Module 5—Wave Theory of Light

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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 lens with convex surfaces refracts rays that are parallel to the principal axis towards a focal point producing a real image as shown below.

 

 

A diverging lens with concave surfaces refracts rays that are parallel to the principal axis outwards such 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:

 

Label	Definition
C	centre of curvature, the radius of the spherical surface of the lens
F	focal point, primary on the object side, secondary on the other side
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 (See "Physics Insight" on page 678 of your physics textbook.)

 

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

 

A simulation will be used to explore the image characteristics for both converging and diverging lenses.

 

SIM 1. Open the simulation and select “Lens” from the dropdown menu.

 

© Fu-Kwun Hwang, NTNU JAVA Virtual Physics Laboratory.  Used with permission.

 

Click the object arrow and move it to various locations on the principal axis. Using the simulation as a guide, draw ray diagrams and describe the image characteristics when the object is located at the positions listed in Table 1 below. The first one is done as an example.

 

Table 1: Using Ray Diagrams to Predict Image Characteristics: Converging Lens

 

Object Position	Ray Diagram	Image Characteristics (real or virtual, attitude, magnification, position relative to lens)
very far away		Image is real, inverted, much smaller than object, and located at F.
outside C
at C
between C and F
at F
inside F

 

SIM 2. Switch the lens in the simulation to a diverging lens by making the focal length negative (). Complete Table 2 below.

 

Table 2: Using Ray Diagrams to Predict Image Characteristics: Diverging Lens

 

Object Position	Ray Diagram	Image Characteristics (real or virtual, attitude, magnification, position relative to lens)
very far away
outside C
at C
between C and F
at F
inside F

 

Physics 30 Field Test Version © 2008 Alberta Education and its Collaborative Partners