T-8 Days and Counting

In-Service officially starts up for us next week Tuesday. Since my wife is at work all week, and my daughter Kira is off on an adventure in New Mexico, I find myself sitting at work, getting ready for work.

Lesson planning for the first few days of school is always exciting and daunting. Exciting to get a new, fresh start. Daunting since those first few days are so important. I feel the pressure of setting a tone. “This is how things are going to work this year!”

It has to be flawless out of the gate. Checking and double checking everything. Making sure everything is set. Its extra exciting/daunting with a new prep as well.

Any way, enough of that. Here are a few things that I have been watching this morning, trying to get ready to go.

I love this discussion by Feynman on the Scientific Method. Its humor filled, and on the money. Which of course basically describes Feynman to a T.

Over the summer I tend to fall behind on my Smarter Every Day watching, so here is a great video set from Destin and also from Veritasium about toilet bowl flushing.

I love it when Destin says,”It’s never been done on the internet. You’re gonna learn something!”

Last Day with Seniors!

Back in June of 1998, I was sitting in one of my last classes as a senior in high school. I don’t remember how, but I won a bottle of bubble solution. In a moment that was not very typical of my personality, I proceeded to blow bubble everywhere I went, for the rest of the day.

Later that day I went to my Engineering Drafting class. This class was taught by an ex-Airborne Ranger, who was a great guy, but not always tolerant of odd behavior.

“What are you doing,” he asked, puzzled by the bubbles.

“Blowing bubbles,” I said, obviously.

“Why?”

“Because I am 18, graduating high school, and I can.”

This got me yet another odd look. This type of answer was a little defiant for me, but I was feeling good about my last day of school. The teacher thought about it for a second.

“Understood. Carry on,” he told me and walked away.

So I told this little story WEEKS ago in Physics class when we started talking about wrapping up high school. It was just a little, funny story about my past, that I like to share with the kids.

2nd hour today, one of my seniors who wrapped things up early, was standing outside my door, blowing bubbles at the door. I stopped lecturing, and just laughed. I walked over to the door, and said:

“Because you can, right?”

“Yep!”

Cool moment.

But then Physics rolled around, and this happened:

photo1 (1)

They all had the bubbles waiting for me. the seniors planned it, one kid brought them in. Anyone who knows me, knows I don’t get teary-eyed, emotional, easily, but this picture really hits me hard. What a great group of kids. I’m going to miss this bunch.

Days 94-98

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,

Let’s begin.

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 use a bunch of different spring types: Snakey Springs, Demo Wave Springs, Giant Slinky’s, and plastic Slinky’s. This allows students to test different things with different medium and tensions.

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!

Some Videos:

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.

Day 93

So today we started 2nd Semester, and decided we would look at some of the stuff we understood, or thought we understood, about the Particle Model of Light.

I began by asking 3 questions:

1) If light behaves like a particle, what should we expect light to do if streams of light particles try to pass through each other? (we never went out of our way to look at or address this during 1st semester)

2) Why does light refract? When will it bend towards the normal or away from the normal?

3) If we shine a beam of light particles through a tiny opening, describe and diagram what happens. If we make the opening smaller, what changes, if anything.

Responses to each of the three questions can be summed up as follows:

1) they should bounce off of each other, and cause scattering of particles. We should be able to see these particles. (some kids said the light intensity should be greater too.)

2) Light speeds up or slows down as it passes into a new medium, and it will bend towards the normal when it slows down.

3) The opening will only allow some light to go through, and we will see a tiny dot of light. If the hole is smaller, then less light goes through, and we see a smaller dot of light.

Well, lets test it out!

Demo 1: I took two lasers (since this would be a really high concentration of light particles traveling together) and had them cross though each other. First, we do not see any scattering. If the light particles were hitting and colliding, then they should collide, scatter, and we would see them. Second, even when we spray disco fog into the beams, we do not see the lasers scattering any light. They just pass through each other.

What does that mean? Well, maybe not much. Students brought up the times I said that light is not a physical particle like a ball. They also mentioned that it was simply a model, and not meant to be taken literally (light = mass particle). It just transfers light energy.

Demo 2: So the setup: I put a big white board leaning up to a table like a ramp. That’s it. If you roll a ball along the table top, at an angle to the ramp, once the ball gets tot he ramp it rolls down the ramp, bending inward towards the “normal.”

Students: Well that did what we expected.

Me: Why is that?

S’s: Well it bent in towards the normal.

Me: But that happens when light enters into a medium with higher index of refraction. This was air and air.

S’s: Yeah, but gravity is always going to speed it up, and so it bends in towards the normal.

