Let's say you have two points, and you want the light to travel from one point to the other, bouncing off a plane mirror along the way. In this lab, the first point is colored red and the other point is colored purple.

Set the positions of points 1 (red) and 2 (purple) by clicking-and-dragging them to the positions you want them to be in the air. Then, click-and-drag the third point (colored green) that is on the mirror - this sets the point on the mirror the light reflects from on its way from point 1 to point 2.

Your goal is to adjust the position of the point on the mirror so that the light takes the smallest possible time to travel from point 1 to point 2, and then you have to think about how this relates to reality. In other words, which of the many possible paths the light could take does the light actually take?