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My question is this : what is the property of transparent material that is used for lens?. I think that it allows the light to pass through ( absorption property) and does not bend the light ( refraction property). But my multiple choice homework has one answer " absorption" and the other " refraction" . So based on my understanding, what is wrong?

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    $\begingroup$ Probably related: physics.stackexchange.com/q/7437 $\endgroup$ – Kyle Kanos Dec 29 '15 at 11:26
  • $\begingroup$ I don't understand your question. Are you asking why we use transparent materials for lenses? Why those materials are transparent? Or something else? $\endgroup$ – ACuriousMind Dec 29 '15 at 12:17
  • $\begingroup$ I don't see why this question shows a lack of effort. The OP states their reasoning - what they think absorption is - in a way that clearly shows the problem: they have a misunderstanding about absorption. As I'm sure we've all had very like experiences, we should all know that such misunderstandings linger very stubbornly notwithstanding even a great deal of research and that a well placed insight from someone else to one's specific question can well be an antidote that demolishes misconceptions in one fell blow. $\endgroup$ – Selene Routley Dec 30 '15 at 0:27
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The allowing of light to pass through is actually the opposite of absorption: absorption is where the energy of the light beam is permanently transferred to the medium and converted e.g. to heat. Ideally, a lens would not absorb light at all, and would interact elastically with it. In practice, one cannot avoid a tiny amount of absorption, but this is a non-ideal effect: "refraction", being the change in propagation speed of the disturbance as a function of position, is the key property that enables a lens to work. You can indeed show that the speed change - varying refractive index with position - implies Snell's law and is what bends light: I show this from Maxwell's equations in this answer here and here

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