Ask Mr. Science
page 28

 
arrow left arrow up arrow right
prev page
 
index
 
next page
 

 

Making music

I was asked to do a science hour in a kindergarten class - normally I work in a 6th-grade class. I decided to do something with music, since I could involve the children. I made this instrument out of bottles. You can tune them by adding water, and talk about how big things make low notes and vice versa. I did some of that in class, with the participation of the kids. We played on the bottles and sang songs. I also brought a bunch of kazoos, and some Raffi music for a singalong. Twenty kindergartners with kazoos is something to experience.

2012

 

About the Earth's interior

(1) The Crust
This is the solid, rocky outer shell of the Earth. Compared to the radius of the Earth (6400 km), the crust is quite thin, ranging from 20 km on the ocean floor to 120 km where there are massive mountain ranges. The crust rides on the mantle, a thick layer of magma.
(2) Mantle
As the magma of the mantle cools off in the vicinity of the crust, cooled magma sinks down and hotter plumes of magma rise up. Pieces of the crust called tectonic plates are pushed around by this slow movement of the mantle. Where plates split apart, fresh magma wells up from below, making new crust. Most of this happens under water along oceanic ridges, but there are places where this happens on land, such as on Iceland. When two plates converge, different things can happen: one plate can slide under the other, as is happening in California, where the plate forming the pacific ocean bottom is sliding under California . On the other hand India is moving North and is crashing into Southern China, and this collision is pushing up the Tibetan plateau and the Himalayan mountains.
(3) Core
The core is mostly made of iron. Since iron is denser than (molten silicate) rock, it sank down to center early in the formation of the Earth. In the outer core the iron is molten, but as you get close to the center, the pressure rises so high that the iron atoms get squeezed in to a solid. The liquid iron in the outer core also circulates, and since iron is a magnetic material, the magnetic field and the iron flow interact in complicated ways. For one thing, the magnetic north pole moves around quite a bit from year to year, and the whole field reverses every 10000 years or so. Having a magnetic field is handy when you need to use a compass, but the real benefit for our earth is that the magnetic field extends far out in space, and acts as a shield to deflect the solar wind - a mighty blast of particles streaming out from the Sun all the time. Without this protection, the solar wind would have gradually blown away our atmosphere into space billions of years ago. Occasionally, some of the solar wind particles overwhelm the magnetic defenses, and sneak in along the magnetic field lines near the top and bottom of the globe. Where they slam into the top of the atmosphere, they make auroras, the Northern (and Southern) lights.

The upshot is that the world as we know it owes much to what happens deep below our feet: plate tectonics, driven by the mantle, shapes all the lands and the oceans, and the magnetic field, driven by the core, protects us from nasty space weather.

November 2013

 

Functions of the skin

[keep warm/cool, keep outside out, keep from drying, heat/cold/touch/pain/itch sensors, symbiosis with bacteria. Who has skin (endo/exo skeletons, terrestrial/aquatic), who has fur (heating/cooling). Functions of pigmentation, vit D, general benefits of sunlight.
January 2014

 

What is total internal reflection?

This question apparently was prompted by a science fair project from the previous year. I decided to do the demo where a (laser) light beam follows a stream of water down into a bucket. I made a fixture for this.


If you look closely, you can see the zigzag path of the light beam

To show how this works, I used a prism and a laser pointer to show the transition between refraction and tota internal reflection as you change the angle of the beam.

Total internal reflection is of course how optical fibers work. I have some broken fibers. Removing protective layers, you can see the glass fiber itself, which is about the thickness of a human hair.

I also have some 1-mm plastic wavelength-shifting fibers. They are bigger than the glass optical fibers, and they also light up along the length. In addition, they light up green even if you use the purple laser. But that's a whole nother story.

In 2023, I got a related question: "Why do things get darker when wet?". First, we observe that this only works for absorbent surfaces: paper, cloth, wood, and not for Al foil, plastic etc. Next I hold the (white) cloth with the wet spot up to the window: now the wet spot is lighter!
This hints at the cause of this effect: the water in between the fibers act as a light guide. Light penetrates further into the material, and is less likely to make it back out again, but you can see that it helps guide light right through the cloth.


Bring:
  • Fixture + bucket
  • Laser pointers (with spare batteries)
  • Towel
  • Funnel
  • Optical fibers
  • Wavelength-shifting fibers
  • 3L bottle with water
  • Container with a little bit of milk
  • Clear rectangular container
  • Prism (in cardboard holder)
  • Tape
  • Pieces of cloth, paper, wood
 
The fixture

Why make a fixture? I have one hour in the classroom, and no advance setup time. I have a small table to work on. This fixture holds all the bits firmly in place, and everything is easily adjustable, so no time is lost. The base plate has a holder for the bottle, rolled from cardboard. A block with notches for rubber bands is attached. The clothespin functions as the switch for the laser. Note I glued a piece of a straw into the bottle, instead of just making a hole. This ensures that the stream of water is (initially) very smooth. The cardboard box that holds everything serves as a raised platform. Since everything gets wet, the fixture gets a coat of paint, and I put some plastic shelf paper on the box.


September 2014





Suggestions, comments, greetings are greatly appreciated.
Just click here
and type away!

arrow left arrow up arrow right
prev page index next page
Back to my home page