Inverse Pythagorean Theorem
Two light sources A and B shine onto a screen located at the triangle's right angle. As you move A and B along the hypotenuse, the green and blue triangles scale and rotate, while their opening angles stay the same. In the limit of a very small screen, those triangles represent light cones emanating from A and B. When the light sources meet at the altitude's foot, the light cones combine into one of a single light source shining onto the screen.