Me: but with light we said it bends towards the normal when the light slows down. Here it bent inward and sped up!

S’s:….oh.

So now there is a bit of a problem with the model, a particle does not behave like we thought it would, so there is a bit of disconnect between our particle model and how light behaved. Still, students argue that it can’t be compared because of mass and gravity issues. Still, it does introduce a bit of dissonance.

Demo 3: A laser beam is shown through two razer blades. At first the students see the small dot of light that they predicted. That’s when I move them closer together, and wham, you get something like this.

That is NOT what was expected. A single stream of particles has been separated into a series of streams of particles. It is not an illusion either. If you use a bright enough laser in a dark enough room, disco fog can be sprayed and students can see the lines of light travel across the room, separating from each other, and the empty spots in-between.

Now the cognitive dissonance is at a high. What does all this mean? maybe a particle model of light is not sufficient to explain how light behaves. We need something more.

But we cannot completely ignore a lot of what we know already.

What else can transfer energy?

Waves.

Day 60-92

Well that was a little bit of a break. What happened? Well a lot of things: Deer Gun Season, Thanksgiving, hunting again, a field trip to Trees for Tomorrow, winter break, Reality Check, the ASVAB, professional development days, and semester 1 final exams.

A colleague of mine, Ryan Peterson of Brillion HS, asked me when I started this blog, “So how long do you think you can keep this up for, with everything else that goes on?”

Well apparently, I found my limit. Still, with a new semester starting up, I thought I would give it another try. So I am going to spend some time here recapping what has been going on in my Physics of Light class over the past 30 days.

Double Lens Debacle: We wrapped this unit up (see last post) and moved on from refraction of light to reflection of light.

Diverging Lens Lab: We also did a diverging lens lab. Using the two lens technique you can find the image placement of the diverging lens:

1) Set up the light source and the diverging lens. Place a Converging lens on the opposite side of the diverging lens. Use a screen to find the real image formed by the system

2) Remove the diverging lens, and move the light source forward until the image reforms on the screen.

3) The light source is now in the location of the virtual image formed by the diverging lens.

A great way to model diverging lenses, and to get kids talking again about why this works, a lead in to diagramming the situation, and why this technique works!

Modeling Plane Mirror: I start this unit out by having students do a small investigation using plane mirrors. The question they get asked is, “how much mirror do you need in order to see all of yourself?” I have some long, plane mirrors, the kind that hang on the backs of doors, so you can see your entire self in it. The kids notice that they can see themselves in it, but upon discussing things, most beleive that they could simply back up from a regular bathroom mirror and see themselves too.

We can’t really test this, as most students agree, the size of a bathroom doe snot allow you to step far enough back for it to work. So I suggest that we use the mirrors we have, and see how much of it we really need. The thought is that as you move backwards, you will use less mirror, and thus be able to see yourself.

Here is the resultant data:

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As you can see, there is no difference in the amount of mirror you need, as the slope comes out to effectively zero. In discussing results though, groups notice that they each got different y-intercepts. Some student usually notices that if you rank the order of y-intercepts as increasing, it is also similar to the increasing height of the test subjects (use a range of students with clearly different heights to aid this).

We find then by measuring peoples heights in centimeters, that the y-intercept is 1/2 the height of the person. So I guess you don;t need a huge, or tall mirror, just one half your height.

This leads us to want to investigate other properties of the plane mirror and the plane mirror lab. I use a setup that was first shown to me by Scott Hertting at Neenah HS. We use CD cases as the plane mirror (I also have a stash of clear DVD cases that work well too) and legos.

By placing the lego in front of the case, you can see the reflection of it in the reflecting surface. This allows you to place the second lego where you see the image, and take your measurements.

From this point on we looked at how to draw images for plane mirrors and also systems of plane mirrors: telescope situations, mirror mazes, etc. I also gave out a diagramming challenge to students:

1) a piece of aluminum square bar, has four reflecting surfaces on the inside of the bar. Diagram a primary, a secondary, and a tertiary image for this situation.

Al Bar Reflection - Zoom Out

2) If an object is placed between two plane mirrors making a 60-degree angle between them, diagram the primary, secondary, and tertiary images formed.

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Bridging Activity from Plane Mirrors to Curved Mirror Systems

In order to get kids thinking about how a curved mirror might work, we use a similar technique to the bridging activity we did to get from refraction to lenses.

Converging Mirrors: Once we get a chance to use a curved surface, we begin by modeling a converging mirror. If you have read my older posts, you know I do not use the terms “Convex” or “Concave.” I find that students get confused that a convex lens and a concave mirror form real images, but a concave lens and a convex mirror form virtual images. Using the terms converging and diverging not only  get the students thinking about how the light behaves as it reflects (or refracts) off of a surface, but it brings the lens and mirror systems together, and makes the student consider one less thing: all converging surfaces create real images, all diverging surfaces make virtual images.

