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Angular Magnification

To understand how a magnifying glass works, we need to first understand what it means when we say something appears larger.

Let me give you an example. Look at the text on the computer screen or on the notes in front of you. Now move closer. The text appears larger. Now move still closer and the text will appear even larger. Clearly, the text has stayed the same size. None the less, it appears bigger because it takes up a larger portion or angle of your view.

Now let us get back to our question. If we draw the object and the virtual image in the above example and measure the angle they cover, we see that the virtual image covers a larger angle and hence appears bigger.

... And that is how a magnifying glass works.


Before next lecture use a ruler to measure the following for each of the ray diagrams which we have drawn:

  1. the focal length,
  2. the object distance,
  3. the image distance,
  4. the object height, and
  5. the image height.

We will use this information to make this lecture more quantitative.

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Selman Hershfield /
Last modified January, 1995.
This work is supported by NSF grant DMR9357474.