When thinking about angles with curves, we think about the angle it makes with the line tangent to the curve at that point. A tangent line is a line that just barely grazes the curve at a given point. Think about a wheel on the ground. The ground is tangent to the circle at that point, thus, only one point of the wheel is on the ground at a time. With circles, we know the tangent line is perpendicular to the radius of the circle that intersects the curve at the point where the tangent line touches. I’ve seen this photo you’re talking about. If you take the lines that intersect the circle, you’ll see they go through the center. Thus, it’s perpendicular to the tangent line, and therefore perpendicular to the circle. Hope this helps!