Rules for drawing ray diagram in lens:
1. Parallel ray rule: A ray parallel to the principal axis passes through the focus in a convex lens, while in a concave lens it appears to diverge from the focus on the same side.
Ray of light passing parallel to principal axis in lenses
2. Focus ray rule: A ray passing through the focus of a convex lens emerges parallel to the principal axis, whereas a ray directed towards the focus in a concave lens emerges parallel after refraction.
Ray of light passing through the principal focus in lenses
3. Optical centre rule: A ray passing through the optical centre of both convex and concave lenses travels straight without any deviation.
Ray of light passing through the optical centre in lenses
Image formation by convex lens:
| Object position | Image position | Nature and size | Ray diagram |
| At infinity | At \(F_2\) | Real, inverted and highly diminished |  |
| Beyond \(2F_1\) | Between \(F_2\) and \(2F_2\) | Real, inverted and diminished |  |
| At \(2F_1\) | At \(2F_2\) | Real, inverted and same size |  |
| Between \(F_1\) and \(2F_1\) | Beyond \(2F_2\) | Real, inverted and enlarged |  |
| At \(F_1\) | At infinity | Image would not be formed |  |
| Between \(F_1\) and \(O\) | On the same side of the lens as the object | Virtual, erect and enlarged |  |
Image formation by concave lens:
| Object position | Image position | Nature and size | Ray diagram |
| At infinity | At \(F_1\) | Virtual, erect, highly diminished and point-sized |  |
| Between infinity and \(O\) |
Between \(F_1\) and \(O\)
|
Virtual, erect and diminished |  |
Applications:
Magnifying glass: A convex lens is used as a magnifying glass. When the object is placed within the focal length, the ray diagram shows that a virtual, erect, and magnified image is formed on the same side of the lens, helping us see small details clearly.
Spectacles: Spectacles use lenses to adjust the path of light rays entering the eyes. Based on ray diagrams, lenses either converge or diverge light rays so that a clear and properly focused image is formed, improving vision.
Projector: A projector uses a convex lens. When the object (film or slide) is placed between \(F\) and \(2F\), the ray diagram shows that a real, inverted, and magnified image is formed on a screen, allowing images to be displayed larger.