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:


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.

photo (2)

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!


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 or on Twitter @mrtschwall


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