experiencias de ciencias inglês

8/6/2019 experiencias de ciencias ingls

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Strange AttractorThe attraction and repulsion of magnets produces entrancing,unpredictable motion.Patterns of order can be found in apparently disordered systems. This pendulum - amagnet swinging over a small number of fixed magnets - is a very simple systemthat shows chaotic motion for some starting positions of the pendulum. The searchfor order in the chaos can be very engrossing.

A ring stand and a clamp.

4 to 6 ceramic magnets (available at Radio Shack).

Paint, masking tape, or correction fluid.

Fishing line or string.

(15 minutes or less)

Put all the magnets together in a stack so that they stick together magnetically. By doingthis, you are orienting the magnets so that all of the north poles point in one direction and allof the south poles point in the other direction. Mark the top of each magnet with paint, tape,or correction fluid, thus identifying all the matching poles.

Use the string or fishing line to hang one magnet from the ring stand so that it is a free-swinging pendulum. You can hang the magnet in any orientation.

Arrange the other magnets on the ring stand base in an equilateral triangle measuring a

couple of inches on a side. Position the magnets so that they all have the same pole up.
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Adjust the length of the pendulum so that the free-swinging magnet will come as close aspossible to the magnets on the ring stand base without hitting them or the base itself. Youcan accomplish this either by changing the length of the string or by adjusting the position ofthe clamp.

(15 minutes or more)

Give the pendulum magnet a push, and watch!

Vary the location and poles of the magnets to develop other patterns. You can arrange themagnets so that all of them have the same pole up, or you can mix them up. Notice that a

tiny change in the location of one of the fixed magnets or in the starting position of thependulum magnet may cause the pendulum to develop a whole new pattern of swinging.

The force of gravity and the simple pushes and pulls of the magnets act together to influencethe swinging pendulum in very complex ways. It can be very difficult to predict where the

pendulum is going to go next, even though you know which magnets are attracting it andwhich are repelling it.

This sort of unpredictable motion is often called chaotic motion. Strangely enough, there canbe a subtle and complex kind of order to chaos. Scientists try to describe this order with

models called strange attractors.

The new sciences of chaos and turbulence are unveiling hidden relationships in nature.Diverse phenomena, such as the patterns of Saturn's rings, measles outbreaks, and the

onset of heart attacks all follow chaotic patterns.

Often, a system that is predictable in the long run shows chaotic variations in the short run.Although it is quite difficult to predict specific daily weather behavior in the San FranciscoBay Area, the overall long-term patterns are generally known. The individual motion ofinsects may be random and insignificant, yet the behavior of the population as a whole canbe analyzed.

As shown in this Snack, a very slight difference in the starting position of the pendulum cangrow to a tremendous difference in the pattern of motion in a short time. This is

characteristic of chaotic systems. Weather scientists recognize this characteristic of chaoswhen they argue over the "butterfly phenomenon." That is, can a butterfly flapping its wings

in China drastically alter the weather in New York?

For further reading, see: Chaos by James Gleick (Viking Penguin, 1988) and The TurbulentMirrorby John Briggs and F. David Peat (Harper Collins, 1990).

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3/22

Look Into InfinityImages of images of images can repeat forever.If you have ever been between two mirrors that face each other, such as in abarber shop or a beauty salon, you will befamiliar with the seemingly endless line

of images fading into the distance. The Look Into InfinitySnack recreates thiseffect.

2 square pieces of plexi-mirror measuring 12 x 12 inches (30 x 30 cm).(Available from plastics stores. Mirror size is not crucial. You can substitute a 12

x 12 inch [30 x 30 cm] glass mirror tile or any two mirrors for the plexi-mirror.See To Do and Notice.)

Some kind of stand. (See Assembly.)

Adult help.


Cut a hole about 1 inch (2.5 cm) in diameter near the center of one of the mirrors. You canuse a hole saw, or you can have this done for you at the plastics store. You can get theeffect without the hole, but the hole gives a more interesting perspective. If you don't care

about the hole, you can use glass mirror tiles instead of a plexi-mirror. You can even createthe hole on a glass mirror tile by scraping away the silver backing.

Stand the mirrors so that their reflecting surfaces face each other and are parallel to eachother. The mirrors can be anywhere from a few inches to a foot apart. You can make awooden stand for each mirror by cutting a slit along the length of the flat side of a piece of 1x 4 inch (2.5 x 10 cm) pine. Then slip the mirror into the slit. You can also stand each mirrorbetween two pieces of wood held together by rubber bands, or place two full softdrink cans(or any heavy objects) so that one is on each side of the mirror, supporting it.

