Bird in the Cage - Stare at color and see it change!

     

Bird in the Cage

 

Stare at color and see it change.

You see color when receptor cells (called cones) on your eye's retina are stimulated by light. There are three types of cones, each sensitive to a particular color range. If one or more of the three types of cones becomes fatigued to the point where it responds less strongly than it normally would, the color you perceive from a given object will change. 

 

• 4 white posterboards or pieces of paper
• Bright red, green, and blue construction or contact paper
• Small piece of black construction or contact paper, or black marking pen
• Scissors
• Glue or glue stick (if you are using construction paper)
• Adult help

 

(30 minutes or less)

Cut the same simple shape, such a bird or a fish, from each of the three colored papers. Glue each shape on its own white board. Leave one white board blank. Cut a small black eye for each bird or fish or draw one in with the marking pen. If you choose a bird as the shape, draw the outline of a birdcage on the blank board; if you choose a fish, draw a fishbowl, etc. (Be creative!) 
 

(15 minutes or more)

Place the boards in a well-lit area. (Bright lighting is a significant factor in making this Snack work well.)
Stare at the eye of the red bird for 15 to 20 seconds and then quickly stare at the birdcage. You should see a bluish-green (cyan) bird in the cage. Now repeat the process, staring at the green bird. You should see a reddish-blue (magenta) bird in the cage. Finally, stare at the blue bird. You should see a yellow bird in the cage. (If you used a fish, try the same procedure with the fish and the bowl.) 
 

The ghostly fishes and birds that you see here are called afterimages. An afterimage is an image that stays with you even after you have stopped looking at the object.

The back of your eye is lined with light-sensitive cells called rods and cones. Cones are sensitive to colored light, and each of the three types of cones is sensitive to a particular range of color.

When you stare at the red bird, the image falls on one region of your retina. The red-sensitive cells in that region start to grow tired and stop responding strongly to red light. The white board reflects red, blue, and green light to your eyes (since white light is made up of all these colors). When you suddenly shift your gaze to the blank white board, the fatigued red-sensitive cells don't respond to the reflected red light, but the blue-sensitive and green-sensitive cones respond strongly to the reflected blue and green light. As a result, where the red-sensitive cells don't respond you see a bluish-green bird. This bluish-green color is called cyan.

When you stare at the green bird, your green-sensitive cones become fatigued. Then, when you look at the white board, your eyes respond only to the reflected red and blue light, and you see a red-blue, or magenta, bird. Similarly, when you stare at a blue object, the blue-sensitive cones become fatigued, and the reflected red and green light combine to form yellow.

You can design other objects with different colored paper and predict the results. Try a blue banana! For smaller versions, you can use brightly colored stickers (from stationery, card, or gift stores) on index cards.

One classic variation of this experiment uses an afterimage to make the American flag. Draw a flag, but substitute alternating green and black stripes for the familiar red and white stripes, and black stars on a yellow field for the white stars on a blue field. For simplicity, you can idealize the flag with a few thick stripes and a few large stars. When you stare at the flag and then stare at a blank white background, the flag's afterimage will appear in the correct colors.

You may also want to experiment with changing the distance between your eyes and the completely white board while you are observing the afterimage. Notice that the perceived size of the image changes, even though the size of the fatigued region on your retina remains the same. The perceived size of an image depends on both the size of the image on your retina and the perceived distance to the object.

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Bernoulli Levitator - Suspend an object in the air by blowing down on it!

 

Bernoulli Levitator

 

Suspend an object in the air by blowing down on it

The Bernoulli principle explains how atomizers work and why windows are sometimes sucked out of their frames as two trains rush past each other. You can choose from two versions of this Snack-- small or large. 

 

Small Snack

• A large wood or plastic thread spool
• An index card
• A pushpin
• Optional: Drinking straws

Large Snack

• A hair dryer or vacuum-cleaner blower
• A stiff paper or plastic plate
• A cardboard box with one side somewhat larger than the plate
• A pushpin

 

Small Snack

(5 minutes or less)

Trim an index card to a 3 x 3 inch (7.5 x 7.5 cm) square. Push the pushpin into the card's center.
If more than one person is going to use this, construct the following sanitary version: Cut a 2 inch (5 cm) long piece of straw for each person. At each person's turn, have him or her push one end of the straw into the hole in the spool of thread. If any straw does not fit, cut a 1/2 inch (6.25 mm) slit near the end of the straw and push it into the spool.

