Disappearing Glass Rods - You can make glass objects disappear.

      

Disappearing Glass Rods

 

You can make glass objects disappear. 

 

Glass objects are visible because they reflect some of the light that shines on them and bend or refract the light that shines through them. If you eliminate reflection from and refraction by a glass object, you can make that object disappear.

• Wesson™ oil. (Regular, not lite.)
• One or more Pyrex® stirring rods or other small, clear glass objects.
• A beaker.
• Optional:  glass eyedropper, glass magnifying lens.

(1 minute or less)
Pour some Wesson™ oil into the beaker.

(15 minutes or more)
Immerse a glass object in the oil. Notice that the object becomes more difficult to see. Only a ghostly image of the object remains. If you do this as a demonstration, keep your audience at a distance to make it harder for them to see the ghost object.

Experiment with a variety of glass objects, such as clear marbles, lenses, and odd glassware. Some will disappear in the oil more completely than others.

You can make an eyedropper vanish before your eyes by immersing it and then sucking oil up into the dropper.
Immerse the magnifying lens in the oil. Notice that it does not magnify images when it is submerged.

When light traveling through air encounters a glass surface at an angle, some of the light reflects. The rest of the light keeps going, but it bends or refracts as it moves from the air to the glass. You see a glass object because it both reflects and refracts light.

When light passes from air into glass, it slows down. It's this change in speed that causes the light to reflect and refract as it moves from one clear material (air) to another (glass). Every material has an index of refraction that is linked to the speed of light in the material. The higher a material's index of refraction, the slower light travels in that material.

The smaller the difference in speed between two clear materials, the less reflection will occur at the boundary and the less refraction will occur for the transmitted light. If a transparent object is surrounded by another material that has the same index of refraction, then the speed of light will not change as it enters the object. No reflection and no refraction will take place, and the object will be invisible.

Wesson™ oil has nearly the same index of refraction (n) as Pyrex® glass (n = 1.474). Different glasses have different indices of refraction. In Wesson™ oil, Pyrex® disappears, but other types of glass &emdash; such as crown glass or flint glass &emdash; remain visible. Fortunately for us, a great deal of laboratory glassware and home kitchen glassware is made from Pyrex® glass.

For most Pyrex® glass, the index matching with Wesson™ oil is not perfect. This is because the Pyrex® glass has internal strains that make its index of refraction vary at different places in the object. Even if you can match the index of refraction for one part of a Pyrex® stirring rod, the match will not be perfect for other parts of the rod. That's why a ghostly image of the rod remains even with the best index matching.

The index of refraction of the oil (and of the glass, too) is a function of temperature. This demonstration will work better on some days than others.

Index of refraction is sometimes called optical density, but optical density is not the same as mass density. Two materials can have different mass densities even when they have the same index of refraction. Though Pyrex® glass and Wesson™ oil have similar indices of refraction, Pyrex® sinks in Wesson™ oil because it has a higher mass density than the oil. Wesson™ oil has a higher index of refraction than water (n = 1.33), but it has a lower mass density and floats on water. The index of refraction depends not only on density, but also on the chemical composition of a material.

You can also make Pyrex® glass disappear by immersing it in mineral oil, which is available from pharmacies or chemical supply houses. However, mineral oil comes in light, medium, and heavy weights, and each variety has a different index of refraction. To match the index of refraction of Pyrex® glass, you'll need a mixture of mineral oils of different weights. To create the proper mixture, place a Pyrex® glass object into a large glass beaker and pour in enough heavy mineral oil to submerge it partially. Slowly add light mineral oil and stir. 

Watch the glass object as you pour. Most Pyrex® glass will disappear when the mixture is about 2 parts heavy mineral oil to 1 part light mineral oil. Notice the swirling refraction patterns as you mix the two oils.

Karo™ syrup is another material that has an index of refraction close to that of glass. Karo™ can be diluted with water to match some varieties of glass.

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Disappearing Act - If you want to stay hidden, you'd better stay still.

 

Disappearing Act

 

If you want to stay hidden, you'd better stay still. 