So at this point we did a converging mirror lab: We used the optics benches (attached to the dynamics tracks) with a converging mirror and a half screen (to get the image location.) If you don’t have the half-screen setups, you can use cardboard to slip into the path of the light, you will get the same results.

Students are actually stunned, but relieved, that the geometry of the situation has not changed, and the models we created for the lens situations still work.

But the plane mirror is different, right? Nope, it also works!

CIMG1961

I love science. If we consider the focal length to be infinity, then di = – do, which is what we should get for the plane mirror!

Diverging Mirror Lab: We can’t just take at face value that the diverging mirror will also work the same as the other surfaces, so lets investigate! We use a setup sort of like the plane mirror lab, but it is a little more challenging to get good data.

photo 1 (7)

Basically if you get the nail (which you can barely see) to be lined up with the virtual image in the mirror, then you can measure the object and image distances. In order to tell if the nail behind the mirror is int he right place, students need to move their heads side-to-side and see if the image stays lined up with the nail behind the mirror. If the nail is in the right spot, you will notice that they stay lined up.

Some kids are awesome at this, others really struggle. I always setup two stations, one that shows what a good result would be, and one that still needs adjustment. That way kids can see what THEY should see. It is helpful.

The lab results here look just like the results for the diverging lens lab. The Lens makers equation works for all lenses and mirrors!

Magic Cylinder Activity: This was not the last thing we did, but it did happen towards the end of the quarter. Basically by stretching an image out so that it matches the curve of a cylinder, the reflection in the cylinder looks like the regular image. It was a fun way to wrap up the particle model of light.

Here are some examples:

So that is it in a nut shell. If you want to know more about any of this, let me know. contact me at tschwaller@shiocton.k12.wi.us or on Twitter @mrtschwall

Day 58 and 59

It took some time to prepare white boards yesterday, so the white board presentation spilled over into today.

These problems were pretty challenging. I will talk here about 3 of them.

the first had the students figure out where to put an object and two lenses (both f = 30 cm) in order to get a final image that is 12x and inverted compared to the original object.

They figured out pretty fast that the first image had to be virtual to get the proper orientation of the final image. What came next though was a lot of trial and error. In order to get the 12x they knew that one lens would have to magnify it part way, and the other finish it. so they tested 3/4, 4/3, 1/12, 12/1, 6/2 and finally 2/6. Of course only the last set up where the virtual image was 2x the object and the final image was 6x the virtual image would work!

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The next question asks the students to figure out two places an object can be placed to get an image that is 8x bigger. That is all it says.

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With the focal length being 30 cm, it means the data on the right is for a real image, and the data on the right is for a virtual image.

Finally we have a situation where the image formed by the first lens is actually beyond the second lens. How do you deal with that? Well, here there is no actual light coming from the first image/percieved object and into the second lens. As such, we can look at it as being like a virtual “object.” and use -do.

Kids didn’t like this idea at first, and even though you can punch it all in the calculator, why did it work? Well the light that should be forming that perceived object, is not yet fully converged. When it passes through the second lens it bends even faster. This forms the image between the 2nd lens and the perceived object.

“But that perceived object is right on the focal point for that second lens. so there should be no image.”

“That would be true IF the perceived object was actually there! Remember once that second lens shows up, it really isn’t there. It is virtually there.”

CIMG1885

Day 57

Today the kids worked on completing their challenge. I was asked if they should use the graphically determined focal length or if they should use what they thought it should be. I let them decide. I personally would have used the graphically determined value. Yes, it might make the math a little messier, but using the data collected for the setup, when you will be using that setup, makes more sense to me.

Results were mixed. 2 groups got 50% error, once group got 0% error, and one had such a slightly divergent lens, that do came out to be about +500 cm. Oops.

Now you might ask, how did we confirm the size of the virtual image? Well, with two lenses of course! I had a lens of known focal length (f = 20 cm.) Once the diverging lens and object were placed, I asked the students for their di.

With di in hand, I set my converging lens 40 cm away from their di. Why 40 cm? well, a lens of focal length = 20 cm has 2F at 40 cm. As we learned before, an object at 2f will form an image that is equal in size. So by placing my converging lens 40 cm away from where their virtual image formed, I could project the image onto a screen, and measure the size of my real image. this would be the same size as the virtual image.

At the end of the hour, I assigned them the task of redoing their double lens problems from last week.