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2 4d W d Figure 6: Determining the radius of a lens from its geometry. If you apply the Pythagorean theorem to the
dashed triangle, you can derive the formula for the radius. 8 Real rays
Virtual rays h f(<0) Figure 7: Determining the focal length of a defocusing lens (negative focal length). 9 Experiment: Diverging Lens Now try to ﬁnd the focus of a plano-concave lens (as shown in Figure 7). This time the light does
not seem to meet! In fact, it spreads apart (diverges). We can still observe (and measure) the focal length
of a diverging lens, however. To do so, you must trace the diverging rays back to a negative focal point
(Figure 7). You may also see faint, real rays in the region of the virtual rays. These are reﬂections and are
not the virtual rays. You can help eliminate those by ensuring that your rays are entering the ﬂat side of
the plano-concave lens. You have to extrapolate the virtual rays by tracing back along the diverging rays
as pictured in Figure 7. This is done most easily by tracing the lens on you pap...
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- Spring '14