 

Some animals blend in with their surroundings so well that they're nearly impossible to see. Only when these animals move can you detect their presence and shape. With this Snack, you can compare what you see when a camouflaged figure remains still to what you see when the figure is moving. 

 

• 2 pieces of dark blue or black construction paper
• Liquid correction fluid.
• A piece of clear plastic the same size as the construction paper. )
• A partner.

(30 minutes or less)
Cut out an animal shape from one of the pieces of paper. Leave a projecting rectangle of paper to serve as a handle (see photo).

Use correction fluid to make a random pattern of dots on both the animal figure and the second piece of paper. The second piece of paper will act as the background for the figure.

Place the figure on the background and cover both pieces of paper with the plastic. The transparent covering keeps the edges of the animal flat against the background.

(5 minutes or more)
View the animal cutout against the background from an arm's length away. It should be very difficult, if not impossible, to detect the shape of the animal. If you can see the edges, move about 6 feet (2 m) away and have a friend hold the animal and the background.

Place the cutout so that you can use the handle to move the animal while it is under the glass or plastic. Notice that this movement makes it easy to detect the presence of the animal and to identify its shape.

By making several different shapes you can make a game of this. Can anyone identify the animal before it moves? Who can identify it first when it moves?

Many animals have patterns of color on their bodies that allow them to blend into the background. These animals are hard to detect when they're still. But when the animals move, you can easily pick them out. That's because humans, as well as many other animals, have specialized brain cells that detect motion. These cells receive information from the light-sensitive cells at the back of the eye.

What animals can you think of that use camouflage to blend into their environment?

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The Dipping Bird - The dipping bird seems to go forever but it's not perpetual motion!

The Dipping Bird

 

The dipping bird seems to go forever but it's not perpetual motion! 

 

A dipping bird is an example of a heat engine. It converts a difference in temperature (between the head cooled by evaporation and the bottom at room temperature) into cyclical motion. 

 

 

 

• Dipping bird (can be obtained from Edmund Scientific, catalog number 53617, $7.95 for a package of 2 birds. You can also try novelty or magic shops.)
• Cup or glass
• Water

 

Wet the bird's head thoroughly with water. Allow enough time for the fuzzy material on the head to absorb water (a few seconds should do it).

Fill a cup or glass with water and place it so that the bird's beak will dip into the water each time the bird tips. You may have to place pieces of wood or cardboard under the cup or glass if it's too short, or get a smaller glass if it's too tall.

Watch the bird go through its cycle. Notice what happens to the liquid inside the bird at different positions in the cycle.

When the bird is manufactured, most of the air is removed from the inside. The gas that remains is largely the vapor from the red liquid, which vaporizes very easily. When the fuzzy coating on the bird's head is wet, water evaporates and cools the vapor inside the bird's head. This condenses the vapor back to red liquid and reduces the pressure in the bird's head. When the fuzzy coating on the bird's head is wet, water evaporates and cools the vapor inside the bird's head. This condenses the vapor back to liquid and reduces the pressure in the bird's head. The bird's head keeps moving.

Since the pressure of the vapor in the bird's body is now higher than the pressure in its head, liquid is forced from the bottom up the tube toward the head. As the liquid moves up the tube, the center of gravity of the bird is raised, and the bird begins to tip around its fulcrum. When the bird finally dips into the water, a clear passage is opened between the head and the body, allowing the pressures to equalize and the liquid to fall back down to the body. The bird returns to the upright position and the whole process repeats.

Each time the bird's beak dips into the water, the fuzzy material absorbs a little water to replace any that has evaporated. This prevents the bird's head from drying out. The bird will continue its cycle until the head dries out, and evaporation can no longer cool it.

In summary, the steps in the cycle are as follows:

1. The bird's head dips and gets wet.
2. Water evaporates from the fuzzy head.
3. The vapor in the bird's head condenses into liquid.
4. Pressure in the bird's head is reduced because the liquid takes up less space than the vapor.
5. Liquid moves up the tube into the low- pressure area in the head; the cycle repeats.
6.  