(5 minutes or more)
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With the reflecting surfaces facing each other, look through the hole into the space betweenthe mirrors. (If you didn't bother making a hole, just look over the top of one mirror.) Youcan also try placing either your finger or some other object between the mirrors.

If you place an object between the mirrors, notice that there is a repetitious pattern in theorientation and spacing of the images. Objects with contrasting colors on the front and back

(such as red and white) show this well. Successive images alternate from front view to backview. If the original object is closer to one mirror than to the other, the distance betweensuccessive images will alternate from close together to far apart - making the images seemto be grouped in pairs, with a front side always facing a front side, or a back side alwaysfacing a back side.

In this light-ray diagram, the solid lines

show the actual path of the light rays; the

dashed lines show the path of the light rays

projected by your brain. You see images

where the dashed lines come together.

This light-ray diagram shows the rays that

come from the front of an object and those

that come from the back. After the first

reflection, you see one image in each mirror

where the dashed lines come together (1).

After the second reflection, you see a

second image in each mirror (2), and so on.

As you look at the images formed in one of

the mirrors on the diagram, notice that thereis an alternation of front and back views,

that the images appear to be grouped in

pairs, and that a front side is always facing a

front side and a back side is always facing a

back side. This corresponds to what you

actually observe in the mirrors.

An interesting variation on the version described involves gluing the two mirrors to a block offoam rubber. Mirrors 6 x 6 inches (15 x 15 cm) work well for this handheld portable version,

but mirror size is not crucial. The foam should separate the mirrors by about 2 to 3 inches (5to 8 cm).

An even simpler handheld version of this Snack uses pieces of sponge as spacers, withrubber bands holding the mirrors together. Use a soft sponge. The cheap ones used forwashing cars work well. One or two holes can be drilled into the mirror.

Squeeze the foam so that the mirrors are not quite parallel to each other. You will see apattern of images that curves off into infinity. Try it, you'll like it!

And if you enjoyed thatone, why not take a look at the Corner Reflector Snack?Snack Home |Teacher Institute Home| Teaching Tools | Use Policy |Contact

Exploratorium| The museum of science, art and human perception
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5/22

Blue SkyNow you can explain why the sky is blue and the sunset is redWhen sunlight travels through the atmosphere, blue light scatters more than theother colors, leaving a dominant yellow-orange hue to the transmitted light. Thescattered light makes the sky blue; the transmitted light makes the sunset reddishorange.

A transparent plastic box, or a large beaker, jar, or aquarium

A flashlight or projector (either a slide or filmstrip projector)

Powdered milk

Polarizing filter (such as the lens from an old pair of polarized sunglasses)

Blank white card for image screen

Paper hole-punch

Optional: Unexposed (black) 35 mm slide or photographic film, or an index

card cut to slide size


Fill the container with water. Place the light source so that the beam shines through the

container. Add powdered milk a pinch at a time; stir until you can clearly see the beamshining through the liquid.


Look at the beam from the side of the tank and then from the end of the tank. You can alsolet the light project onto a white card, which you hold at the end of the tank. From the side,the beam looks bluish-white; from the end, it looks yellow-orange.

If you have added enough milk to the water, you will be able to see the color of the beamchange from blue-white to yelloworange along the length of the beam.
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If you want to look at a narrower beam of light, use a paper hole-punch topunch a hole in the unexposed black slide or in a piece of 35 mm film, or

even in an index card cut to size. Place the slide, film, or index card in the projector.(Do not hold it in front of the lens.) Focus the projector to obtain a sharp beam.

The sun produces white light, which is made up of light of all colors: red, orange,yellow, green, blue, indigo, violet. Light is a wave, and each of these colors corresponds to adifferent frequency, and therefore wavelength, of light. The colors in the rainbow spectrumare arranged according to their frequency: violet, indigo, and blue light have a higherfrequency than red, orange, and yellow light.

When the white light from the sun shines through the earth's atmosphere, it collides with gasmolecules. These molecules scatter the light.

The shorter the wavelength of light, the more it is scattered by the atmosphere. Because ithas a shorter wavelength, blue light is scattered ten times more than red light.

