In the GUI below, click on the button "select ray" to select a ray. When this button has been selected, left click to select the endpoint of the ray, and right click to select another point which determines the direction of the ray.

To select a line segment, click the "select line" button. Then, left click to choose the first point of the line and right click to choose the second point of the line. Once you've filled in rayReflect in rayReflect.js, it should show the reflection