Isoptic

In the geometry of curves, an isoptic is the set of points for which two tangents of a given curve meet at a given angle. The orthoptic is the isoptic whose given angle is a right angle.

Without an invertible Gauss map, an explicit general form is impossible because of the difficulty knowing which points on the given curve pair up.

Example

Take as given the parabola (t,t²) and angle 90°. Find, first, τ such that the tangents at t and τ are orthogonal:

<math>(1,2t)\cdot(1,2\tau)=0 \,<math>

<math>\tau=-1/4t \,<math>

Then find (x,y) such that

<math>(x-t)2t=(y-t^2) \,<math> and <math>(x-\tau)2\tau=(y-\tau^2) \,<math>

<math>2tx-y=t^2 \,<math> and <math>8t x+16t^2y=-1 \,<math>

<math>x=(4t^2–1)/8t \,<math> and <math>y=-1/4 \,<math>

so the orthoptic of a parabola is its directrix.

External links

• Mathworld
• Jan Wassenaar's Curves
• isoptic, orhtoptic in French but with good illustrations