Peoples Physics Concepts

16.7. Lens http://www.ck12.org

16.7 Lens

• Describe how light behaves when passing through converging and diverging lenses.


• Use ray tracing diagrams and calculate image distances and magnification using the lens’ maker’s equation.

Students will learn how light behaves when passing through converging and diverging lenses. Students will also

learn how to do ray tracing diagrams and calculate image distances and magnification using the lens’ maker’s

equation.

Key Equations

1

f

=

1

do

+

1

di

; The len’s maker’s equation

Wherefis the focal length of the lens,dois the distance of the object from the lens anddiis the distance the image

is formed from the lens.

M=

−di

d 0

The size of an object’s image is larger (or smaller) than the object itself by its magnification,M. The level of

magnification is proportional to the ratio ofdianddo. An image that is double the size of the object would have

magnificationM=2.

Guidance

• For lenses, the distance from the center of the lens to the focus isf. Focal lengths for foci behind the lens are
  positive in sign. The distance from the center of the lens to the object in question isd 0 , where distances to the
  left of the lens are positive in sign. The distance from the center of the lens to the image isdi. This number is
  positive for real images (formed to the right of the lens), and negative for virtual images (formed to the left of
  the lens).


• Lenses, made from curved pieces of glass, focus or de-focus light as it passes through. Lenses that focus
  light are calledconverginglenses, and these are the ones used to make telescopes and cameras. Lenses that
  de-focus light are calleddiverginglenses.


• Lenses can be used to make visual representations, calledimages.


• Thefocal length,f, of a lens or mirror is the distance from the surface of the lens to the place where the light
  is focused. This is called thefocal pointorfocus. For diverging lenses, the focal length is negative.


• For converging lens, one can find the focal point by simply holding a piece of paper near the lens until a distant
  image is formed. The distance from the paper to the lens is the focal point.