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Last update Oct 2008



How does the eye work? Actually, the real question I got was 'why does people's eyesight go bad?', but I can't answer the second one without answering the first one.
Out of cardboard, paper and glue, and using a big lens from a hand-held magnifying glass, I built a working model of the eye.
Here is how to. With the model, you can take the eye apart piece by piece: take off the eyelids, the cornea, the iris. You can see the real image formed on the retina (upside down of course). This works best of course when you can point the eye at a window so you get a bright image. This is always a great hit.

[the model cheats a bit because in a real eye the image forming is done mostly by the shape of the cornea, and the lens is only a corrective element; If anyone has a good way of showing this, I'd like to know.]

model of the eye Anyway, eyesight can go bad in a lot of ways, and you can explain what can go wrong with each of the components, as well as the stuff in between them, namely the aqueous humor in between the cornea and the iris, and the vitreous humour between the lens and the retina. Not included here is the brain, where more things can go wrong with your vision.

A related question came up: why do people have different color eyes? Basically to block the light and to make the inside of the eye as dark as possible. I noted that people with ancestry in Northern climates don't need as much pigment, since on average there is less light than in the tripics. See the last link below.

In 2004, I brought in the eyeball to answer the question "Is the night sky blue or black?". In this context, I talked about the retina, the rods (light/dark perception, not very sharp, but very sensitive), and cones (only in the center, color-sensitive, but not very sensitive in low light). The week before, we had done the blue sky game. The sky is blue due to scattering and the light intensity has nothing to do with color. I concluded that the night sky not seen as dark blue, because (human) color vision does not work in low light.

Things you can do at home:

  • Make an upside-down image using a magnifying glass.
  • Find your blind spot: follow the link below
  • Look in the mirror and see your iris contract, dilate as you brighten/dim the lights
  • Look closely at the color patterns of your iris

Links:



How do clouds form? I did 2 demonstrations: one 'cloud in a bottle', where you pressurize a pop bottle that has a spoon of water in it, and then when the cork blows, the bottle is filled with fog. Kids love explosions!
I use a #4 rubber stopper with a hole in it, a nylon union and some plastic tubing, and tire valve with most of the rubber cut off on the other end of the hose. Put a spoonful of water in the bottle, and jam the stopper in as hard as you can. With your bicycle pump, start pressurizing the bottle. You can occasionally shake the bottle to ensure that the water splashes around and keeps the air in the bottle saturated. The pressurization will raise the temperature in the bottle. If you pop the cork immediately, the temperature will drop back to where you started from - no fog. What you have to do is pump up the bottle, and wait a while so it cools off again to room tempetature. Usually I talk and wave the bottle around, in order to dissolve water, and cool the bottle. After a while, the bottle has the same temperature as the room, and now, when you depressurize, the temperature drops and fog forms. You can squirt it out and see it against a dark background.

The other demo was the 'collapsing bottle' where you use a 2-liter plastic pop bottle, fill it with steam, and then have the air pressure crush the bottle completely flat as it cools back down.

I took a 2-liter plastic pop bottle, and put a little bit of water in it (about 1/4 cup). Put it in the microwave oven with the top off and let the water come to a boil. As the water boils, the steam will completely drive out all the air that was in the bottle. Quickly open the oven door and put a stopper into the bottle. Careful! the bottle is hot, and so is the steam blowing out of it. With the stopper in place, take the bottle out and let it cool down to room temperature. Since the air was all driven out, and the steam will eventually condense back into less than 1/4 cup of water, the air pressure in the room will crush the bottle completely flat. Also observe that sometimes the pressure in the bottle falls so fast that the water inside will start boiling again, albeit at a temperature less than 100°C.

[Why do I use a rubber stopper and not the original plastic screw cap? I noticed that the plastic of these bottles shrinks when heated to 100°C. Sometimes the shrinkage is so bad that the original cap does not fit very well anymore.]

How does this relate to clouds? Here in Santa Fe we're at the foot of the Sangre de Christo mountains. The kids know that when you go up the mountain, your ears pop (pressure drop) and it gets colder. So as you go up, there comes a point where the temperature drops to where fog (clouds) form. This height is the same everywhere, and therefore the bottom of summer clouds is flat, and the tops are billowy.

10 Feb 99, Oct 2006



click for bigger picture
How are you able to hear yours and other people's voices?

This question was about how sounds travel. I had done an hour on sound before (in last year's 3rd grade class), and I used some of the same things. I decided this time to start with the 'slinky demo', which demonstrates (among other things) the propagation of compression waves. This one can be found in many other places, but this is what I did: I had a 5' pole which I hung in front of the blackboard with 2 cardboard brackets. The 'slinky' spring hangs from a number of string loops, and is pulled out with string loops, which are hooked over paperclip hooks taped near the ends of the stick.
Look here for construction details and pictures.
You can show that if you bunch up a bit of spring near the end and then let go, the compression wave travels to the other end, and bounces back. You can also try to show that two waves starting at opposite ends can travel through each other. All this is pretty much what goes on with sound waves in air. Now you have to make the leap from there to sound waves.

I brought in an old 78 rpm turntable I found at a garage sale. I have a few 78's, and the grooves on these things are big enough that you can see them with a magnifying glass. Next I take a sheet of paper and a straighpin. Put the pin through the paper twice. When you put the pin in the groove, you can hear the music. You can add a few folds to the paper to improve things. Here is a better view of the paper. The kids can hear the music, and feel the vibrations in the paper. I also brought a vinyl record, where you can almost not see the grooves anymore even with the magnifying glass. Of course by the time vinyl came around, there was electronic amplification, so the amplitude (the size of the wiggles) can be smaller, so that in turn the grooves can be packed closer together. We also played the vinyl record on the grammophone with the paper and pin. Clearly a 'DO NOT DO THIS AT HOME' -type activity. Since the amplitude is smaller, the sound is not as loud. No matter what you have on your record, on a 78rpm turntable everything sounds like the chipmunks. In fact this is the part they remembered several weeks later.