Large Snack

(5 minutes or less)

Cut the flaps off the top of the box, and turn the box so that the opening faces to the side. Put the side of the box that is larger than the plate on top, and cut a hole in the center slightly smaller than the outlet of the hair dryer or vacuum hose. Stick a pushpin through the center of the plate.

Small Snack

(5 minutes or more)
 

Hold the card against the bottom of the spool with the pushpin sticking into the spool's hole. The pushpin keeps the card from drifting off to the side.Blow strongly through the hole in the top of the spool and let go of the card. If the card falls at first, experiment with different sized cards or spools until you can make the card hang suspended beneath the spool.

Large Snack

Turn on the blower and direct it down through the hole. If you use a vacuum cleaner, be sure to use it as a blower. If you use a hair dryer, turn the heat off if you can. (If you can't, the hair dryer may overheat and automatically turn off. It will work again as soon as it cools down.)Bring the plate up toward the hole from below. Contrary to what you might expect, as the plate approaches the hole it will be sucked up and held in place by the air blowing down. The pushpin should keep the plate from drifting off to the side. 
 

When you blow into the spool or through the box, the air goes through the opening, hits the card or plate, and accelerates outward. The energy needed to accelerate the air comes from the energy stored as compression of the gas, so the gas expands, and its pressure drops.

As air (or any other fluid) accelerates, its pressure drops. This is known as the Bernoulli principle. In the small version of this Snack, the air rushing between the spool and the card exerts less pressure on the card than the still air underneath the card. The still air pushes the card toward the spool and holds the card up against gravity. In the larger version, the same principle is at work, holding the plate up against the hole in the box.

In an atomizer, or perfume sprayer, you squeeze a rubber bulb to squirt air through a tube. Because of the Bernoulli principle, the air rushing through the tube has a lower pressure than the surrounding atmosphere. Atmospheric pressure forces the perfume up an intersecting tube into the low-pressure airstream. The perfume is pushed out of the tube and sprays into the air as a fine mist.

The air rushing through the space between two moving trains also has a lower pressure, due to the Bernoulli principle. Sometimes, the higher pressure, stationary air inside each train forces some of the trains' windows out of their frames.

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Benham's Disk - A rotating black-and-white disk produces the illusion of color!

   

Benham's Disk

A rotating black-and-white disk produces the illusion of color.

When you rotate this black-and-white pattern at the right speed, the pattern appears to contain colored rings. You see color because the different color receptors in your eyes respond at different rates.

• Posterboard or cardboard
• Glue stick or other suitable adhesive
• Pattern disk (provided here)
• Access to a copy machine
• A black marking pen
• A rotator You can use a turntable, variable speed electric drill, hand drill, portable electric mixer, or electric screw driver. Attach the disk with adhesive Velcro™, or if a drill with a chuck is used, a bolt can be used as a shaft, with two nuts to hold the disk. You can also reduce the size of the disk on a copy machine, then mount it on the flat upper surface of a suitable toy top, or you can devise your own spinner for the disk. Try spinning the mounted disk on a pencil point, or on a pushpin stuck into a pencil eraser.
• Adult help

 

(15 minutes or less)

Copy the pattern disk in the drawing provided as a separate page, here and mount it on a cardboard backing with the adhesive. If your copier does not make good solid blacks, fill in the black areas with a black marking pen. You can reduce or enlarge the pattern disk if you like.

Attach the mounted disk to a rotator. 
 

(15 minutes or more)
Spin the disk under bright incandescent light or sunlight. (Fluorescent light will work, but there is a strobing effect that gives the disk a pulsating appearance and makes it harder to look at.)

Notice the colored bands that appear on the disk. Look at the order the colors are in. What color do you see at the center? What about the next few bands?

Reverse the direction of rotation and compare the order of colors again, from the center of the disk to the rim.

Try varying the speed of rotation and the size of the pattern, and compare the results with your initial observations. 
 

