I am having difficulty accepting an illustration from a physics textbook. The illustration is attached. For part (b), the authors state that hec is 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 is tangential to 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?