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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.]
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:
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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.
Dust devil on Mars:
Storm on Saturn
Storm on Jupiter
10 Feb 99, Oct 2006, Jan 2011... 2017
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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.
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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.
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>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 >>
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Some links:
Apr 99, Oct 2008
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This one naturally came up last week during the session about
sound waves, and we measured the speed of sound using only the
following:
picture 1
picture 2
picture 3
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- big can
- spoon
- watch or alarm clock with second hand
- tape measure
- calculator
- clipboard
- marker flag
- open space with a wall on one end
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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
Links:
1 April 99, October '07, September '08
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Science in action!
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
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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).
Links:
20 March 2002
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