An interesting extension is to paint the bottom chamber of the bird black. An essential requirement to make the bird dip is to get the head cooler than the body. Normally this is accomplished by evaporation of water from the head. By painting the body black and exposing the bird to a hot lamp or to sunlight, the body will become warmer than the head. In this way, you can either enhance the normal operation of the duck, or get it to operate without wetting the head at all.

Don rathjen has measured the power output of a dipping bird by attaching it to a windlass and using it to raise paper clips. He managed to extract a nanohorsepower of work from his dipping duck. (A nano-horsepower is about a microwatt.)

References

1. Mentzer, Robert, "The Drinking Bird - The Little Heat Engine That Could," The Physics Teacher, February 1993.

2. Bent, Harry, and Harold Teague, "The Hydro-Thermal-Dynamical Duck; A Sketch of His Uses in the Classroom and the Laboratory," Journal of College Science Teaching, September 1978.

3. Rathjen, Don, "Duckpower," Exploring, Winter 1994, pp. 7 - 8. ("Duckpower" shows how to use the dipping bird as a heat engine to lift a weight, and discusses the work and the horsepower involved. Exploring is the quarterly magazine of the Exploratorium.)

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Diffraction - Light can bend around edges

 

Diffraction

 

Light can bend around edges. 

 

Light bends when it passes around an edge or through a slit. This bending is called diffraction. You can easily demonstrate diffraction using a candle or a small bright flashlight bulb and a slit made with two pencils. The diffraction pattern, the pattern of dark and light created when light bends around an edge or edges, shows that light has wavelike properties.

 

 

 

• 2 clean new pencils.
• A piece of transparent tape. (Any thin tape will do.)
• A candle.

OR

• a Mini-Maglite® flashlight (available for under $15 in many hardware stores). Do not substitute other flashlights.

OR

• A flashlight bulb for a Mini-Maglite®, two AA batteries, a battery holder (available from Radio Shack), and two clip leads.
• Optional: pieces of cloth, a feather, plastic diffraction grating, a metal screen, a human hair.
•  

(5 minutes or less) Light the candle or, if you are using a Mini-Maglite®, unscrew the top of the flashlight. The tiny lamp will come on and shine brightly. You can also make your own bright point source of light by attaching the Mini-Maglite® flashlight bulb to the battery holder with the clip leads. Be sure you put two AA batteries in the battery holder.
Wrap one layer of tape around the top of one of the pencils, just below the eraser.

(15 minutes or more) If you measure distances on the diffraction pattern, you can calculate the wavelength of light emitted by the candle or bulb.

Place the light at least one arm-length away from you.
Hold the two pencils vertically, side by side, with the erasers at the top. The tape wrapped around one pencil should keep the pencils slightly apart, forming a thin slit between them, just below the tape. Hold the pencils close to one eye (about 1 inch [2.5 cm] away) and look at the light source through the slit between the pencils. Squeeze the pencils together, making the slit smaller. Notice that there is a line of light perpendicular to the slit. While looking through the slit, rotate the pencils until they are horizontal, and notice that the line of light becomes vertical.

If you look closely you may see that the line is composed of tiny blobs of light. As you squeeze the slit together, the blobs of light grow larger and spread apart, moving away from the central light source and becoming easier to see. Notice that the blobs have blue and red edges and that the blue edges are closer to the light source.

Stretch a hair tight and hold it about 1 inch (2.5 cm) from your eye. Move the hair until it is between your eye and the light source, and notice that the light is spread into a line of blobs by the hair, just as it was by the slit. Rotate the hair and watch the line of blobs rotate.

Look at the light through a piece of cloth, a feather, a diffraction grating, or a piece of metal screen. Rotate each object while you look through it.

The black bands between the blobs of light show that there is a wave associated with the light. The light waves that go through the slit spread out, overlap, and add together, interacting in complex ways to produce the diffraction pattern that you see. Where the crest of one wave overlaps with the crest of another wave, the two waves combine to make a bigger wave, and you see a bright blob of light. Where the trough of one wave overlaps with the crest of another wave, the waves cancel one another out, and you see a dark band.

