Lenses and focal length
A converging lens bends parallel rays of light so that they meet at its focal point, and the focal length measures how far away that point is.
Practise this conceptA converging lens bends light rays inward so that rays entering parallel to its axis all meet at a single point on the other side, called the focal point. Focal length is simply the distance from the lens to that point — short focal lengths mean the lens bends light sharply, long focal lengths mean it bends light only gently.
That "meet at a point" behaviour is specifically a property of parallel rays. Light rays that enter a lens at an angle, spreading out from a nearby object rather than arriving parallel from far away, converge to a different point off to the side — not the focal point itself — which is exactly how a lens forms an image of something other than a distant object.
A lens's focal length depends on both its curvature and the material it's made from, since different glasses bend light by different amounts. Swap the glass for a different type while keeping the same curved shape, and the focal length changes even though the lens looks identical.
Key ideas
Requires: Snell's law
Formulas
| Symbol | Name | Unit |
|---|---|---|
| focal length | m (metre) | |
| distance | m (metre) | |
| distance | m (metre) |
Common misconceptions
- A converging lens with a shorter focal length bends light less than one with a longer focal length.
- All the light passing through a lens converges to the focal point, regardless of whether the rays started out parallel.
- A lens's focal length is the same regardless of the material it is made from.
Resources
Part of these UT courses
This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.
- The physical world viewLOFY.01.002
- OpticsLOFY.01.008
- Physics practical III — opticsLOFY.01.124
- Materials physics practical III — opticsLOFY.02.007