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## 16 November 2012

### Ray diagram practice sheet

Our conceptual classes need to be able to handle ray diagrams for converging and diverging mirrors and lenses.  That's really only six different diagrams:

• Converging lens with an object inside the focal point
• Converging lens with an object outside the focal point

• Diverging lens*

• Converging mirror with an object inside the focal point
• Converging mirror with an object outside the focal point

• Diverging mirror*
*The ray diagrams for diverging instruments are essentially the same no matter where the object is located.

Since this is conceptual physics, I don't ask them ever to use the thin lens or magnification equation to predict the location and size of an image.  We use ray diagrams, and then estimate distances based on the scale of the diagram.

In preparation for our exam, I handed out this practice sheet.  It presents the six different situations above, with an appropriately-sized mirror or lens, with focal and center points already labeled, and an object already drawn.  If a student can fill out this sheet correctly, he's ready for any question I can throw at him on the exam.

You can use these diagrams as a basis for your own questions or your own review sheet:  one idea is to change the focal length in the text of each problem, so that each student has a different focal length.  They will see, then, if they check their answers with each other, how the diagram can look the same but the different scale leads to different values for image and object distance.

You can also copy the diagrams into "paint" or some graphic design and manipulation program.  That will allow you to change the diagram itself, perhaps by moving the object closer or farther from the lens, or changing the focal lengths.  Every time I need to construct a question about lenses or mirrors, I use these diagrams as a template to adjust whatever parameters I need to adjust.

GCJ

#### 1 comment:

1. These are great templates to work from! Thanks! I like to have my students engage in healthy competition, so after they complete these, I have them create their own for other teams of students. Once they get comfortable, they often try to trip each other up by trying crazy combinations - the only caveat - they have to demonstrate a correct solution to me before I let them turn another team loose on their problems.