A thin lens, similar to that found in the human eye, is a circular piece of transparent material with a spherically shaped surface.  With any curved lens, light rays will be refracted at various angles depending on where they contact the curved surface.  There are two types of thin lenses.

 

A  converging or convex lens  with convex surfaces refracts rays that are parallel to the principal axis toward a focal point producing a real image (as shown in the diagram).

              


Converging Lens:  a lens that refracts rays travelling parallel to the principal axis inward to the focal point of the lens

Diverging Lens:  a lens that refracts rays travelling parallel to the principal axis outward so that they appear to have originated or passed through the virtual focal point of the lens


 



 

A diverging or concave lens with concave surfaces refracts rays that are parallel to the principal axis outward so that they appear to have originated from a virtual focal point.  Therefore, a diverging lens can only produce a virtual image.

 


 

The following table summarizes all the relevant labels for each ray diagram. For more information, review page 678 in the text.

Label

Definition

C (or 2F)

centre of curvature, the radius of the spherical surface of the lens

F

focal point, primary focus on the object side of concave and virutal primary focus on the object side for convex.

O

optical centre of the lens

principal axis

imaginary line that is perpendicular to the lens surface at the optical centre

ray 1

travels parallel to the principal axis and is refracted through, or appears to have passed through, the principal focus

ray 2

travels through, or appears to have passed through, the secondary focus and emerges parallel to the principal axis

ray 3

passes straight through the optical centre

 

As with ray diagrams used for curved mirrors, many rays produce the image but only a few are required to identify the image characteristics.  The three rays used in the illustrations can be used with any ray diagram to predict the image characteristics.

 

Recall the following image characteristics from Lesson 3.

Image Characteristic

Definition

magnification

ratio of the size of the image to the size of the object

attitude

vertical orientation of the image relative to the object

position

where the image and object are located relative to the reflecting surface

real or virtual image

a real image forms where light rays converge, a virtual image forms where light rays "appear" to have converged or originated