The angle at which the light bends is proportional to the wavelength of the light. Red light, for instance, has a longer wavelength than blue light, and so it bends more than blue light does. This different amount of bending gives the blobs their colored edges: blue on the inside, red on the outside.

The narrower the slit, the more the light spreads out. In fact, the angle between two adjacent dark bands in the diffraction pattern is inversely proportional to the width of the slit.

Thin objects, such as a strand of hair, also diffract light. Light that passes around the hair spreads out, overlaps, and produces a diffraction pattern. A piece of cloth or a feather, which are both made up of many smaller, thinner parts, produce complicated diffraction patterns.

In a dimly lit room, look at a Mini-Maglite® bulb with one eye (a candle will not work). Notice the lines of light radiating out from the light source, like the seeds radiating out from the center of a dandelion. Propose experiments to find the origin of these lines. For example, rotate the light source, and notice that the lines of light do not rotate. Rotate your head, and notice that the lines do rotate. Hold your hand or an index card in front of your eye so that it doesn't quite block your view of the light source. Notice that you still see a full circle of lines radiating out from the light source. The lines of light are spread out onto your retina by imperfections in the tissues of your cornea.

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Diamagnetism - Push me a grape

Diamagnetism

 

Push me a grape.

 

A grape is repelled by both the north and south poles of a strong rare-earth magnet. The grape is repelled because it contains water, which is diamagnetic. Diamagnetic materials are repelled by magnetic poles. 

 

• Two large grapes
• Drinking straw
• Film canister with lid
• Push pin
• Small knife or razor blade
• Neodymium magnet

 

Insert the push pin through the underside of the film canister lid and put the lid on the canister so that the point of the pin is sticking out.

Find the center of the drinking straw and use the knife to cut a small hole, approximately 0.5 cm x 1 cm. (You can also use the hot tip of a soldering gun to melt a hole.)

Push one grape onto each end of the straw. Balance the straw with the grapes on the point of the push pin; the point of the pin goes through the small hole on the straw.


side view

Bring one pole of the magnet near the grape. Do not touch the grape with the magnet.


The grape will be repelled by the magnet and begin to move slowly away from the magnet.

Pull the magnet away and let the grape stop its motion.

Turn the magnet over and bring the other pole near the grape. 

The grape will also be repelled by the other pole; the grape is repelled by both poles of the magnet.

Ferromagnetic materials, such as iron, are strongly attracted to both poles of a magnet.

Paramagnetic materials, such as aluminum, are weakly attracted to both poles of a magnet.

Diamagnetic materials, however, are repelled by both poles of a magnet. The diamagnetic force of repulsion is very weak (a hundred thousand times weaker than the ferromagnetic force of attraction). Water, the main component of grapes, is diamagnetic.

When an electric charge moves, a magnetic field is created. Every electron is therefore a very tiny magnet, because electrons carry charge and they spin. Additionally, the motion of an orbital electron is an electric current, with an accompanying magnetic field.

In atoms of iron, cobalt, and nickel, electrons in one atom will align with electrons in neighboring atoms, making regions called domains, with very strong magnetization. These materials are ferromagnetic, and are strongly attracted to magnetic poles.

Atoms and molecules that have single, unpaired electrons are paramagnetic. Electrons in these materials orient in a magnetic field so that they will be weakly attracted to magnetic poles. Hydrogen, lithium, and liquid oxygen are examples of paramagnetic substances.

Atoms and molecules in which all of the electrons are paired with electrons of opposite spin, and in which the orbital currents are zero, are diamagnetic. Helium, bismuth, and water are examples of diamagnetic substances.

If a magnet is brought toward a diamagnetic material, it will generate orbital electric currents in the atoms and molecules of the material. The magnetic fields associated with these orbital currents will be oriented such that they repelled by the approaching magnet.

This behavior is predicted by a law of physics known as Lenz's Law. This law states that when a current is induced by a change in magnetic field (the orbital currents in the grape created by the magnet approaching the grape), the magnetic field produced by the induced current will oppose the change.