Different people see different intensities of colors on this spinning disk. Just why people see color here is not fully understood, but the illusion involves color vision cells in your eyes called cones.

There are three types of cones. One is most sensitive to red light, one to green light, and one to blue light. Each type of cone has a different latency time, the time it takes to respond to a color, and a different persistence of response time, the time it keeps responding after the stimulus has been removed. Blue cones, for example, are the slowest to respond (have the longest latency time), and keep responding the longest (have the longest persistence time).

When you gaze at one place on the spinning disk, you are looking at alternating flashes of black and white. When a white flash goes by, all three types of cones respond. But your eyes and brain see the color white only when all three types of cones are responding equally. The fact that some types of cones respond more quickly than others -- and that some types of cones keep responding longer than others -- leads to an imbalance that partly explains why you see colors.

The colors vary across the disk because at different radial positions on the disk the black arcs have different lengths, so that the flashing rate they produce on the retina is also different.

The explanation of the colors produced by Benham's disk is more complicated than the simple explanation outlined above. For example, the short black arcs that are on all Benham's disks must also be thin, or no colors will appear. 
 

Benham's disk was invented by a nineteenth-century toymaker who noticed colors in a black-and-white pattern he had mounted on a top. Toy spinning tops with Benham's disks are still available in the Exploratorium Store and in toy stores.

The three different color sensors in a color television camera also have different latency and persistence times. When a color television camera sweeps across a bright white light in its field of view, it often produces a colored streak across the television screen.

When your eye scans a black-and-white pattern containing fine detail, you will sometimes see subtle colors. For more information, see the book Seeing the Light, by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986).

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Balancing Stick - Does it matter which end is up?

Balancing Stick

 

Does it matter which end is up?

The distribution of the mass of an object determines the position of its center of gravity, its angular momentum, and your ability to balance it! 

 

• One 1/2 inch (1.25 cm) wooden dowel, approximately 3 feet (90 cm) long.
• A lump of clay. (such as electrical tape). 

 

(5 minutes or less)
Place a lump of clay about the size of your fist 8 inches (20 cm) from the end of the dowel.

(5 minutes or more)
Balance the stick on the tip of your finger, putting your finger under the end that's near the clay. Now turn the stick over and balance it with the clay on the top. Notice that the stick is easier to balance when the clay is near the top.

The dowel rotates more slowly when the mass is at the top, allowing you more time to adjust and maintain balance. When the mass is at the bottom, the stick has less rotational inertia and tips more quickly. The farther away the mass is located from the axis of rotation (such as in your hand), the greater the rotational inertia and the slower the stick turns. An object with a large mass is said to have a great deal of inertia. Just as it is hard to change the motion of an object that has a large inertia, it is hard to change the rotational motion of an object with a large rotational inertia.

You can feel the change in inertia when you do the following experiment. Grab the end of the dowel that's near the clay. Hold the dowel vertically, and rapidly move the dowel back and forth with the same motion you would use to cast a fishing line. Next, turn the dowel upside down, and hold it at the end that is farthest from the clay. Repeat the casting motion. Notice that it is much harder to move the dowel rapidly when the clay is near the top. The mass of the stick has not changed, but the distribution of the mass of the stick with respect to your hand has changed. The rotational inertia depends on the distribution of the mass of the stick.

As an alternative, do not demonstrate the Snack in advance. Instead, give a group of people the clay and dowel separately, and challenge them to see who can balance the dowel the longest. Let them discover the role of the clay.

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Balancing Ball - Suspend a ball in a stream of air!

Balancing Ball

 

Suspend a ball in a stream of air

A ball stably levitated on an invisible stream of air is a dramatic sight. When you try to pull the ball out of the airstream, you can feel a force pulling it back in. You can alsofeel that the airstream is being deflected by the ball. This Snack shows one of the forces that give airplanes lift. 
 

Small Snack

• A hair dryer (blower)
• A spherical balloon or table tennis ball
• Tissue paper
• Optional: a stand for the blower

Large Snack

• A vacuum cleaner
  It should come with a reversible hose, like a Shop Vac has, so it can be used as blower.
• A light-weight vinyl beach ball
• Tissue paper
• Optional: A stand for the hose

 

None required. Note, though, that you can make a large or a small Snack (see "Materials"). Depending on the blower you choose, some experimentation may be necessary to find a satisfactory ball. You might want a partner to help you, or you can devise some sort of stand for the blower. That way, your hands will be free to experiment with the ball in the airstream.

