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For part (b), the authors state thathecis a right triangle.

Also, the wavefront (cg) is clearly not parallel to the wavefront (he).

However, isn't this impossible?

If point (c) on the wavefront is part of (cg), which istangentialto a circle whose center is point (e), wouldn't (he) necessarily be parallel to (cg) (given that angle (hec) is a right angle) ?

To rephrase my question: how can angle (ecg) not be 90 degrees, given Huygens' principle?

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# Derivation of Snell's law from Huygens' principle

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