Blue light also has a frequency that is closer to the resonant frequency of atoms than that ofred light. That is, if the electrons bound to air molecules are pushed, they will oscillate with anatural frequency that is even higher than the frequency of blue light. Blue light pushes onthe electrons with a frequency that is closer to their natural resonant frequency than that ofred light. This causes the blue light to be reradiated out in all directions, in a process calledscattering. The red light that is not scattered continues on in its original direction. When youlook up in the sky, the scattered blue light is the light that you see.

Why does the setting sun look reddish orange? When the sun is on the horizon, its lighttakes a longer path through the atmosphere to your eyes than when the sun is directlyoverhead. By the time the light of the setting sun reaches your eyes, most of the blue lighthas been scattered out. The light you finally see is reddish orange, the color of white light

minus blue.

Violet light has an even shorter wavelength than blue light: It scatters even more than bluelight does. So why isn't the sky violet? Because there is just not enough of it. The sun puts

out much more blue light than violet light, so most of the scattered light in the sky is blue.

Scattering can polarize light. Place a polarizing filter between the projector and the tank.Turn the filter while one person views the transmitted beam from the top and another viewsit from the side. Notice that when the top person sees a bright beam, the side person will seea dim beam, and vice versa.

You can also hold the polarizing filter between your eyes and the tank and rotate the filter to

make the beam look bright or dim. The filter and the scattering polarize the light. When thetwo polarizations are aligned, the beam will be bright; when they are at right angles, thebeam will be dim.

Scattering polarizes light because light is a transverse wave. The direction of the transverseoscillation of the electric field is called the direction of polarization of light.

The beam of light from the slide projector contains photons of light that are polarized in alldirections. horizontally, vertically, and all angles in between. Consider only the verticallypolarized light passing through the tank. This light can scatter to the side and remainvertically polarized, but it cannot scatter upward! To retain the characteristic of a transversewave after scattering, only the vertically polarized light can be scattered sideways, and onlythe horizontally polarized light can be scattered upward. This is shown in the drawing.

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7/22

Charge and CarryStore up an electric charge, then make sparks.Are you tired of electrostatic experiments that just won't work? This experiment

will produce a spark that you can feel, see, and hear. You rub a Styrofoam platewith wool to give it a large electric charge. Then you use the charged Styrofoam to

charge an aluminum pie pan. The entire apparatus for charging the aluminum plateis called an electrophorus, which is Greek for charge carrier. An even larger charge

can be stored up in a device called a Leyden jar, made from a plastic film can.

For the Electrophorus:

A Stryofoam dinner plate (Acrylic plastic sheets also work well, as will old

LP records)

A piece of wool cloth (Other fabrics may work, but wool will definitelywork.)

A disposable aluminum pie pan

A Styrofoam cup

Hot glue gun or masking tape

For the Leyden jar:

A plastic 35 mm film can

A nail slightly longer than the film can

Some aluminum foil.

Tap water

Optional: A neon glow tube (available from Radio Shack)


Electrophorus:
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Tape or hot-glue the Styrofoam cup to the middle of the inside of the pie plate. (Mosthousehold glues won't work because they dissolve Styrofoam.) Place the pie pan on top ofthe upside-down Styrofoam plate or a piece of acrylic plastic.

Leyden Jar:

Push the nail through the center of the lid of the film can. Wrap aluminum foil around thebottom two-thirds of the outside of the film can. You may tape the aluminum foil in place. Fillthe film can almost full with water. Snap the lid onto the can. The nail should touch thewater.


Rub the Styrofoam plate with the wool cloth. If this is the first time you are using theStyrofoam in an electrostatic experiment, rub it for a full minute.

To charge the pie pan follow the next steps exactly:

1. Place the pie pan on top of the charged Styrofoam plate.

2. Briefly touch the pie pan with your finger. You may hear a snap and feel a shock.

3. Remove the pie pan using only the insulating Styrofoam cup (see photo). You may have to

hold the Styrofoam plate down with your other hand.

The pan is now charged.

Discharge the pan by touching it with your finger. You will hear a snap, feel a shock, and, ifthe room is dark, see a spark. To make the largest spark, have the pie plate at least one footaway from the Styrofoam plate. You can also discharge the pie pan through a neon glowtube. Hold one of the two metal leads of the tube in your fingers and touch the other lead tothe pie pan. The electric spark will go through the neon and make a flash that is easilyvisible. After charging the Styrofoam once, you can charge the pie pan several times. The piepan is portable and can be used for many electrostatic experiments.

