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This question already has an answer here:

I came across this image when researching refraction in lenses. I can't understand why the light does not refract at the points which I have marked with burgundy arrows even though it is crossing a boundary between mediums or varying optical densities?Mentioned Image

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marked as duplicate by Floris, David Hammen, Bill N, peterh, Kyle Kanos Mar 11 '17 at 20:31

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ See this earlier question and my answer thereto for more about this. Yes, the diagram is badly drawn. It often is. $\endgroup$ – Floris Mar 11 '17 at 16:41
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    $\begingroup$ Possible duplicate of Interference at the focus of a convex lens or a concave mirror $\endgroup$ – Floris Mar 11 '17 at 17:42
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    $\begingroup$ I agree, thank you for the link to the other question; it's told me what I needed to know. $\endgroup$ – Sylith Mar 11 '17 at 18:19
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    $\begingroup$ I do not think this is a duplicate of the proposed question, so I have voted to leave it open. I do not see how the other question (about interference) answers the issue about refraction at the points indicated with burgundy arrows. I do not understand why you @Sylith think that question answers what you are asking here. $\endgroup$ – sammy gerbil Mar 11 '17 at 20:31
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    $\begingroup$ I agree with sammy gerbil, the two questions are entirely different, but the other just has an answer that is in depth enough that it happens to answer this question as well. If people are going to easily find the answer to this specific question, then this question should remain unless a previous question is found which specifically asks about the drawing of these lens diagrams. $\endgroup$ – Ryan Franz Mar 11 '17 at 20:38
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The diagram is misleading. In ray diagrams it is conventional to depict the lens as a plane perpendicular to the axis of the lens (in 2D, the vertical dotted line). Then rays only change direction when they meet such planes. It is also a convention to use the paraxial approximation so that all rays parallel to the axis are refracted through the forward focal point of the lens.

This illustration depicts the physical lens superposed on the ray diagram. Those are two separate ways of looking at the refraction of light through the lens.

In reality you are correct. Rays passing through the physical lens are refracted at both front and rear faces. For convex lenses, parallel rays further from the optical axis cross that axis before reaching the focal point.

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  • $\begingroup$ Thank you; this has cleared up some of my misconceptions. $\endgroup$ – Sylith Mar 16 '17 at 16:41

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