The bright blue iridescence of the wings of this insect results from the way light refracts through the thin layered membranes of the wings.
When you look at the sunlight reflected off this black insect’s wings at just the right angle, they blaze bright blue. The phenomena is called iridescence, and results from the way different wavelengths of light refract through the wing membrane. Blue light is of just the right wavelength that the light reflected off the top of the membrane and the light that’s refracted through the membrane constructively interfere. The Natural Photonics program at the University of Exeter has an excellent page detailing the physics of iridescence in butterflies (Lepidoptera), and the history of the study of the subject.
Surface tension supports the water strider on the surface of the still water of the creek.
Down at the creek the water striders are out. They can stand, walk and jump on the surface of the water without penetrating the surface because of the force of surface tension that causes water molecules to stick together — it’s the same cohesive force that make water droplets stick to your skin. I got a decent set of photos to illustrate surface tension.
The water striders still create ripples on the surface of the water, even though they never break the surface.
The green canopy that over hangs the creek allows for some nice photographs.
One of the middle-school projects is to build a little solar water heater. By simply pumping water through a black tube that’s sitting in the sun, you can raise the temperature of the water by about 15°C in about 15 minutes.
The solar water heater in action.
Next year I want to try building an actual solar water heater, similar to the passive air heater my students built two years ago, with the tubing in a greenhouse box to see just how efficient we can make it.
Crayfish camouflaged against the rocks in the creek.
One of my favorite things about the Fulton School campus is the little creek that runs along the boundary. It’s small, dynamic, and teeming with life.
The crayfish are out in force at the moment. Some of the high-schoolers collected one last fall and it survived the winter in our fish tank (also populated with fish from the creek).
They are quite fascinating to observe; wandering around the sandy bed as if they own the place; aggressive with their pincers occasionally; but then darting backward amazingly fast if they feel threatened.
The one in our tank has just molted a second time, so now we have two almost perfect exoskeletons sitting around the science lab.
Crayfish exoskeleton. From pincers to tail the skeleton is approximately 10cm long.
The area around the Sun is blue on Mars because the gasses in the thin atmosphere don't scatter much, but the Martian dust does (it scatters the red). Image via NASA.
The dust in Mars’ atmosphere scatters red, while the major gasses in Earth’s atmosphere (Nitrogen and Oxygen) scatter blue light. Longer wavelengths of light, like red, will bounce off (scatter) larger particles like dust, while shorter wavelengths, like blue light, will bounce of smaller particles, like the molecules of gas in the atmosphere. The phenomena is called Rayleigh scattering, and is different from the mechanism where different molecules absorb different wavelengths of light.
Ezra Block and Robert Krulwich go into details on NPR.
Blue sky in the upper right, but the dust scatters the red light.
The discussion started off with projectiles. If you’re floating in space — zero gravity — and throw something, like a basketball, away from you, you’ll be pushed off in the opposite direction. In fact, if you throw something that has the exact same mass as you do away from yourself, you’ll move off in the opposite direction with the exact same speed as the thing you threw.
Then I brought up rockets, and how they’re expelling gas to move them forward. I think it was the phrase, “expelling gas” that did it. The next question, which the student brought up somewhat circumspectly, sidling around the issue and the language, was (more or less), “So if you expel gas in space will you move off in the other direction?”
The simple answer, appropriate to that stage of the discussion, was, of course, “Yes.”
Which lead to to, “What about spitting?”
“Yes.”
“What about, you know, peeing?”
“Yes, except …”
At that point I thought it would be wise to rein it in a little, and make a further point about the whole action-reaction thing.
“You see, if you expel anything, wouldn’t it just be stuck in your spacesuit with you? Then you’re not really expelling it, it’s still attached to you, so you wouldn’t really move. What would be more useful would be to collect it in something like a spray can or a squeeze bottle. Then you can just squirt it out opposite the direction you want to go in to control your movement.”
This produced a moment of thoughtful silence as they figured out the logistics.
Notes
I thought this was a useful conversation to have. The students were interested and animated. And I believe it’ll be memorable too.
An artist's concept depicts the Deep Space 1 probe with its ion engine operating at full thrust. Image via NASA.
P.S.: I’d wanted to talk about ion drives, which operate on the same reaction principle, but are much cooler (after all they’re used in Star Trek). Instead of burning fuel to create the propulsive force ion drives generate an electric field that ejects charged particles; we’d been talking about ions and charged particles earlier in the day. However, I decided on the day that it would just complicate what was a new issue. I’ll probably bring it up this week though as we recurse through Newton’s Laws.
PhD Comics does a wonderful job of explaining of sub-atomic particles: what we know, what we don’t know. What’s particularly great about this video is that it goes into how physicists are using the Large Hadron Collider to try to discover new particles: by making graphs of millions of collisions of particles and looking for the tiniest of differences between different predictions of what might be there.
I also like how clear they make the fact that science is a processes of discovery, and what fascinates scientists is the unknown. Students do experiments all the time and if they don’t find what they expect — if it “doesn’t work” — they’re usually very disappointed. I try my best to let them know that this is really what science is about. When your experiment does not do what you want, and you’re confident you designed it right, then the real excitement, the new discoveries, begin.
We successfully harvested and processed three chickens during last week’s interim. It was my first time going through the entire process, but fortunately we had a very experienced guide in Dr. Samsone who also happens to be a vet.
The interim focused on where food comes from (students also saw the documentary “King Corn”), and the cleaning of the chickens was tied into our Biology students’ study of anatomy (I’d done fish and squid before). Unfortunately, I was unable to find someone who knew how to read the entrails so we could tie the process into history and language arts as well.
Student holds a kidney. A heart is in the background.
When we were done with the processing and analysis, Mr. Elder cooked the chickens on our brand new grill (which worked quite well he says). The chickens were free-range (donated by Ms. Eisenberger), but a little on the old side, at about 7 months old; the chickens you buy at the grocery are somewhere around 2 months old.
Dr. Samsone recommended that next time we raise the chickens ourselves from chicks, which I’d love to try, but I suspect would run into some serious resistance from the students. We’d only had the chickens we harvested for five minutes before they’d all been given names. Raising chickens from chicks would bring a whole new level of anthropomorphizing.
Chicken on the grill. The culmination of the interim.
References
Being new to the chickens, I spent a bit of time researching how it is done.
Ken Bolte, from the Franklin County Extension of the University of Missouri, recommended the University of Minnesota’s Extension site on Home Processing of Poultry (the page on evisceration provided an excellent guide), as well as Oklahoma State’s much briefer guide (pdf).
Dr. Samsone recommended the series of videos from the Featherman Equipment Company. Videos are particularly useful for novices like myself.