Charge the Leyden jar by touching the charged pie pan to the nail while holding the Leydenjar by its aluminum foil covering. You can make several charge deliveries by recharging the

pan before touching it to the nail. Discharge the jar by touching the aluminum foil with onefinger and the nail with another. Watch for a spark.


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When you rub the Styrofoam plate with a wool cloth, you charge it negatively. That'sbecause the Styrofoam attracts electrons from the cloth. Often, a plate fresh from thepackage will start with a positive charge. If it does, you will have to rub the plate longenough to cancel this initial charge before you can begin building a sizable negative charge.By using an electroscope (such as the one you can build with the ElectroscopeSnack, youcan determine whether the Styrofoam is positively or negatively charged. Styrofoam is aninsulator; it will hold its charge until it is discharged by current leaking into the air or along amoisture film on the surface of the Styrofoam

When you place the pie pan on the Styrofoam, the electrons on the Styrofoam repel theelectrons on the pan. Since the electrons can't leave the pie pan because it is completelysurrounded by insulating air and Styrofoam, the pan retains its neutral charge. If you touchthe pie pan while it is near the Styrofoam, the mobile electrons will be pushed off the panand onto you. The electrons make a spark as they jump a few millimeters through the air toreach your finger. The air in the spark is ionized as the moving electrons knock otherelectrons off air molecules. The ionized air emits light and sound. You can also feel the flowof electrons though your finger.

After the electrons leap to your finger, the pan has a positive charge. Physicists say the panhas been charged by induction. You can carry the positively charged pan around by itshandle and carry the positive charge to other objects. If you bring the positive pan near yourfinger again, or near any object that can be a source of electrons, the pan will attractelectrons, creating a second spark.

The low-pressure neon gas in a neon glow tube is easier to ionize than air that is atatmospheric pressure. If you discharge the pan through a neon glow tube, the spark willmake a bigger flash of light.

When you touch a positively charged pie pan to the nail on the Leyden jar, electrons from

the nail flow onto the pie pan. The resulting positive charge on the nail attracts electronsfrom your body through your hand onto the aluminum foil of the jar. The Leyden jar will then

have a positive center separated from a negative foil outside by the insulating plastic of thefilm can. If you touch one finger to the foil and bring another finger near the nail at the

center of the Leyden jar, a spark will jump as the negative charges are attracted through youto the positive nail. The beauty of the Leyden jar is that it can store charges from several

charged pie pans, thus building up to a larger, more visible, more powerful (and morepainful) spark.

The Leyden jar is the forerunner of the modern-day capacitor. It was invented in 1745 at theUniversity of Leyden by Pieter Van Musschenbroeck. Early Leyden jars were larger than aplastic film can and could hold more charge. The inventor discharged one through himselfand wrote, "My whole body was shaken as though by a thunderbolt." At another time, aLeyden jar was discharged through 700 monks who were holding hands. The charge causedthem to simultaneously jump slightly off the ground.

To give the Styrofoam plate a positive charge, try rubbing it with a plastic bread bag. Tryrubbing it with other cloths, too. Try charging the Leyden jar in reverse. That is, whileholding the nail, touch the aluminum foil with the pan.
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10/22


Circles of Magnetism IYou can make a magnetic field that's stronger than theearth's!Compass needles are little magnets that are free to rotate. Compasses allow us toobserve the direction of a magnetic field. Normally, they respond to the earth'smagnetic field, orienting themselves parallel to magnetic field lines. If we create amagnetic field that is stronger than the field of the earth - for example, by using

electric currents - a compass needle will orient itself parallel to the new field.

A 6- or 12-volt lantern battery.

A 1 foot (30 cm) length of heavy wire that is rigid enough to stand by

itself. (You can use the wire from a coat hanger.)

A Tinkertoy set for building the stand (or another improvised stand).

A flat, rigid support surface measuring approximately 6 x 6 inches(15 x 15 cm). (This can be made of posterboard or even a manila file folder.)

It should have a hole in the center of it that is large enough for the wire to passthrough.

4 or 6 small compasses, measuring about 1 inch (2.5 cm) in diameter.

2 electrical lead wires with alligator clips at both ends (available at RadioShack).

Adult help.

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11/22

Construct a Tinkertoy stand (or the equivalent), and lay the flat support surface in positionon the stand. (See the photo and the diagram above.)

If the coat hanger wire is painted or varnished, scrape the coating off to expose about 1 inch(2.5 cm) of bare metal at each end.

Insert the wire through the hole in the flat support surface, and support the wire vertically in

the stand, as shown in the photo and diagram.

