Lenses form images by refracting light.
Image Formation by Convex Lens
- Take a convex lens. Find its approximate focal length.
- Draw five parallel straight lines on a table such that the distance between the successive lines is equal to the focal length of the lens.
- Place the lens on the central line such that the optical centre of the lens lies just over the line.
- The two lines on either side of the lens correspond to F and 2F of the lens respectively. Mark them 2F1, F1, F2 & 2F2 respectively.
- Place a burning candle far beyond 2F1 to the left. A clear sharp image is formed on a screen placed at F2.
- Repeat this activity by placing the object just behind 2F1, between F1 & 2F1, at F1, between F1 & O.
Nature, Position, and Relative Size of the Image Formed by a Convex Lens for Various Positions of the Object
| Position of the Object | Position of the Image | Relative Size of the Image | Nature of the Image |
|---|---|---|---|
| At infinity | At F2 | Highly diminished, point-sized | Real and inverted |
| Beyond 2F1 | Between F2 & 2F2 | Diminished | Real and inverted |
| At 2F1 | At 2F2 | Same size | Real and inverted |
| Between F1 & 2F1 | Beyond 2F2 | Enlarged | Real and inverted |
| At focus F1 | At infinity | Infinitely large or highly enlarged | Real and inverted |
| Between F1 & O | On the same side of the lens as the object | Enlarged | Virtual and erect |
Image Formation by Concave Lens
- Place a burning candle on one side of a concave lens.
- Look through the lens from the other side and observe the image. The image will not be obtained on a screen but can be observed through the lens.
- Move the candle far away from the lens. The image becomes highly diminished.
- A concave lens always gives a virtual, erect, and diminished image, irrespective of the position of the object.
Nature, Position, and Relative Size of the Image Formed by a Concave Lens for Various Positions of the Object
| Position of the Object | Position of the Image | Relative Size of the Image | Nature of the Image |
|---|---|---|---|
| At infinity | At F1 | Highly diminished, point-sized | Virtual and erect |
| Between infinity & O | Between F1 & O | Diminished | Virtual and erect |
Image Formation in Lenses Using Ray Diagrams
- For drawing ray diagrams in lenses, any two of the following rays are considered:
- A ray of light parallel to the principal axis: After refraction from a convex lens, passes through the principal focus on the other side of the lens. For a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.
- A ray passing through a principal focus: After refraction from a convex lens, emerges parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, emerges parallel to the principal axis.
- A ray passing through the optical centre: It emerges without any deviation.
Ray Diagrams for Image Formation in a Convex Lens
- The ray diagrams for position, size, and the nature of the image formation in a convex lens for a few positions of the object are shown below (Refer the table):
Ray Diagrams for Image Formation in a Concave Lens
- The ray diagrams for the image formation in a concave lens for various positions of the object (Refer the table):
Sign Convention for Spherical Lenses
- It is the same as the sign convention used in spherical mirrors.
- The rules are applied for signs of distances, but the measurements are taken from the optical centre of the lens.
- According to the convention, the focal length of a convex lens is positive and that of a concave lens is negative.
Select a Topic 👇
1. Reflection of Light 2. Spherical Mirrors 3. Image Formation by Spherical Mirrors 4. Sign Convention, Mirror Formula & Magnification 5. Refraction of Light 6. Refraction through a Rectangular Glass Slab 7. Refractive Index, Refraction by Spherical Lenses 8. Image Formation by Lenses 9. Lens Formula & Magnification, Power of Lenses