Help index
Zoom Lens Activity Help

Help is available on the following topics:

Structure of this activity

Ray tracing

Varifocal lens

Zoom lens


Structure of the program
Structure of this activity

The activity is broken down into four "pages". The purpose of each of these is as follows:

1. Introduction.

A general introduction to the program

2.  Ray tracing

Practice ray tracing with between 1-3 lenses of variable position and focal length.

3.  Varifocal lens

Examine the operation of the varifocal lens.

4.  Zoom lens

Examine the operation of the zoom lens.

To access each of these simply click the mouse (left-hand button) on the tab containing your desired destination.

Notice that once the lenses are set, the settings remain the same when you move from page to page. This makes it easier to compare behaviours of similar lens systems.


Ray tracing

The first time you open the Ray tracing page the lenses are set to produce a magnified upright image of the object. In this section you can vary the position and focal length of the three lenses, you can also remove the lenses by clearing the check-box associated with the lens.


Varifocal lens

A varifocal lens system is one in which the position of the first and third lens are fixed. In addition, the focal lengths of all three lenses are fixed. Once the lenses are set to appropriate values for position and focal lengths, these are fixed by pressing the Start button. Make sure that all three lenses are in for the varifocal lens experiment as the experiment cannot function with any missing.

Having fixed the lenses, the only variable is the position of the middle lens. Use the buttons to vary the position of this middle lens. The graphs will show plots of Image magnification and position versus Position of lens 2.

A freely variable version of this experiment can be set up using the Ray tracing page. Just switch back to the Ray tracing page and vary lens 2's position by either using the buttons or by dragging the lens. Watch the movement of the image.

When on the varifocal page and having fixed the lenses, if you wish to change the lenses then choose Options|Restart from the menu bar.


Zoom lens

A zoom lens system is one in which the position of the first lens is fixed. In addition, the focal lengths of all three lenses are fixed. Once the lenses are set to appropriate values for position and focal lengths, these are fixed by pressing the Start button. Make sure that all three lenses are in for the zoom lens experiment as the experiment cannot function with any missing.

Having fixed the lenses, the only variable is the position of the middle lens. Use the buttons to vary the position of this middle lens. As you move the middle lens, the computer wiggles the third lens to try to move the image back to the original location. Sometimes this is not possible - in these cases it may be that the system chosen is not viable as a zoom lens.

The graphs will show plots of Image magnification, image position and position of the final lens versus Position of lens 2.

When on the zoom lens page and having fixed the lenses, if you wish to change the lenses then choose Options|Restart from the menu bar.


Graphs

The graphs can be rescaled by either choosing Options|Rescale... from the menu bar or by clicking on the graph you want to rescale. Note that the only checking that is done before a rescale is to see that the maximum value is greater than the minimum value. So be warned that you may choose values that means that none of your points will show on the graph! rescaling is also a good idea if you have moved through the range a few times. Particularly with the zoom lens since the nature of the zoom mechanism is to move lens 3 until the final image position is within a small fraction of a percentage of the original position. This small percentage gives a bit of flexibility in the absolute positioning of lens 2 - hence you may end up with more than one point at a specific position of lens 2. Rescaling to the same values redraws the graph and so eliminates the additional points - only keeping the last point for each plotted position of lens 2.