(5 minutes or more)
Blow a stream of air straight up. Carefully balance the ball above the airstream. Pull it slowly out of the flow. Notice that when only half the ball is out of the airstream, you can feel it being sucked back in. Let go of the ball and notice that it oscillates back and forth and then settles down near the center of the airstream.

With one hand, pull the ball partially out of the airstream. With the other hand, dangle a piece of tissue paper and search for the airstream above the ball. Notice that the ball deflects the airstream outward. On the large version of this Snack, you can actually feel the deflected airstream hit your hand.
Tilt the airstream to one side and notice that the ball can still be suspended.

Balance the ball in the airstream and then move the blower and the ball toward a wall (try the corner of a room). Notice the great increase in height of the suspended ball. 
 

When the ball is suspended in the airstream, the air flowing upward hits the bottom of the ball and slows down, generating a region of higher pressure. The high-pressure region of air under the ball holds the ball up against the pull of gravity.

When you pull the ball partially out of the airstream, the air flows around the curve of the ball that is nearest the center of the airstream. Air rushes in an arc around the top of the ball and then continues outward above the ball.

This outward-flowing air exerts an inward force on the ball, just like the downward flow of air beneath a helicopter exerts an upward force on the blades of the helicopter. This explanation is based on Newton's law of action and reaction.
Another way of looking at this is that as the air arcs around the ball, the air pressure on the ball decreases, allowing the normal atmospheric pressure of the calm air on the other side of the ball to push the ball back into the airstream.

People immediately raise several questions when they hear the second explanation:

Why does air flowing over a surface in an arc exert less pressure on that surface? To answer this question, consider a rider in a roller coaster going over the top of a hill at high speed. The force that the rider exerts on the seat decreases as the rider goes over the top of the hill. In the same way, the air that arcs around the side of the ball exerts less force on the ball.

Why does air follow the surface of the sphere? Consider what would happen if the air did not curve around the ball. An "air shadow" would be formed above the ball. This air shadow would be a region of low pressure. The air would then flow into the low-pressure air shadow. So the air flows around the ball.

An alternative explanation is provided by the Bernoulli principle. If you pull the ball far enough out of the airstream, then the air flows over only one side of the ball. In fact, the airstream speeds up as it flows around the ball. This is because the middle of the ball sticks farther into the airstream than the top or bottom. Since the same amount of air must flow past all parts of the ball each second, it must flow faster where it is pinched together at the middle. The Bernoulli principle states that where air speeds up, its pressure drops. The difference in pressure between the still air and the moving air pushes the ball back into the center of the airstream.

When you approach a wall with the balanced ball, the highpressure region under the ball becomes a region of even higher pressure. The air that hits the bottom of the ball can no longer expand outward in the direction of the wall. The higher pressure drives the ball to a greater height.

This exhibit illustrates one of the reasons that airplanes fly. A flat wing will fly if it is tipped into the wind, so that it forces air downward. Newton's third law tells us that for every action there must be an equal and opposite reaction: The reaction to the downward force of the wing on the air is the upward force of the air on the wing. You can feel this lifting force if you hold your hand out the window of a moving car and tip your hand so that it forces the air downward.

A wing that is curved on top will deflect air downward and produce lift even if it isn't tipped. The explanation for this is essentially the same as the one given in this Snack. The wing collides with air, creating a region of high pressure in front of the wing. This high pressure produces drag, which is always associated with lift. The high-pressure air in front of the wing accelerates air over the curved surface of the wing. This air then flows downward behind the wing. Airplanes fly because their wings throw air downward.

It is sometimes said that air must flow faster over the curved top surface of a wing than over the flat bottom. This is said to happen because the air has to meet up again at the far end of the wing, and since the air traveling over the curved path must travel farther, it must travel faster. This is not true. Two parcels of air that start together, then split to flow over different sides of a wing, do not, as a rule, rejoin at the far end of the wing.

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