A Geometry QuestionCmaybe have something to do with specular reflection
Let us assume that a billiard ball which strikes a flat wall will bounce
off in such a way that the two lines of the path followed by the ball
(before and after the collision) make equal angles with the wall. Consider
$n$ lines $D_1,D_2,...,D_n$ in the plane, and points $A,B$ on the same
side of all of these lines. In what direction should a billiard ball be
shot from $A$ in order that it arrive at $B$ after having bounced off each
of the given lines successively? Show that the path followed by the ball
in this case is the shortest broken line going from $A$ to $B$ and having
successive vertices on the given lines.
Special Case. The given lines are the four sides of rectangle, taken in
their natural order; the point $B$ coincides with $A$ and is inside the
rectangle. Show that, in this case, the path travelled by the ball is
equal to the sum of the diagonals of the rectangle.
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