# Parabolic Properties from Pieces of String

I am pleased to announce that my latest paper, “Parabolic Properties from Pieces of String,” has now been published in Math Horizons. This was a really fun project for me. As I describe in the paper, I started wondering if it was possible to convince a student who hadn’t learned calculus yet that string art from two line segments traces a parabola. Not only was I able to come up with a way of demonstrating this without calculus, but I was also able to (1) prove that a quadratic polynomial satisfies the focus-directrix property of a parabola, which is the reverse of the usual logic when students learn conic sections, and (2) prove the reflective property of parabolas. I was really pleased with the final result, and am very happy that this was accepted for publication.

Due to copyright restrictions, I’m not permitted to freely distribute the final, published version of my article. However, I am able to share the following version of the article.

The above PDF file is an Accepted Manuscript of an article published by Taylor & Francis in College Mathematics Journal on February 24, 2022, available online: Full article: Parabolic Properties from Pieces of String (tandfonline.com)

# A New Derivation of Snell’s Law without Calculus

Last week, I posted that my latest paper, “A New Derivation of Snell’s Law without Calculus,” has now been published in College Mathematics Journal. In that previous post, I didn’t provide the complete exposition because of my understanding of copyright restrictions at that time.

I’ve since received requests for copies of my paper, which prompted me to carefully read the publisher’s copyright restrictions. In a nutshell, I was wrong: I am allowed to widely distribute preprints that did not go through peer review and, with extra restrictions, the accepted manuscript after peer review.

So, anyway, here it is.

The above PDF file is an Accepted Manuscript of an article published by Taylor & Francis in College Mathematics Journal on January 28, 2022, available online: Full article: A New Derivation of Snell’s Law Without Calculus (tandfonline.com).

# A New Derivation of Snell’s Law without Calculus

I’m pleased to say that my latest paper, “A New Derivation of Snell’s Law without Calculus,” has now been published in College Mathematics Journal. The article is now available for online access to anyone who has access to the journal — usually, that means members of the Mathematical Association of America or anyone whose employer (say, a university) has institutional access. I expect that it will be in the printed edition of the journal later this year; however, I’ve not been told yet the issue in which it will appear.

Because of copyright issues, I can’t reproduce my new derivation of Snell’s Law here on the blog, so let me instead summarize the main idea. Snell’s Law (see Wikipedia) dictates the angle at which light is refracted when it passes from one medium (say, air) into another (say, water). If the velocity of light through air is $v_1$ while its velocity in water is $v_2$, then Snell’s Law says that

$\displaystyle \frac{\sin \theta_1}{v_1} = \displaystyle \frac{\sin \theta_2}{v_2}$

I was asked by a bright student who was learning physics if there was a way to prove Snell’s Law without using calculus. At the time, I was blissfully unaware of Huygens’s Principle (see OpenStax) and I didn’t have a good answer. I had only seen derivations of Snell’s Law using the first-derivative test, which is a standard optimization problem found in most calculus books (again, see Wikipedia) based on Fermat’s Principle that light travels along a path that minimizes time.

Anyway, after a couple of days, I found an elementary proof that does not require proof. I should warn that the word “elementary” can be a loaded word when used by mathematicians. The proof uses only concepts found in Precalculus, especially rotating a certain hyperbola and careful examining the domain of two functions. So while the proof does not use calculus, I can’t say that the proof is particularly easy — especially compared to the classical proof using Huygens’s Principle.

That said, I’m pretty sure that my proof is original, and I’m pretty proud of it.

# Engaging students: Graphing a hyperbola

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Biviana Esparza. Her topic, from Precalculus: graphing a hyperbola.

B2. How does this topic extend what your students should have learned in previous courses?

Prior to learning about conics and hyperbolas in precalculus, students should be able to identify different shapes and figures and learn to identify cross sections of prisms, pyramids, cylinders, cones, and spheres, from geometry class. In algebra 2, students learn to write quadratic equations and learn vocabulary such as vertex, foci, directrix, axis of symmetry, and direction of opening, all which are used when dealing with hyperbolas as well.

