The focal length of a converging (convex) lens is the distance from the centre of the lens to the principal focus (focal point). It is measured along the principal axis.
The principal focus
When parallel rays of light pass through a converging lens, they are refracted and converge to a single point on the other side of the lens. This point is the principal focus (F). Every converging lens has two focal points, one on each side, at equal distances from the lens.
What affects focal length
A fatter (more curved) lens bends light more strongly and has a shorter focal length. A thinner (less curved) lens has a longer focal length.
Focal length and magnification
The focal length determines how the lens forms images:
| Object distance | Image |
|---|---|
| Beyond 2F | Real, inverted, smaller than object |
| At 2F | Real, inverted, same size |
| Between F and 2F | Real, inverted, larger than object |
| At F | No image (rays emerge parallel) |
| Inside F | Virtual, upright, larger (magnifying glass) |
Finding focal length experimentally
Point the lens at a distant object (such as a window or tree far away). Move a screen behind the lens until a sharp image forms. The distance from the lens to the screen is approximately the focal length, because light from a distant object arrives as nearly parallel rays.
Power of a lens (Extended)
Power = 1 / focal length (in metres). The unit is dioptres (D). A short focal length means a high power (strong lens).
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