Arrange the compasses in a circle on the support surface as shown in the diagram.

Attach one clip lead to each battery terminal. but donot attach the other ends of the lead

wires to the coat hanger wire yet.

Observe the compass needles around the wire as when there is no current passing throughthe wire. Rotate the support surface. What happens to the compass needles? They will point

north, orienting themselves so that they are parallel to the earth's magnetic field. (Note: a

few of your compasses may point south! Inexpensive compasses that are exposed to astrong magnet will sometimes become magnetized in the reverse direction. It's nothing toworry about, though - just keep in mind which end of your compasses points north.)

Attach the clip leads to the ends of the coat hanger wire where it has been scraped. Watchwhat happens to the compass needles as current passes through the wire. If the electricalcurrent is large enough, each compass will point in a direction tangent to a circle centered onthe wire.

Rotate the support surface again. What happens to the compass needles this time? Thecompasses will continue to point in a direction tangent to a circle centered on the wire.

Don't leave the clip leads connected too long, because the electric current will rapidly drain

the battery. A few seconds should be long enough to make good observations.

Switch the clip leads to the other terminals of the battery. What happens? The compassneedles will reverse direction when the electrical current reverses direction.

Compass needles line up with magnetic fields. Since the earth is a magnet, a compass willnormally line up with the earth's magnetic field. Because opposite magnetic poles attract, themagnetic north pole of the compass points toward the magnetic south pole of the earth. (Themagnetic south pole of the earth is located in northern Canada! That is not a misprint. Thesouth pole of the earth magnet is near the geographic north pole.)


12/22

The electric current passing through the wire creates a magnetic field that is stronger thanthe earth's field (in a region close to the wire). You can visualize the shape of this new fieldas a set of concentric circles surrounding the wire. Each of these circles has its center at thewire.

The closer to the wire you are, the stronger the magnetic field. The compass needles align

themselves with the total magnetic field at each point, the sum of the earth's field and thatof the wire. Since the magnetic field from the wire is significantly larger than that from theearth, each needle ends up pointing essentially in the direction of the magnetic field of thewire.

When you reverse the current, the direction of the magnetic field also reverses, and theneedles dutifully follow it.

To find the direction of the magnetic field made by an electrical current, use a techniquecalled the righthand rule.

Place your right hand with the thumb parallel to the wire carrying the current. Point your

thumb in the direction of the electrical current in the wire. (Remember: The electric currentflows from the plus side of the battery through the wire to the minus side.) Wrap yourfingers around the wire. Your fingers will now point in the direction of the magnetic fieldaround the wire. If there are compasses near the wire, they will point in the same directionas your fingers.

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13/22

Circles of Magnetism IVTwo parallel, current-carrying wires exert forces on each

other.When an electric current flows through a wire, a magnetic field is created aroundthe wire. If you place two current-carrying wires near each other, the magneticfield around each wire exerts a force on the current flowing in the other wire.These forces can push two current-carrying wires apart, or pull them together.

One 6-volt lantern battery or an equivalent current supply.

2 electrical lead wires with alligator clips at both ends (available at Radio

Shack). Tinkertoys or wood for a stand.

Masking tape or transparent tape.

Light aluminum foil.

Adult help.


Make a stand from wood or Tinkertoys (see the photo on page 14 and the diagramsbelow), or build a stand of your own design from available materials.

Cut a strip of aluminum foil measuring about 2 feet (60 cm) long and 1/2 inch (1.3 cm) wide.Tape one end of the foil strip to your support. Run the strip down and back up to thesupport, making a loop, then tape the other end in place. Be sure the ends of the strip donot touch.

Attach one clip lead to each battery terminal, but do not attach the other ends of the leadwires to the strip yet.
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Touch the two clip leads to the ends of the foil strip. The descending and ascending portionsof the loop will repel each other. The closer you can hang the descending and ascending

portions of the loop to each other - without allowing them to touch - the larger the repulsion.

Now hang the foil strip from the support with the two ends overlapping, so they make a good

electrical contact. Connect one of the clip leads to these overlapping ends. Separate the twosides of the loop and briefly touch the other clip to the bottom of the loop. Notice that the

sides of the loop are attracted to each other when the current flows. (This step requires alittle coordination and a delicate touch to clearly demonstrate that it is the current flow in the

strips that makes them move together and not forces that you create when you touch theclip to the bottom of the loop.)

A current-carrying wire generates a magnetic field that circles the wire (See the "Circles ofMagnetism I"Snack.)

