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The definition of refraction which I found on wikipedia is

In physics, refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium. enter image description here

But in the below case, there is no change in direction of light. So, is this also refraction? enter image description here

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I really feel like I genuinely understand your question. In essence you are asking "do we count it as refraction when there is no refraction in virtue of the angle of incidence being 90 degrees (or zero degrees, however you look at it).

The example you give is an example of something that is vacuously true. It's a bit like a child claiming that he ate all of his vegetables because there were in fact, no vegetables served on his plate this evening.

I suppose that you are correct that there is no refraction, but only in virtue of the fact that the angle of incidence is 90 degrees. But in physics, we try to avoid conundrums involving vacuous statements like this. They don't really improve our understanding of the how and why of the physical world. It's a bit like asking "if a tree falls in the forest, but no one is there to hear it, does it make a sound". The question, by its very construction, is designed to generate endless debate rather than move our understanding forward.

Your question is a great one - it shows inquisitiveness and curiosity, and shows your interest in pushing the envelope. When I was working on my undergraduate degree in physics I used to ask questions like this all the time.

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Actually refraction is the phenomenon in which speed of light changes when it passes from one medium to another. The change in direction can be explained by Fermat's principle which you can check on Wikipedia.

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  • $\begingroup$ So, why didn't Wikipedia mention that? $\endgroup$ – SarGe Jun 16 at 6:56
  • $\begingroup$ Read Fermat's principle on Wikipedia. $\endgroup$ – user262060 Jun 16 at 7:18
  • $\begingroup$ They should have mention in the "Refraction" article itself. $\endgroup$ – SarGe Jun 16 at 8:21
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Refraction describes the change of direction of a light beam in geometrical optics. Since your the light beam does not change it's propagation direction, there is no refraction.

I guess you are somehow mixing the phenomena and its explanation: The change of angle is a phenomena. Its explanation is that the speed of light changes according to $c_0/n$. We are not allowed to reverse this "logic".

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