Activity Description

This activity consists of a sequence of four steps. In the first step a beam of parallel rays can be dragged around in front of a convex lens. The students can observe the location of the focal point and that it is independent of the location of the beam. The location of the focal beam is marked with a red dot. In the second step, the beam is replaced by a point source. as the point source is moved around, the students observe which rays emitted by the point source pass through the focal point marked as a red dot. In the third step, the following figure showing different locations where an object will be placed. The students then predict the order from left to right of the corresponding images formed. In the last step, the object is dragged to each location and the question is resolved by noting the location of the image for each.

Target Conceptual Difficulty

The focal point is a property of the optical element. The focal point and the image location are generally different.

PowerPoint

Ordering Task - Image Formed by a Convex Lens

Simulations

Ordering Task - Image Formed by a Convex Lens

Presentation Notes

To begin the simulation click on the link marked Start. Drag the source around the screen and have the students observe the location of the focal point and that it doesn't change as the beam is moved. Mark the location of the focal point by dragging the red dot. Next click on the Point Source link. Drag the point source around. Have the students observe which rays from the point source reflect through the focal point. Also have the students observe how the location of the image of the point source varies as it is moved around. Next click on the Locations link to show the students three locations at which the object will be placed and have them list the order from left to right at which the images will be formed. Finally, click on the Object link and drag the object to each location and resolve the question. Note that the object starts at the far left of the simulation and the image is marked with a 1.

Student Results

This was not a very successful activity. From the students' previous experience with the ordering task with the concave mirror, they knew that the answer for all real images was incorrect, but they could not construct a correct answer. The main difficulty of this activity stemmed from the inclusion of both real and virtual images in the ordering task. In the future, this activity should be broken into smaller steps.

Answering on their own.

1.) Arrange the location of the images formed when the object is placed at the three points indicated from left to right.

Responses(percent) (count)
The image is always formed at the focal point 0% 0
A then B then C 13.64% 3
B then A then C 36.36% 8
C then B then A 18.18% 4
C then A then B 27.27% 6
Some other order 4.55% 1
Totals 100% 22

Following small group discussion.

2.) Arrange the location of the images formed when the object is placed at the three points indicated from left to right.

Responses(percent) (count)
The image is always formed at the focal point 0% 0
A then B then C 4.55% 1
B then A then C 31.82% 7
C then B then A 50% 11
C then A then B 9.09% 2
Some other order 4.55% 1
Totals 100% 22

Pen and Paper Actvivities

Pen and Paper version of this activity

Follow up tipers from homework assignments, test reviews and test items