Principal focus and focal length
A converging lens bends a bundle of rays travelling parallel to its principal axis so that the rays meet after passing through the lens.
- The principal axis is the reference line through the optical centre, perpendicular to the lens.
- A principal focus, , is the point on the principal axis where rays that entered parallel to the axis are brought together.
- A converging lens has a principal focus on each side, depending on the direction in which light travels.
- The focal length, , is the distance from the optical centre to a principal focus.
- For otherwise similar lenses, greater curvature corresponds to a shorter focal length.
Useful ray rules
- A ray through the optical centre of a thin lens continues undeviated.
- A ray parallel to the principal axis is refracted through the principal focus on the far side.
- A ray directed through the principal focus on the near side emerges parallel to the principal axis.
Real and virtual images
The image type depends on whether the refracted rays themselves meet or only appear to come from a point.
| Real image | Virtual image |
|---|---|
| The light rays actually converge at the image position. | The light rays do not meet there; tracing them backwards gives the apparent image position. |
| Can be projected onto a screen. | Cannot be projected onto a screen. |
| For a real object and a single converging lens, the image is inverted. | When the object is inside the focal length of a single converging lens, the virtual image is upright. |