2F1 passes through the optic center without any deviation and the other ray which is parallel to the principal axis after refraction passes through the focus. These two refracted rays meet at 2F2. Thus, when an object is placed at 2F1 of a convex lens, inverted and real image of the same size as the object is formed at 2F2 on the other side of the lens.
When the Object is Placed Between F1 and F2
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The image is
Let us consider two rays coming from the object. The ray which is parallel to the principal axis after refraction passes through the lens and passes through F2 on the other side of the lens. The ray passing through the optic center comes out of the lens without any deviation. The two refracted rays intersect each other at a point beyond 2F2. So, when an object is placed between F1 and 2F1 of a convex lens the image is formed beyond 2F2.
When the Object is Placed at F1
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The image is -
Here again we consider two rays coming from the top of the object. One of the rays which is parallel to the principal axis after refraction passes through F2 and the other ray which passes through the optical center comes out without any deviation. These two refracted rays are parallel to each other and parallel rays meet only at infinity. Thus, when an object is placed at F1 of a convex lens, the image is formed at infinity and it is inverted, real and magnified.
When the Object is Placed Beyond 2F1
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The image is -
- formed between F2 and 2F2
The ray parallel to the principal axis after refraction passes through F2 and the ray which passes through the optical center comes out without any deviation. The refracted rays intersect at a point between F2 and 2F2. The image is inverted, real and diminished.
When the Object is placed at Infinity
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The image is -
When the object is at infinity, the rays coming from it are parallel to each other. Let one of the parallel rays pass through the focus F1 and the other ray pass through the optical center. The ray which passes through F1 becomes parallel to the principal axis after refraction and the ray which passes through the optical center does not suffer any deviation.
The table below gives at a glance the position, size and nature of the image formed by a convex lens corresponding to the different positions of the object and also its application.
| Position of the object | Position of the image | Nature of the image | Size of the image | Application |
| Between O and F1 | on the same side of the lens | Erect and virtual | Magnified | Magnifying lens (simple microscope), eye piece of many instruments |
| At 2F1 | At 2F2 | Inverted and real | Same size | Photocopying camera |
| Between F and 2F1 | Beyond 2F2 | Inverted and real | Magnified | Projectors, objectives of microscope |
| At F1 | At infinity | Inverted and real | Magnified | Theatre spot lights |
| Beyond 2F1 | Between F2 and 2F2 | Inverted and real | Diminished | Photocopying (reduction camera) |
| At infinity | At F2 | Inverted and real | Diminished | Objective of a telescope |
