- 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.
|Figure 1: Convex Lenses|
|Figure 2: Concave Lenses|
|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.|
|Principal Axis ||The principle axis of a lens is the line joining the centres of curvature of its surfaces.|
|Principal focus, F ||The principal focus of a lens is the point on the principal 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 and the principal 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.