Friday, April 22, 2011

Making Virtual Images Real

We go through geometric optics like this:

Plane mirrors
All images are:
  • virtual
  • upright
  • same size as the object
  • the same distance behind the mirror that the object was in front of it
This makes these pretty easy - lots of absolutes.  The only difficult thing is making sure to measure perpendicular to the mirror plane, especially if I get tricky and tilt the mirror on the page.  (Just tilt the paper, people!  It's a cheap trick - don't let me get away with it!)

Convex Mirrors
All images are:
  • Upright
  • Virtual
Some things start to change:
  • All of the images are behind the mirror, but the distance varies.  The images are always between the mirror and the focus
  • The images are all diminished, but the magnification depends on the object distance
Look out for an upcoming post on graphing the magnification and image location as a function of the image distance for these mirrors!

Concave Mirrors
Here, there are few absolutes.  The images are real, virtual, diminished, enlarged, near, far, upright, and inverted, depending on the object location.

We do a lab that combines ray tracing and Pasco light sources, mirrors, screens, and tracks.  To compare the tracing data and the experimental data (the focal lengths are different), we scale everything by the focal length (graphing M vs. d_o/f, d_i/f vs. d_o/f, etc.).

Everything goes swimmingly until they get to the virtual images in the lab.  How to find the location?  They're virtual because there are no light rays there, so they can't be projected.

We previously did a lab using a pane of glass (with light on one side of the room and darkness on the other); they put lab a lab weight on the light side and moved an identical weight around on the dark side until - from every viewing angle - the two lined up.  That's how they knew where the image was; a single observer can only point in the direction of an image, so two are required to fix the image's location to a single point in space.  Your two eyes rock at this. 


We use the same strategy here.  We use the lab mirror (a 100 mm concave mirror on a stand), and a thin magic marker for the object.  Moving around a second marker behind the mirror until it lines up with the image from every viewing angle will find the image location. 

Can't quite picture it?  I threw this video together today.  Please don't laugh.  Click through the thumbnail to see it!

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