236+Manual+09+Optics+in+2D

This time the light does not seem to meet in fact it

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Unformatted text preview: — W+d 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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