Tuesday, June 23, 2015

Samsung's "Transparent" Safety Truck

They finally did it: a truck with an index of refraction n = 1.0. Well, no. But still, this uses readily available technology to do something very groovy.

This seems like a great idea. It should be mandated for any large vehicle (trucks, RVs) on two-lane roads.

Samsung Safety Truck (Versión en Español)

Samsung Safety Truck (Versión en Español)

Fragile floating rainbow whales

These ephemal bubble clouds flourish where the breezes are gentle and humidity is high. Small children delight in seeing them, but are also all to eager to destroy them. Still though, these amoeba-like examples of fluid dynamics, surface tension, and thin film interference are mesmerizing.

Giant Stinson Beach Bubbles

I especially love the longitudinal shot from behind the bubble master. The disintegration is magical.

Monday, June 22, 2015

Down Periscope!

Using the loosest form of the word, I am the advisor of our high school's AVBotz Robotics Team. These students are self- or peer-taught and exceed the electronics knowledge I can bestow on them in a regular Physics classroom. They have built an autonomous underwater vehicle (AUV) and compete in the International RoboSub competition every summer against universities such as CalTech, Cornell, Penn State, ASU, etc. (Notice I said universities!)

When the sub is being water tested some students are on the deck editing code on their computers at a safe distance from the pool while others are in the water to manipulate the sub and task props. At a recent water test we frequently asked the swimmers what the sub was doing since we couldn't see it in detail from above the water. The poor swimmers had to duck underwater then return up to say "It's heading straight," duck down and up again to say, "Now its dropped a foot," etc. as the sub was going through maneuvers.

It is probably the first time in my life I thought, "I need a periscope."

Although it wasn't the first time I had thought of building one. The Exploratorium's "Square Wheels" book of demonstrations by Don Rathjen and Paul Doherty has a project called "Periscope With A Twist." The instructions explain how to make a PVC periscope you can twist and see how the orientation of the image changes. I knew I would have to modify the design because I wanted to have part of it underwater. As light moves from air to water it refracts or bends because light waves travel more slowly in water. If I had used the original design the mirror I was looking into would give me a view of the water level within the tube but would not allow me to see underwater. I needed to seal the mirror with something transparent at least on the end that would go into the water. I decided sealing both sides would be best so I wouldn't ever accidentally dip the unsealed end in the water.

So I was off to TAP Plastics and got two of my favorite planar mirrors for a few dollars each. I was planning on cutting some transparent scrap to seal off my ends when I found pre-cut circles 4 inches in diameter. It required me to up my PVC pipe size but I found 4 inch solid PVC drainage pipe and two 90 degree elbows to fit. The pipe only came in 10 foot lengths so for awhile I awkwardly maneuvered my son in a shopping cart through Home Depot while holding the pipe vertically. (FYI the fine-toothed hack saw in the molding aisle is the best cutting option if you didn't bring a truck.) The TAP Plastics bill was $10; the pipe was $10 but I have enough for two more periscopes at least and the two elbow joints were less than $7. That makes the total cost of the raw materials to be about $20 per periscope.

I was pleasantly surprised to find that because I had increased the pipe size I did not have to cut the elbow joints to hold the mirrors as in the original plans. The mirrors fit nicely into the pipe and I was able to hot glue them in place (A). The hot glue job is not pretty; it was difficult to glue a rounded corner of a planar mirror to the inside a PVC pipe on a curve when I wedged all four corners in at once (B). But they seem secure! The 4 inch circles fit nicely into the elbows and a bit of hot glue secured them (C). There was not enough hot glue for me to feel that they were waterproof though. I used window caulking around the circles and the ends seem water proof (D). I used PVC glue between the elbows and the straight piece. I added some more window caulking along the straight pipe and elbow seam even though it was glued (E).

I had a periscope! Of course I had to test it out in a pool and my daughter was happy enough to help me out. As I expected the periscope is very buoyant, it is almost 4 feet of air filled tube after all. If you hold one end in water only one elbow will be submerged; it takes some force to hold more of the periscope under water if you would like to view objects deeper. If I had not been holding the periscope for my daughter it would have risen up too high for her to look through.

I do plan to make another periscope that is not sealed and not glued in place so that I can use it as the original project plans intended. There may also have to be a third one built for my kids; they don't like watching me make toys for school that they don't get to keep. Explaining this periscope alongside an unsealed one will bring up refraction, planar mirrors, image orientation, buoyancy and more!

Saturday, June 13, 2015

A giant eyeball… in your classroom!

When we talk about the refraction of light in physics it can seem abstract to students which surprises me since it is everywhere in their daily life. I like to bring up the convex lens of the human eye as an example and despite having a lot of AP Biology and anatomy students, they really haven't put the two concepts together. I model the way the eye works to my class by focusing light from outside our classroom onto a piece of white paper through a convex lens. I usually send a student outside to move around so the rest of the students can see his/her image and darken the room. The comments they make in disbelief are similar year to year: "He's upside down!" or "OMG its moving!" or my personal favorite, "She's in color!"

This year I added my own version of the Exploratorium's eye model exhibit using an old laundry room light. Since it was translucent white already, I masked off all but the top and spray painted it black. The original exhibit has a two foot diameter plastic sphere about the same shape that is clear except for a white colored portion the size of what I left white.

