Converging and Diverging Lenses

What is a Lens?✴ Lens - transparent object used to refract (bend) light in a predictable way

✴ Usually made of glass or plastic

✴Two basic lens shapes:

Converging (convex) Diverging (concave)

Converging LensesThickest in middle and thinnest at edges

Parallel light rays converge through a single point - focal point

FOCAL POINT

FO PA

- exact centre of lensOptical centre (O):

- horizontal line that goes through O and F

Principal axis (PA):

- point after lens where light rays converge

Primary Focus (F): - point before lens that light rays travel through

Secondary Principle Focus (F’):

Converging Lenses

F’2F’ 2F

Diverging LensesThinnest in the middle and thickest at edges

Parallel light rays diverge (spread apart) and do not meet at focal point

Imaging Rules1) Ray parallel to principal axis is refracted through principal focus (F)

2) Ray through secondary principal focus (F’) is refracted parallel to principal axis

3) Ray through optical centre (O) continues straight through without being refracted

Describing Image CharacteristicsS - size (bigger, smaller, same size)

A - attitude (upright, inverted)

L - location (b/w F and 2F, at 2F, beyond 2F, behind lens)

T - type (real, virtual)

Beyond 2F’

At 2F’

Between F’ and 2F’

Important NoteAs object gets closer to lens, image gets larger and further away

For all object positions before F’, image is inverted and real

Lens Characteristics and Images

Many factors affect the characteristics of images:

Lens Type: converging or diverging

Lens Thickness

Lens Optical Density (n)

Lens Thickness

Focal length decreases as lens thickness increases because refracted rays bend more

Lens Optical Density (n)

Focal length decreases as lens optical density increases because refracted rays bend more