Ordering Task - Image Formed by a Concave Mirror

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 concave mirror. 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 observes 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 Concave Mirror

Simulations

Ordering Task - Image Formed by a Concave Mirror

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

The task as originally written proved to be too difficult. No student correctly gave the correct order of B then A then C either before or after small group discussion. The answer tht 68% of the students gave before small group discussion and 75% gave after mall group discussion is the corrct answer provided all of the images are real. A future improvement is to split this into several tasks: one with only real images; one with only virtual images; and then one with both real and virtual images. 

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 20% 4
A then B then C 10% 2
B then A then C 0% 0
C then B then A 65% 13
C then A then B 5% 1
Some other order 0% 0
Totals 100% 20

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 10% 2
A then B then C 10% 2
B then A then C 0% 0
C then B then A 75% 15
C then A then B 5% 1
Some other order 0% 0
Totals 100% 20

Pen and Paper Actvivities

Pen and Paper version of this activity

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

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Copyright 1999, 2000, 2001, 2005 - 2009 by Paul Williams
Last Updated: August 21, 2009