Lenses
- There are 2 types of lenses, namely the
- Convex lens
- Concave lens
- Convex lenses are thickest through the middle, concave lenses are thickest around the edge, but several variations on these basic shapes are possible, as shown in figure 1.
- Light rays passing through a convex or converging lens are bent towards the principal axis, whereas rays passing through a concave or diverging lens are bent away from the principal axis.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUeZRc3zhT3P2AVeRrHBhC8zWuiKSe8vmdZNMQCUQdPylkBV0JUZQ8X9K4IHWOa-vqP6G2wMRTTmimfZosPtXkY1v-bHkoiAzLMTdDYPHAS5ocxbI7XZyDrpjNye-OlBzpzqxNEVMyeTY/s200/Picture37.png) |
Figure 1: Convex Lenses |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjcyfKIUkB5HU7_hwndLyzzUSr4qiJdNGGnHo-jNXNDekW8-dLx-OTZ4fbzchW7Seag-bgU12VnuJc2jr-1MvyUu9aEGt1UO0c8vcwe3trNEJDfhRcHoM5e5R9E5qDgxH4J7HhCppIjwj0/s200/Picture38.png) |
Figure 2: Concave Lenses |
Important Terms
Optical centre, P |
Light passing through the central block emerges in the same direction as it arrives because the faces of this block are parallel. P marks the optical centre of the lens. |
Principle Axis |
The principle axis of a lens is the line joining the centres of of curvature of its surfaces. |
Principle focus, F |
The principle focus of a lens is the point on the priciple axis to which all rays originally parallel and close to the axis converge, or from which they diverge, after passing through the lens. |
Focal length, f |
The focal length of a lens is the distance between the optical centre an the principle focus. |
Rays of light can pass through a lens in either direction, so every lens has two principal foci, one on each side of the optical centre.