When a current flows in a magnetic field, the field exerts forces on that current. (See the"Motor Effect" Snack.) So each current-carrying wire in this Snack generates a magnetic field

at the position of the other wire and thus exerts a force on the current in the other wire. Twoparallel wires will either attract or repel each other, depending on the direction of current

flow in each wire. If both currents flow in the same direction, the wires will attract; if theyflow in opposite directions, they will repel.

The forces produced on the aluminum foil are small. This is because the electrical currentflowing through the foil is small, only a couple of amperes. Larger currents produce largerforces. The Exploratorium exhibit, for example, uses wires carrying 400 amperes, whichproduces forces that are more than 10,000 times stronger than the forces you produce withthis Snack.

The ampere, the fundamental unit of electrical current, is defined by the force exerted byone wire on another. The definition of the ampere is as follows: A current of 1 ampereflowing in each of two infinitely long parallel wires separated by 1 meter will produce anattractive force of 2 x 10-7 newton on each 1-meter length of wire. For comparison, a force of1 newton is approximately the weight of a quarterpound of hamburger.

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Colored ShadowsNot all shadows are black.When two different-colored lights shine on the same spot on a white screen, thelight reflecting from that spot to your eyes is called an additive mixture because itcontains the colors from both lights. We can learn about human color perception byusing colored lights to make additive color mixtures.

White surface. (A white wall, white posterboard, or white paper taped to

stiff cardboard works well. Do not use a beaded or metal slide projectionscreen.)

Red, green, and blue lightbulbs or floodlamps, one of each color.

Sylvania #11 colored lightbulbs or General Electric Dichrocolor DichroicFloodlamps (150 watt) work well. We have even obtained excellent results withclear-colored Christmas tree lights. Smaller or dimmer bulbs are fine fortabletop use by a few students, but larger, brighter bulbs allow a larger-scaledemonstration.

3 light sockets of any type or arrangement that will get the light from thethree bulbs simultaneously directed onto the same area of a white surface.

Any solid object such as a pencil, ruler, correction fluid bottle, finger, etc.

Adult help.

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Set up the bulbs and screen in such a way that the light from all three bulbs falls on thesame area of the screen and all bulbs are approximately the same distance from the screen.For best results, put the green bulb in between the red and the blue bulbs.

Turn on the lights, and adjust the positions of the bulbs until you obtain the "whitest" lighton the area of the screen where the three lights mix. For best results, make the room asdark as possible.

Place a narrow opaque object, like a pencil, fairly close to the screen. Adjust the distancefrom the screen until you see three distinct colored shadows.

Remove the object, turn off one of the colored lights, and notice how the color on the screenchanges. Then replace the object in front of the screen and notice the color of the shadows.

Move the object close to the screen until the shadows overlap. Notice the color of thesecombined shadows.

Repeat the previous step with a different light turned off while the other two remain on, andthen a third time so you have tried all combinations. Repeat again with only one color at atime on, and then with all three on. Vary the size of the object and the distance from thescreen. Try using your hand as an object.

The retina of the human eye has three receptors for colored light: One type of receptor ismost sensitive to red light, one to green light, and one to blue light. With these three color

receptors we are able to perceive more than a million different shades of color.

When a red light, a blue light, and a green light are all shining on the screen, the screenlooks white because these three colored lights stimulate all three color receptors on yourretinas approximately equally, giving us the sensation of white. Red, green, and blue aretherefore called additive primaries of light.

With these three lights you can make shadows of seven different colors: blue, red, green,black, cyan (blue-green), magenta (a mixture of blue and red), and yellow (a mixture of redand green). If you block two of the three lights, you get a shadow of the third color: Blockthe red and green lights, for example, and you get a blue shadow. If you block all threelights, you get a black shadow. And if you block one of the three lights, you get a shadowwhose color is a mixture of the two other colors. If the blue and green mix, they make cyan;red and blue make magenta; red and green make yellow.


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If you turn off the red light, leaving only the blue and green lights on, the lights mix and thescreen appears to be cyan, a bluegreen color. When you hold the object in front of this cyan

screen, you will see two shadows: one blue and one green. In one place the object blocksthe light coming from the green bulb and therefore leaves a blue shadow; in another place it

blocks the light from the blue bulb to make a green shadow. When you move the objectclose to the screen you will get a very dark (black) shadow, where the object blocks both

lights.

When you turn off the green light, leaving the red and blue lights on, the screen will appearto be magenta, a mixture of red and blue. The shadows will be red and blue.