This demonstration would not work if I did not have a convex lens with a focal length that matched the height of the light. Luckily I had a magnifying glass that was very close and focuses an image on the white portion is held just outside the bottom of the light. If I push the magnifying glass right against the bottom of the light the image is not focused and it models how slight changes in focal length can create vision problems. (Although the focal length isn't changing the slight position change of the lens means that the retina isn't at the focal length and thus produces a fuzzy image.)

From there I show some basic images of nearsighted and farsighted lenses and discuss how their focal length varies. I ask students to decide if a convex or concave lens would fix them and we discuss how they would adjust the focal length by making it longer or shorter. Its interesting to take a poll of the class and ask who is nearsighted and farsighted; there are more farsighted than nearsighted people and it's rare to have a near-sighted student in class. Before I ask the students though I try to guess if they are nearsighted or farsighted based on the appearance of the eyes. A nearsighted person will have eyes that appear smaller since a concave lens is used to correct it; although it is easier to see if the sides of their face are closer together than the rest of their face when viewed through their glasses. A farsighted person will have eyes that appear larger since a convex lens is used to correct it.

And to bring in your more literary students you can reference the optics mistake in The Lord of the Flies. The character Piggy is described as being nearsighted (myopic) which would require corrective concave lenses. Early in the book Piggy's glasses are used to start a fire, something that could only be done by focusing sunlight from a convex lens. Tell your students to bring that up to their English teachers; it's cross-curricular learning!

Color subtraction—reflected color

They say necessity is the mother of invention and it's very true for teachers. I found myself wanting to do a reflected color (color subtraction) lab in my Conceptual Physics class that was observation based and introductory before a lecture. An internet search found nothing I could use so I had to come up with something myself.

NOTE: Any reflected color demonstrations or labs have to be in utter darkness. Any light from an outside source can be reflected off your object and will not produce the results you want.

At first I wrote one that used my light ray boxes and shone light through filters on a paper that students would color with Mr. Sketch markers. When I tried the experiment myself though, I wasn't getting the results I wanted using the filters. I turned to my trusty Inova Microlights in red, green and blue and found that they worked well when shone on the same paper but I didn't have enough for lab groups nor the time to get to an electronics warehouse that carried them locally. It was the night before I wanted to do the lab and I felt like a first year teacher not knowing what I would do the next day.

I thought if I didn't have a light source in the colors I wanted then perhaps I could make them using my computer. I started making images in the colors I wanted and trying to project them before I thought, "Someone must have done this already." Sure enough I found an app called Color Light Changer (free version) which did exactly what I needed it to. The program allows you to choose colors using either HSV, RGB or HEX color systems. To produce as colors with the right amount of red, green and blue as possible and for easy student manipulation I chose RGB. I set it up to produce red, yellow, green, cyan, blue, and magenta with little transition time. [See also RGB Colors and RGB Explorer.]

Now I had a reliable light source in the colors I wanted but I had to still find somethings that were the right colors to reflect that light. Since my kids were in bed as I was contemplating this as I put away their toys and found myself looking at exactly what I needed. First I used a set of four wooden balls that were red, green, yellow and blue. With the help of my patient husband that just wanted to go to bed I took this video of the four wooden balls under the changing light.

I only had one set of the wooden balls which wouldn't work for eight lab groups and I really wanted my students to try it themselves. I was pleased with the results but found that I wanted the other two primary pigments, magenta and cyan. I found one or two Duplos pieces in magenta but since we purposely don't have many "girl" Duplos I didn't think I'd have enough. But after dumping my kids' entire Duplos collection out onto the floor I managed to find enough magenta and a surprise store of cyan! Although humorously heterogenous, each primary pigment and secondary pigment was represented for each of my lab groups.

At this point I was promising we would be able to go really soon ... just as soon as I made a second video using the Duplos pieces:

For the actual lab the next morning students either used their own phones after downloading the app or used one of a few Samsung tablets that I borrowed from another classroom and preloaded with the app. Each group was able to observe the reflected color from the Duplos and of course anything else they could get in front of the light. It ended up being simple to do, relatable and more importantly for any light experiment it worked like it was supposed to.

Color addition egg

I enjoy teaching color and light not just because its fun hands-on physics but because I get to blow my students' minds. There is a special pep in your step on days you get to mess with them and tell them their elementary school teachers lied to them. My color and light lecture days are almost as fun as Van de Graaff demo day when I shock them repeatedly; almost.

Students "oooh and ahhh" over color addition; they never believe that red and green make yellow until they see it and some of them not even then. I have a few favorite color addition demos: "Colored Shadows" from The Exploratorium Snackbook; a Mysterious Glowing Ball from Educational Innovations; and this year a color changing egg. I found this at a local toy store and bought it for my son for Easter. It cycles through colors and I ask my students how it is able to create six different colors. Most students come to realize that there are only three LEDs in the egg; sometimes only one is on and sometimes two.

Since my daughter broke the first one we bought (she wanted to play with her brother's toy—she replaced it with coins from her piggy bank) I plan to take it apart so I can show my students the inner LED lights without the translucent white egg exterior. At less than $10 from a variety of sources (Amazon, etc.) it is a cheap and easy demonstration of color addition. Below is a video I took of it cycling through colors although it is better to see it in person.