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_ THE _7a- 2006 V20 B - TAKE-HOME EXP 7a Light Imagined as...

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TAKE-HOME EXP. # 7a Light Imagined as Traveling in Straight Lines: Sun Images in the Shade of a Tree. Our ordinary senses imply that light travels in a straight line. In fact, Isaac Newton was so convinced of light's straight-line travel that he proposed a particle model for light, saying: "Are not the rays of light very small bodies [particles] emitted from shining substances? For such bodies will pass through uniform mediums in straight lines without bending into the shadow,. . . " This experiment examines phenomena that rely completely on explanations that assume straight-line travel for light. Optics studied with this assumption is called "geometric optics", because the drawings showing such behavior for light look like the figures one can find in a geometry book. Straight-line propagation of light energy appears so commonly in our experience that our brain assumes it is always the case—and will even produce the illusion of image located from where it thinks the light came from. For example, call to mind your own image behind plane mirrors, where no light from you is present! Sun Images In The Shadow Of A Tree * . These observations again rely on straight- line propagation of light. That is, we will assume we can trace the progress of light by drawing a straight-line "ray". This experiment can lead to rough measurements of the diameter of the Sun, given its distance from Earth. To do so with a simple measurement is again a remarkable feat showing the power of good observations and some mathematics. An approximate geometric model of straight-line light travel has proven to be enormously useful. * Much of this write-up and experiment on sun images under trees is inspired by observations written up in a wonderful classic book by M. J. G. Minnaert called "Light and Color in the Outdoors". First published around 1937 in Dutch, it has just been newly translated and revised by Len Seymour and published in paperback by Springer-Verlag, 1996. An earlier inexpensive version was published by Dover in 1954 and may still be available, but it lacks photographs.
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PINHOLE IMAGES OF THE SUN THE#7a-2 ____________________________________________________________________ Any tree with relatively many small leaves will work. Many local areas produce very good images of the Sun. One area is the table area underneath the leafy trellises in front of the Nugget; take a look at the sun images on the tabletops. Another area is the tree near the southwest entrance of PH1. Another is the shade of the small tree outside the office of the Dean of the College of Natural Sciences, between PH1 and PH2. Another is a heavily shaded area between buildings PH2 and PH3, opposite the front of the Bookstore, although the Sun should be fairly high in the sky. Many trees and bushes can produce Sun images; just look carefully at the sidewalks.
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