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ray-line diagram showing light bending in the center of a lens

As light enters a denser medium from a rarer medium, it bends towards the normal. Why does the light bend after passing through point F & not E?

Fred
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S.M.T
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    Similar earlier question on [physics.se]: Why doesn't a light ray bend again when emerging from a lens? – Ilmari Karonen Nov 25 '21 at 19:46
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    it doesn't, it's a simplification – njzk2 Nov 25 '21 at 22:18
  • Refraction depends on the index of refraction. While that may be affected by density, density itself does not cause refraction (unless you mean optical density, in which case saying just "density" is misleading). – Acccumulation Nov 26 '21 at 07:08
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    This is a 'thin lens' approximation. As the lens is 'thin', it doesn't really matter whether the refraction takes place at the two faces of the lens (which is what actually happens) (see NMech's diagram), or the centre of the lens (which is a close enough simplification). Optics is hard enough for most students to grasp, so it's made easier for them by just having one deflection point, to make diagrams easier to draw. – Neil_UK Nov 26 '21 at 20:45

2 Answers2

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It changes on both E and the exit point of the beam (see image below).

enter image description here

source

My understanding is that refraction occurs at any boundary that there is a change of the wave propagation velocity.

The image in the original post is a simplification which is used either as an introductory image or to describe other aspects where what happens inside the lens is not important.

NMech
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  • Should be E and the exit point (unlabelled), not E and F – g.kertesz Nov 25 '21 at 19:50
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    Yes you are right. My eyesight is not what it used to be. I'll correct it. – NMech Nov 25 '21 at 20:28
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    More to the point, OP's diagram is commonly used when introducing and illustrating the assumptions of the thin lens approximation. – J... Nov 25 '21 at 21:54
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    Your image is also a simplification... Optics is a weird rabbithole of ever complicating simplifications, all of which refuses to make sense untill you start reading some QM, at which point the world you realize that the universe refuses to make sense, in general. – Stian Nov 26 '21 at 09:15
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    @StianYttervik I agree. I can't argue with what you are saying. – NMech Nov 26 '21 at 09:20
  • "Focal point" is not unique ... It is a "function" of the "height" of incident ray. – Antonio51 Dec 01 '21 at 10:05
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You are right, rays refract at both the entry and the exit points. That drawing is just a simplified diagram, frequently used in textbooks about geometrical optics. It helps to illustrate concepts like parallel rays, focal point, object distance, image distance etc. where we do not really care what happens inside the lens.

g.kertesz
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