I saw a diagram that showed a light beam hitting a curved glass block (half circle) and nothing happened until it hit the flat surface, in which it refracted. Why doesn't it also refract when it hit the curved surface. I also tried this in real life with the same instruments and it proved to be so. So why?


Image of diagram: https://postimg.org/image/6kukkz3it/

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    $\begingroup$ I'm guessing your light beam was thin and directed radially? $\endgroup$ Oct 17, 2016 at 3:47
  • $\begingroup$ @WetSavannaAnimalakaRodVance I don't know if the case is circumstantial as this question was also part of the report in my class. $\endgroup$
    – John Doe
    Oct 17, 2016 at 3:48
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    $\begingroup$ Of course light refracts on curved surfaces in general: that is how lenses work. So the most important take-away here is how you know from the drawing that the light is normally incident. Con you see that? $\endgroup$ Oct 17, 2016 at 17:26

2 Answers 2


Refraction is proportional to the angle away from the normal (away from right angles to the surface). In this case the angle is zero (the light enters perpendicular to the surface) so there is no refraction.


Well, your light ray must have hit the curve at an exact point where the angle of incidence was zero, (the light ray was perpendicular to the exact point at which it hit the semi circle), which means there was no refraction.

But in general, a ray of light can bend when it hits a curved surface, it just depends at what angle it hits, and what place on the curve it hits. The thing about a curved surface is that each of its points is at a different angle and thus has a different normal and thus refracts light coming from the same direction differently. On the diagram it just happened to be hitting straight on, so no angle of incidence, so no refraction (as I've already said). (I think, don't take this answer for sure).


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