How has this topic appeared in pop culture?

The sport of baseball originates back before the Civil War and has come to be known as America’s pastime. On average, 110 balls are used in a major league baseball game, because the balls are usually tossed out if they’ve touched the dirt. Baseballs have a rubber or cork center, wrapped in yearn, and covered with leather sown together tightly by 108 stitches of red string. The stitches are in a hyperbola shape if looked at from a certain angle and depending on how the pitcher has held the stitches, different pitches are thrown.

E1. How can technology be used to effectively engage students with this topic?

Desmos is a great, interactive website that has many activities that can be used in the classroom. One of the activities it has is called Polygraph: Conics. The Desmos activity is similar to the board game Guess Who? in which students are in pairs and will ask yes or no questions to guess the graph of a hyperbola or ellipse of their choosing. This activity encourages students to make good questions and use precise vocabulary and academic language when describing conics, specifically over ellipses and hyperbolas, so that they can win the game.

# Engaging students: Graphing a hyperbola

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Rebekah Bennett. Her topic, from Precalculus: graphing a hyperbola.

Hyperbolas are one of the hardest things to find within the real world. Relating to students, the hyperbola is popularly known as the Hurley symbol; A widely known surf symbol that is now branded on clothes and surf boards. It is also used widely in designs to create patterns on large carpets or flooring. They can also be used when building houses to make sure that a curve on the exterior or interior of the house is mirrored exactly how the buyer wants. Hyperbolas can be found when building graphics for games such as the game roller coaster tycoon. This is a game where several different graphics must be formed so that any type of roller coaster can be created. Also, when playing the wii or xbox Kinect, hyperbolas are used within the design of the system. Since both game systems are based on movement and there are several different types of ways someone can move, the system must have these resources available so that it can read what the person in doing. Hyperbolas are commonly found everywhere with some type of design.

To explore this topic, I would first show the students this video of the roller coaster “Fire and Ice” which is in Orlando, Florida at Universal Studios. This roller coaster was created so that when the two roller coasters go around a loop at the same time, they will never hit, making for a fun, adventurous time. This is what a hyperbola simply is; every point lies within the same ratio from focus to directrix. During the video point out the hyperbolic part of the roller coaster which is shown at the 49-51 second mark.

Now after watching the video, the students would be given about 8 minutes to explore by themselves or with a partner, how to create their own hyperbola. The student can use any resources he/she would like. Once the students have had enough time to explore, the teacher would then have the student watch an instructional video from Kahn Academy.

The video is very useful in teaching students how to graph a hyperbola because the instructor goes through step by step carefully explaining what each part means and why each part is placed where it is in the function. The video is engaging to the students since they don’t have to listen to their teacher say it a million times and then reinforce it. This is also helpful for the teacher because the student hears it from one source and then it is reinforced by the teacher, giving the teacher a second hand because it’s now coming from two sources not just one.

After the video, the students can now split up into groups of at least 3 and create their own “Fire and Ice” roller coaster from scratch. They will have the information from the video to help them know how to create the function and may also ask questions. The student may create their hyperbola roller coaster anyway they would like, using any directrix as well. But keep in mind that you would probably want to tell them it needs to be somewhat realistic or else you could get some crazy ideas. Once all the groups are finished, they will present their roller coaster to the class and be graded by their peers for one grade and then graded by the teacher for participation and correctness.

From previous math courses, the student should already know the terms slope and vertex. The student should’ve already learned how to graph a parabola. Everything that a student uses to graph a parabola is used to graph a hyperbola but yet with more information. Starting from the bottom, a parabola is used because all a hyperbola technically is, is the graph show a parabola and its mirrored image at the same time. From here the student learns about the directrix, which is the axis of symmetry that the parabola follows. The student will now be able to learn about asymptotes which are basically what a directrix is in a hyperbola function. This opens the door to several graphs of limits that the student will learn throughout calculus and higher math classes.