From: Kevin J Doyle (k-sub-d@ix.netcom.com)
Date: Tue May 08 2001 - 16:49:43 PDT
Message-ID: <001201c0d819$94b13f80$9ce76ed1@oemcomputer> From: "Kevin J Doyle" <k-sub-d@ix.netcom.com> Subject: Lens Equation vs Reality Query Date: Tue, 8 May 2001 16:49:43 -0700
HI Everyone!
I just did a lens experiment in class today with generally excellent results. However, using a lens with a focal length of 15 cm and a candle at a distance of 20 cm my classes got very consistent results that did not agree with the lens equation. If you calculate out the image distance for that scenario you get di = 60cm. My students consistently found a sharp image at 40 cm. I'm frankly a little stumped. The rest of the results worked great. The only explanations I can come up with are the following
1) The curvature isn't consistent over the whole lens so that as the object gets closer to the lens the edge affects play a larger role and change the effective focal length. This seems a bit off mark to me.
2) Slightly more satisfying but still not quite right might be that as the object approaches the focal point more of the light from the object is simply not hitting the lens. Particularly the classic ray diagram line that passes through the focal point on the object side. This would men that you are forming an "image" with light that does't follow the classic treatment in ray diagram or equation.
I'm a little ata loss as to how to help my sutdents understand this. THey've accepted as plausible my abovetwo suggestions but none of us is satisfied yet. Any help out there?
Thanks
Kevin
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