Recently I've added this demo, using my laptop, and the school's projector. I installed Audacity, a free sound editor and recorder. You can record sounds, and then zoom in on the wave forms. Just with your voice you can show that notes that are one octave apart, have wavelengths that are twice as large (or small).

bigger >>

Apr 99, Oct 2008



If we keep using the same amount of water, when will we run out? Here in the desert Southwest, this is a very relevant question, especially at a time when the snowpack on the mountains is far below what it is supposed to be, and we're probably facing a summer with water restrictions.

No demos for this topic. We talked about the water cycle, aquifers, and why it is not easy to make drinking water out of all that sea water. Here in New Mexico it is clear that there is a very limited amount of water, and that we can not indefinitely use more than we receive.

9 Mar 99



What is the speed of sound?
This one naturally came up last week during the session about sound waves, and we measured the speed of sound using only the following:
 
  • big can
  • spoon
  • watch or alarm clock with second hand   
  • tape measure
  • calculator
  • clipboard
  • open space with a wall on one end
picture 1
picture 2
picture 3

Actually, I had 4 sets of cans, spoons, clipboards etc. The last time I also prepared a form to write down the measurements The last item on the list, the space with the wall, is of course the hardest to obtain. At Wood-Gormley Elementary, where I did this with a 6th-grade class, there is a large playground in front of the school building, as can be seen in this picture.

We have two measurement problems here:
1) How do you measure something very small - the time between a bang on the can and the echo - if all you have is a watch or alarm clock to keep time, and
2) How do you measure something very big - the distance to the wall - if all you have is a tape measure?
I posed an analogous question of the first case: how would you measure the thickness of a piece of paper if all you have is a ruler? One way to do this is to try to measure a pack of 500 sheets, which you can do pretty well with a good ruler, and then divide the answer by 500. In our case, we will do something similar. We BANG on the can repeatedly at such a rate that the little bang of the echo falls precisely halfway between the big BANGS:

BANG    bang    BANG    bang    BANG    bang    BANG    bang   
If you beat too slowly, the echo follows too fast :
BANG bang        BANG bang        BANG bang        BANG bang       
and if you beat too fast, the echo is too close to the next beat :
BANG       bang BANG       bang BANG       bang BANG       bang

The human ear is pretty sensitive to these things, and with a little practice the kids will get the hang of it. Now what you do is have someone beating the can, and have everyone with a watch see how many beats there are in say 30 seconds or a minute.

I did three tries; I counted 35 beats in 33 seconds, 30 in 28 and 30 in 26. This means the time per beat is (33+28+26 seconds)/(35+30+30 beats) = 0.92 seconds/beat.

To measure the distance to the wall, which in our case was almost 100 feet, you can either do it the slow way, by having them measure the whole distance with a yardstick or a 12-foot tape measure, or you can carefully measure the distance of 10 steps, and then step off the distance and do the multiplication.

Count your steps to the wall, and again on the way back. I got 97 steps one way, and 96 steps back, average 96.5 steps or 289 feet.
The speed of sound is now 4x289 feet/0.92 seconds = 1260 ft/sec or 384 m/s. The 'real' speed of sound is only about 11% smaller. Not a bad result.

When we did it with the whole class, we got a whole bunch of measurements. Here they are, plus some more tips

1 April 99, October '07, September '08



Science in action!
Is the speed of sound the same in warm and cold air?
I got this question over the internet, and here is how you can investigate this. You need the following:

  • two glass (beer) bottles    
  • a fridge
  • an oven

Take the bottles, and blow on them to make a note. If the bottles are from the same sixpack, they will make exactly the same pitch tone. [My empty bottles of Red Hook Ale made a G below middle C on the piano].
Now put one bottle in the freezer, and the other bottle in a warm oven. Not too hot, you don't want to burn your fingers or your lips when you take it out. A brown glass bottle left in direct sun will get warm enough.
After a while, take the bottles out and blow on them again. What do you hear? When I did it, the tones were 1/2 note different!
OK, someone will say that the warm bottle expanded, and the cold one shrunk a little, so you'd expect the cold bottle to go up in pitch and the warm one to go down. Of course you get a difference. But here's the thing: the difference went the other way! Cold bottle: lower note - warm bottle: higher note.Why is that? The bottles do in fact change in size as they get colder or warmer, but this is a very small effect. The change you hear is due to the fact that the speed of sound in air depends on the temperature of the air. In cold air, the speed of sound is slower than in warm air. The note you make whan you blow on a bottle is made by pressure waves going from the top and bouncing off the bottom of the bottle back up to the top. Only those notes that 'fit in the bottle' such that the pressure of the wave going down coincides with the pressure wave coming back up, can be made. The pitch of the tone depends on how fast the waves can travel through the air in the bottle: warm=fast and cold=slow.

How big a difference is 1/2 note on the piano? An octave is when the frequency doubles. An octave is evenly divided into 12 half-note intervals. Therefore the ratio of frequencies of two notes that are 1/2 note apart is the 12th root of 2, which is 1.06. Therefore the speed difference between 0°C and 40°C is 6%. (I am guessing at these temperatures).

20 March 2002



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Last update October 2008 - HvH
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