When you turn off the blue light, leaving the red and green lights on, the screen will appear

to be yellow. The shadows will be red and green.

It may seem strange that a red light and a green light mix to make yellow light on a whitescreen. A mixture of red and green light stimulates the red and green receptors on the retinaof your eye. Those same receptors are also stimulated by yellow light --- that is, by lightfrom the yellow portion of the rainbow. When the red and green receptors in your eye arestimulated, whether by a mixture of red and green light, or by yellow light alone, you will seethe color yellow.

Find out what happens when you use different colored paper for the screen. Try yellow,green, blue, red, purple, and so on.

If you let light from the three bulbs shine through a hole in a card that is held an appropriatedistance from the screen, you will see three separate patches of colored light on the screen,

one from each lamp. (Make the hole large enough to get a patch of color you can really see.)If you move the card closer to the screen, the patches of light will eventually overlap and

you will see the mixtures of each pair of colors.Snack Home |Teacher Institute Home| Teaching Tools | Use Policy |Contact

Exploratorium| The museum of science, art and human perception
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Soap BubblesCreate geometric art with soap films.Using pipe cleaners and drinking straws, you canmake three-dimensional geometric frames: cubes,tetrahedrons, or shapes of your own design. Whenyou dip these frames in a soap solution, the soapfilms that form on the frames are fascinating andcolorful.

Plastic drinking straws.

Pipe cleaners (available at school supply, hobby, or party stores).

A small bucket or container for the bubble solution. The container must belarge enough so that bubble frames are entirely covered when they are dipped.

Bubble solution (You can use a commercial solution like Wonder Bubbles,

or use the Exploratorium's recipe: To each gallon (3.8 1iters) of water add 2/3cup (160 ml) of Dawn or other dishwashing liquid and 1 tablespoon (15 ml) of

glycerine, available at your local pharmacy. Bubble solution works best if it isaged at least a day before use.)


Form frames using the drinking straws for the straight pieces. Connect two straws at acorner by inserting a doubled pipe cleaner into the end of each straw. In places where three

straws meet, fold the pipe cleaners as shown in the diagram. Attach a pipe cleaner handle toyour frame.

Try constructing cubes or tetrahedrons, or just let your imagination run wild. Mix the soapsolution in the bucket. Make sure that you have enough solution to fully cover the frames

when they are dipped.

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Dip the frames into the soap solution, and observe the fascinating geometrical shapes thatthe soap films form. Also notice the shimmering colors in the soap film.

As you lift your frame out of the solution, the soap film flows into a state ofminimumenergy. The soap film is in a state of minimum energy when it's covering the least possibleamount of surface area. The intricate shapes you see inside the frame represent theminimum area the soap film can cover. You may notice that a soap film will sometimes takeon different shapes when you dip the frame into the solution again and again. That's becausethere may be more than one way for the soap film to form a minimum surface area.

When light waves hit the soap film, they reflect and interfere with each other. Thisinterference causes the shimmering colors you see. White light is made of many differentcolors. When white light shines on the soap film, some light waves reflect from the frontsurface of the film and some reflect from the back surface of the film. When these two setsof reflected waves meet, they can add together, cancel each other out, or partially cancel,depending on the thickness of the film and the initial color of the light. When light waves of aparticular color meet and cancel each other, then that color is subtracted from white light.For example, if the red light waves cancel, then you see white light minus red light, whichyou perceive as blue-green light.

Plastic bar straws, which have a smaller diameter than regular drinking straws, hold the pipe

cleaners more tightly. But bar straws are more expensive and are sometimes harder to get.If you can't find them at grocery or liquor stores, try restaurant or party supply stores.


Size and DistanceA clueless way to determine the size of an object.
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20/22

You can trick your brain into thinking that two similar objects of different sizes arereally the same size. This is done by removing clues to the actual size and distanceof the objects. You can then compare what you see when you limit yourinformation to what you see when you have complete information.

A shoe box with lid. 2 straws.

2 coins of different sizes (for instance, 1 quarter and 1 dime).

Modeling clay or Fun Tak reusable adhesive.

Construction paper that contrasts with the inside of the shoe box.

A piece of posterboard or similar-weight cardboard, a little larger than the

end of the box.

A knife or scissors.

Adult help.


