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Light changes its speed, because the new medium is either more, or less dense; but why does it change direction?

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  • $\begingroup$ Something to do with light taking the path with the shortest time. Imagine a point in the medium at the end of the line. To reach this light can go straight through or in the way shown. Since light travels faster in air, light via path shown would be first to reach the point. $\endgroup$ – sku Mar 11 '18 at 2:33
  • $\begingroup$ see also physics.stackexchange.com/q/59607 $\endgroup$ – sku Mar 11 '18 at 2:39
  • $\begingroup$ Answer to your question is the Fermat's principle. Light prefers to travel along the path which would take the shortest time. I will provide you a link. It would give you a great idea about what and how the Fermat principle proceeds. Here's a link: feynmanlectures.caltech.edu/I_26.html $\endgroup$ – Physicpsycho Mar 11 '18 at 5:30
  • $\begingroup$ Light does not behave like the left picture either. $\endgroup$ – Pieter Mar 11 '18 at 8:40
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Think of continuous wave fronts perpendicular to the direction of travel. At the left of the left image the wave front at a point is slowed down while the wave front at a point to the right of the left image is not slowed. As the wave front penetrates the refractor it is therefore distorted so that the front which was originally a horizontal line moving down becomes a diagonal line moving to the left.

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