Diffraction and interference around us

For years and years this has been my least favorite part of school’s curriculum. Too much messy drawing on the board, too much mathematical formulas, and from student’s point of view this was just empty talk. I naturally tried and tried to explain this to my students as best as I could, but even with experiments, that I could demonstrate during the class, I’ve felt that my students been missing the point. They would just try to memorise whole thing without any critical thinking, and let’s face it, what kind of a teacher would I be to let that happen? Most of my colleagues agreed that this is not an easy part of curriculum which it’s hard to be understood even by older students at faculties, and that it is meant only for top of class.

Now, don’t get me wrong. I am not saying that this was unnecessary part of school’s curriculum. This is fundamental part of physics. This was one of the first mistakes made by Newton. And shockingly this is where quantum mechanics starts (thank you Feynman). So I really needed to help my students learn more about this, but how?

So what do I need to do to help my students understand diffraction and interference?

Well, here is my plan, step by step. I already did something similar, with lessons from Waves and Acoustics, and results were amazing. My students loved it, their grades were higher and I even managed to save time, as it turned out that students can learn much faster by themselves than I was able to teach them. So actually this method proved to be faster, more active, more fun and student’s results were higher.

So what teacher does not dream about this kind of activity?

STEP 1: More active students

What if we switch roles? What if I let students teach other students? And I don’t mean in classical way where student would stand in front of the class and act as a teacher. I wanted my students to teach each other the same way like they are group studying before tests (I am sure you have noticed groups of kids in hallways preparing for the next class). What I did was to split my students into groups and made sure that each group has at least one gifted student.

During next classes students worked in groups. They prepared lessons at home, and then discussed about it during the class.

Just look at this scene

My role was simple. I followed them, answered all the questions and uncertainties they had, and asked them questions to make sure they understood correctly.

I have to be honest and say that this is not a new idea. This is a kind of combination of two methods Peer learning and Flipped classroom.

STEP 2: Prepare materials

You can’t teach without materials. But if you just try to search the web for diffraction and interference you would probably get lost in the sea of information. So how can I steer my students through this oceans of information?

This is one of the cons of flipped classroom. It requires a lot of preparation from teacher. I decided to use a tool called SUTORI. This allowed me to create a presentation with all the videos, experiments, explanations… I needed and wanted my students to see. Of course SUTORI was my choice, you are welcome to use whatever else you like.

In my diary (quick note this is not original diary, this one is translated to English, since original was in Serbian, so it would not help you) you will find more detailed instructions. But keep in mind that is meant for students , and for teachers there is a lot more to be done before performing this activity in classroom.

STEP 3 Experiments, not demonstrations

This is my favorite part. I did not just show my students phenomenon I actually made them do this during lessons.

We showed diffraction in differents surfaces (papers, pencil’s leds, CD…)

Example of diffraction on left and regular laser spot on right

Diffraction grating may not be that expensive after all. I bought one used for telescopes and after little cutting ended with 4 gratings. Enough for the whole class. During the lesson on diffraction on grating we used simple experiment (we used cloth pin instead of holder).

But my favorite experiment was the next one.

Did you know you could determine thickness of human hair by measuring position of maximums? (I prefer explanation in this link but it is in Serbian) What I did was: brought three small lasers in class, sticky tape, scissors, graph paper, rulers and measuring tape. Students had to figure out for themselves how will they set up an experiment!!!

And their ideas worked pretty good. Here is what you can do with one hair, one euro laser, some books and a sticky tape.

No need for expensive equipment when you have students like this.

If you wonder was it good enough just look at next picture.

Perfect picture of diffraction on something as small as human hair

Finally, when introducing single slit diffraction we decided to, once again, use something unusual. Two pencils (make sure to use one with rubbers cause that is what makes a little slit between them)

Improvised single slit, but works just as great

One more advice, this time you’ll need stronger laser (we bought 1W green one for 10 euros, but I don’t think it’s nearly as strong as it says), and distance has to be at least 6 meters , but the end result is shown in following picture.

Interference pattern using two pencils

Step 4 Presenting and grading

Grades are important. It’s not just that my principle, school supervisor, parents and others demand grades. This is also a part of seeing how much did your students learn. Well I decided to give them, a little boost before test. Half of their grade was achieved during experiments. You can see more in rubric I created. This meant that I had to observe them during classes, notice which students are active and which not, and help them to get more into it. After all, teamwork is one of the soft skills we try to develop.

At same time it’s just as equally important to see what have your students learned. And that is why I decided to give them a test. In order to prepare them better I created an online test, that was there just for them to see how well they understood so far.



So let’s sum up what I actually did?

Students, split in groups, performed experiments in class and during that process learned theory. During classes students learned from each other, discussed, solved problems… Teacher was in a role of mentor, guiding students, helping to clear uncertainties, and prepared material for the class. All material needed was prepared in SUTORI and can be used for next year as well. For more details look at my plan for teachers.

So what are pros of this method?

  • More active students
  • More fun in classroom
  • Better quality of knowledge students gain
  • Less preparation for test
  • Developing problems solving, teamwork and other skills

And what would be cons?

  • Students need more time to prepare at home
  • Most of the students are new to this idea
  • Teachers have to pay more focus in classroom
  • Teachers have to spend more time in preparing materials

In other words you work hard but you get good results. I am aware that this activity demands high level of knowledge among students (17 -18 years in my case) but there aren’t many STEM activities for students of that age so I hope this helps someone.

Remember, no matter what lesson you choose, you can always prepare classes by this model.

2 thoughts on “Diffraction and interference around us

  1. Very interesting post! I also teach Physics at high school and I was fascinated by this topic for many years now. You did a great job, and the teaching resource you created is a valuable one! I’d like to share with you my experience regarding Young’s double slit experiment, as well as some ideas of exploring interference on thin films in white light. There are so many experiments easily to perform related to this chapter!

  2. Thank you. I was looking for someone for a project, maybe eTwinning or on some other platform.

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