Remove the box lid. At the center of one end-panel of the shoe box, cut a hole large enoughfor one-eyed viewing (approximately 1/2 inch [1.3 cm] diameter). Then cut out a window,approximately 3 x 5 inches (8 x 13 cm), with the viewing hole in the center. Replace thecardboard in the window from which it was cut, and tape it in place along its bottom edge toform a hinge inside the box (see diagram).

At the other end of the box, make a hole for each straw approximately 1/2 inch (1.3 cm) toeach side of the center of the panel.

Cut the piece of posterboard so that it is the same height as the box and about 2 inches (5cm) wider than the box. Fold back 1 inch (2.5 cm) on each side to make two flaps. Make two

holes in the main portion that coincide exactly with the two holes in the end of the box. Tapeor glue the flaps to the inside sides of the box, using them as spacers to position this piece 1

inch (2.5 cm) from the end with the holes (see diagram).

Insert the straws into the holes. The double set of holes will keep the straws properly

aligned.

Using the quarter and dime as templates, cut one circle of each size from the constructionpaper. Make sure the color of the construction paper contrasts strongly with the color of theinside of the box. Use clay to mount the circles on the ends of the straws inside the box.



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With the cover on the box and the hinged window closed, close one eye and look through theviewing hole at the circles. Lift the end of the box cover closest to you and slide it away fromyou until adequate viewing light reaches the inside of the box. Push or pull on the strawsfrom the outside rear of the box until the mounted circles appear to be the same size. Notethat your depth perception is almost nonexistent: both circles look essentially the samedistance away, and it is very difficult to judge which circle is closest to you.

Fold the window down and look with both eyes to see the actual positions of the circles. (Youmay have to move your head back a little from the box to get both circles focused.) Notethat depth perception is now a factor and that the circles no longer look the same size or thesame distance away.

Position the small disk a couple of inches closer to you than the large disk. With both eyes

open, look through the window at the disks. Notice that you have no trouble establishingtheir size and distance. Now close one eye. You may notice that it is much more difficult to

tell whether the small disk is now actually a small disk that's close to you, or a large diskthat's far away. You can also use the dime and quarter by themselves, without building the

box, to illustrate the same principle. Close one eye. Holding one coin in each hand, movethem toward or away from your viewing eye until they appear to be the same size. A solid-

color background gives less distraction than an irregular background. A very bright, solid-colored background works best, so that the coins appear essentially in silhouette and their

features cannot be easily distinguished.

Large, distant objects can appear to be the same size as small, nearby objects. Undernormal viewing conditions, with both eyes open, you have the ability to perceive depth. Iftwo objects appear to be the same size, but you know that one is farther away than theother, your brain tells you that the distant object is larger.

When one eye is closed, your depth perception is impaired. In the case of the circles, you

can't tell how far away either of the circles really is. Since they are not actually the samesize, this means that, for the smaller one to look the same size as the larger one, it will haveto be closer to you than the larger one.

With both eyes open, you can gather more information and more points of view, and so you

can make more accurate judgments about an object's size, shape, and distance from you.

There is a pattern on the pupa of the butterfly Spalgis epius that looks like the face of arhesus macaque monkey. Even though the pupa is only half as wide as a human fingernail, itstill seems to scare away predatory birds who mistake it for a more distant, and thereforelarger, monkey.


Real Image
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A snack version of Mirror MirageCreate the image of an object in space using a $2 ornament.

4" silvered plastic Christmas tree ornament

These ornaments are available in the months preceeding Christmas from Michael's CraftStores.

Using a band saw cut the top off one half of the ornament creating a circular hole that isabout 3.5 cm in diameter. You will be trimming only 0.5 cm off the top. Then trim a slice 1.8cm off the bottom of the same half. Remove the little bits of plastic carefully with a softcloth. Place the trimmed half inside the unaltered half.

Place an object, such as a pushpin, inside the sphere. View the image at an angle nearlyhorizontal to your eye. Locate the image of the object that has the same orientation as theobject. Touch the image with a finger.

You are seeing an image formed by two concave mirrors facing one another. The object isplaced at the center of the bottom mirror. The curvature of the mirrors is such that theobject is at the focal point of the top mirror.

When light from a point on the object hits the top mirror, it reflects in parallel rays. These

parallel rays hit the bottom mirror and reflect so that they reassemble to form a pointlocated at one focal length from the bottom mirror. The mirrors are placed so that the focalpoint of the bottom mirror is located at the hole in the top of the device. The end result isthat light from every point on the object is assembled into an image in the hole.

The ray diagram may help explain this effect.

ByHeidi Strahm Black

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