What you need to do
Mark a spot near the middle of the piece of paper. This mark will be the focus of the parabola.
Along the bottom of the piece of paper, mark a series of equidistant points. In this example, we’ve used 13 dashes. If you’re using lined paper, then it is already marked, and you could use every alternate line. For ease, the marks can be labelled, with 0 directly under the focus.
Repeat the process, this time using the marks labelled -1 and 1. These scores are also perpendicular bisectors of the lines joining the focus points on the directrix. They’re tangents to the parabola, and start showing the edge of the curve.
By the time you’ve repeated this process a number of times, the parabola becomes very clear. It’s even clearer if you look at the other side of the piece of paper.
Try experimenting with making the focus point at different heights on the page. How will the parabola look if you place the spot above the centre of the piece of paper?
What happens if the spot is placed below the centre of the paper?