Measuring
Measuring the focal length of a converging lens using a distant object
- Point the lens at a distance object outside the window 鈥 for instance a tree or building. The greater the distance the better. Rays of light from this object are taken to be parallel as the object is assumed to be at infinity.
- Move the position of the screen behind the lens until a sharp image is formed on it.
- Parallel rays of light form an image at the principal focus of a lens, and so the screen is at the principal focus. The distance between the optical centre of the lens and the screen is the focal length, f.
- Measure this distance with a 30 cm rule and record in a suitable table.
- Repeat the process several times and calculate the average focal length.
Diverging (or Concave) lenses
A diverging lens is thinner in the middle than it is at the edges.
This causes parallel rays to diverge.
They separate but appear to come from a principle focus F on the other side of the lens.
In a ray diagram, a diverging lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.