We will now discuss the formation of different types of images by a convex lens when the object is placed
1. At infinity
2. Beyond 2F
3. At 2F
4. Between F and 2F
5. At the focus F
6. Between the focus F and optical centre C
1. Image formed by a convex lens when the object is placed at infinity
When the object is placed at infinity, the two rays AO and BD running parallel to the principal axis get refracted at point O and D respectively and intersect each other at the principal focus. Therefore, in this case the image A’B’ is formed at the focus, which is highly diminished (point sized), real and inverted.
Why convex lens is called converging lens?
A convex lens is called converging lens because of its ability to converge a parallel beam of light on a point called principal focus.
2. When the object is placed beyond 2F
When an object is placed beyond the centre of curvature then a ray of light AO which is parallel to the principal axis, pass through the focus F along the direction OF after refraction. While the other ray AC pass through the optical centre C and goes straight without any deviation according to the rule (2). These two refracted light rays intersect each other at point A’, between the focus F and centre of curvature 2F on the other side of the lens. In this way, a diminished, real and inverted image A’B’ is formed.
3. When the object is placed at 2F
When the object is placed at the centre of curvature of a lens then a ray of light AO which is parallel to the principal axis after refraction pass through the focus F along the direction OF. While the other ray AC pass through the opticl centre C and goes straight without any deviation. These two refracted light rays intersect each other at point A’, on the other side of the lens at the centre of curvature 2F. so, the image A’B’ formed in this case is at the centre of curvature, of same size as the object, real and inverted.
4. When the object is placed between the focus F and the centre of curvature 2F
When an object is placed between the focus and centre of curvature of a convex lens then a ray of light AO which is parallel to the principal axis after refraction pass through focus F along the direction OF. While the other ray AC pass through the optical centre C and goes straight without any deviation. These two refractesd light rays intersect each other at point A’, beyond the centre of curvature 2F on the other side of the lens. So, in this case the image A’B’ formed is larger then the object, real and inverted.
5. When the object is placed at focus F
When an object is placed at the principal focus F, a ray of light AO which is parallel to principal axis after refraction pass through the focus along the direction OX. While the other ray AC pass through the optical centre and goes straight without any deviation along the direction CY. In this case, as is clear from the Figure given below, both the refracted rays: OX and CY are parallel to each other, so these rays cannot intersect each other, hence the image will be formed at infinity. That is why, the image A’B’ formed in this case will be highly enlarged, real and inverted.
6. When the object is placed between the focus F and optical centre C
When an object is placed between the principal focus and optical centre of a convex lens, then a parallel ray of light AO passes through the focus after refraction along the direction OX. while the other ray AC pass through the optical centre and goes straight without any deviation along the direction CY. But, in this case the two refracted light rays i.e. OX and CY are diverging away from one another, so these cannot intersect each other to form a real image on the right side of the convex lens. Thus, the refracted rays OX and CY are extended backward by dotted lines. On extending back, these rays appear to intersect at point A’. Hence, the image A’B’ formed in this case is a virtual image which is formed on the same side of the lens behind the object. Also the image formed is erect and highly enlarged.
Table of Summary of Image Formed by a Convex Lens
| S No. | Position of Object | Position of Image | Size of Image | Nature of Image |
| 1. | At infinity | At the focus | Highly diminished, Point sized |
Real and inverted |
| 2. | Beyond 2F | Between the focus and centre of curvature | Diminished | Real and inverted |
| 3. | At 2F | At the centre of curvature | Same size | Real and inverted |
| 4. | Between F and 2F | Beyond the centre of curvature | Enlarged | Real and inverted |
| 5. | At the focus F | At infinity | Highly enlarged | Real and inverted |
| 6. | Between the focus F and optical centre C | Behind the object | Enlarged | Virtual and erect |
Test Your Understanding and Answer These Questions:
- Explain formation of different types of images by a convex lens.
- What should be the position of an object in order to make its highly diminished image at principal focus of a convex lens?