A small note: As I play catch up, I am without many images to upload. I shot some high sped footage of the wave interference patterns, but they are at school. So, I posted some videos at the end of this post. Hope you enjoy it,
So after busting up the Particle Model of Light, and coming up with waves as a possibility, we decided on a prudent action: lets investigate the dickens out of wave properties first. It was of some concern that we may have figured out that light isn’t exactly like particles, had we spent ore time finding ways that the model might not work.
Of course this provided a really great teachable moment; not just for Physics, but science in general. Going back to the start of the year, I had the kids do a reading from a excerpt from one of Feynman’s books. In it he talks about the need to grow our understanding, that some times we might have to use a wrong idea, that works in some instances, before we can move to a greater, deeper understanding. I believe his example dealt with the Law of Conservation of Mass.
Any ways, we talked a bit about how the Light Model still has some really great aspects, and does work in many ways. If we had decided to move on to some other study in Physics, we could have just walked away and been fine. But since we saw a light phenomenon that could not be explained by this model, and we are still talking about light, we have to make changes or additions to our LIGHT model, which is really the point of the class.
So, we began to talk about waves. I did not restrict this investigation to EM waves, rather I pulled together ideas from the Modeling Curriculum for both Mechanical Waves and the Wave Model of Light materials. I think it is worth the time to look at mechanical waves (though we do not dive into compression waves) as investigating waves in this manner, gives us something concrete to see and manipulate.
So we spent about 3 days investigating a bunch of different things with waves: wave motion, reflections (fixed and free end), waves entering or exiting mediums, interference of waves, and speed of waves.
I tell them things I want to them to try, and then they have to document what they did, how they did it, and what they got for results. It amounts to 8 tasks all together once broken down. With 5 groups, each group had 3 tasks to do, and then we do a big white board museum at the end to share information. I then summarized it with them after wards.
From the investigations we develop an understanding of fixed-end and free-end reflections (important for standing wave formation, and also behaviors at boundaries), Interference patterns (important for standing waves and also basic wave interactions), and some ideas about things that affect the speed of waves.
The speed of waves is really one of the things that I want to end with in this investigation. A couple of groups have the task of seeing what things may or may not have an effect on wave speed. This list usually involves tension of the medium, the material it is made of, and how long the medium is. Some of the groups will run little test, but they can be inconclusive. For example, if you stretch a spring and make it longer, you are also changing the tension in the spring. So timing a pulse as it travels does not really work here.
This issue also really traps the groups that are tasked with finding a method of finding the wave speed. Most go right to a method used in mechanics: lets make a pulse and see how long it takes to travel from one end to the other. However, if you start changing parameters, you run into the above problems.
So to close we talk about ways to get a distance/length measurement (meters) and a time measurement (seconds), so we can do this. Inevitably a student will shout out that we could use wavelength for one, but we do need to push forward to get to frequency. I mentioned that this property of the wave has units of 1/seconds, and thankfully someone said, “Well then we’ll graph the inverse of it, and solve that problem.”
Looks like we have the start of a lab!
A recent Smarter Every Day post that deals with pasta and how it breaks. I have done this a million times, and just thought it was annoying that pieces always flew everywhere. Now it is just cool.
And for Sci-Fi junkies or comic book geeks out there, the much un-hyped trailer for the new Fantastic Four. I for one am interested.