Try fruits other than grapes; a fruit such as watermelon, which has a high water content, works well. Cut the fruit into grape-sized chunks.

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Descartes' Diver - To paraphrase the French philosopher Rene Descartes: "I sink, therefore I am."

 

Descartes' Diver

To paraphrase the French philosopher Rene Descartes: "I sink, therefore I am."

Squeezing the sides of a plastic soda bottle changes the fluid pressure inside. Changes influid pressure affect the buoyancy of a Cartesian diver made from an eyedropper or a Bic™ pen. The diver floats, sinks, or hovers in response to pressure changes. There are two different versions to choose from here. 
 

Eyedropper Diver

• An eyedropper.
• A tall drinking glass.
• Room-temperature water.
• One 2-liter soda bottle with screw-on cap.
• Optional: Thin, flat bottle (an empty dish washing liquid or shampoo bottle, for instance).

Bic™ Pen Diver

• A Bic™ ballpoint pen with transparent plastic body.
• Pliers.
• A small lump of modeling clay the size of a pea.
• A tall drinking glass or wide-mouthed container.
• One 2-liter soda bottle with screw-on cap.
• Room-temperature water.
• Optional: Thin, flat bottle (an empty dish washing liquid or shampoo bottle, for instance).

Eyedropper Diver

(5 minutes or less)
Fill the tall drinking glass with room-temperature water. Gradually draw water into the eyedropper until the eyedropper floats in the glass with its top barely above the surface.
Fill the soda bottle almost to the top with room-temperature water. Transfer the eyedropper into the soda bottle. Be careful not to change the amount of water in the dropper while doing this. Screw the cap onto the bottle tightly.

Bic™ Pen Diver

(5 minutes or less)
Remove the ink cartridge from the pen with a pair of pliers: It will come out easily. Notice that the empty pen body is open at one end and plugged at the other. Attach a small amount of clay around the outside of the tube near the open end, without plugging the hole. This is just for weight.

You can use another bit of clay to plug the small air hole in the side of the tube, or you can leave the air hole unplugged, allowing the water to rise higher in the tube. If you like, you can also saw the tube off to a shorter length to make a smaller diver. If you shorten the tube or leave the hole open, you will need less clay to adjust the diver's buoyancy.

Test and adjust the diver by placing it open-end-down in the drinking glass or other wide-mouthed container. Add or remove clay until the diver floats with about 1/4 inch (6.25 mm) sticking out of the water.

Fill the soda bottle almost to the top with room-temperature water. Place the diver open-end-down in the almost-full bottle, and screw the cap on tightly.

Squeeze the soda bottle to make the diver sink, rise, or hover at any depth. You also want to test your diver's responses in a thin, flat bottle, such as a bottle that originally contained dishwashing liquid or shampoo. To add a little spice, you can decorate the top of the eyedropper so that it becomes a diver with a funny face, or find small, hollow, open-bottomed toy figures to use as divers. You can also decorate the bottle. Use your imagination and creativity!

The Greek philosopher Archimedes was the first person to notice that the upward force that water exerts on an object, whether floating or submerged, is equal to the weight of the volume of water that the object displaces. That is, the buoyant force is equal to the weight of the displaced water.

As you squeeze the bottle, you increase the pressure everywhere in the bottle. The higher pressure forces more water into the eyedropper, compressing the air in the eyedropper. This causes the dropper to displace less water, thus decreasing its buoyancy and causing it to sink. When you release the sides of the bottle, the pressure decreases, and the air inside the bulb expands once again. The dropper's buoyancy increases, and the diver rises. If you look carefully, you can see the level of water changing in the dropper as you vary the pressure on the bottle.

If you use a thin, flat bottle, squeezing on the wide sides of the bottle will increase the pressure inside the bottle, but squeezing on the narrow sides will cause the volume of the bottle to expand and the pressure inside to decrease. If you use such a bottle, adjust the weight or water content of a Cartesian diver so that it barely floats. When this diver reaches the bottom of the bottle, it will stay there, even when you stop squeezing on the wide sides. You must squeeze the narrow sides to drive the diver to the surface. It will then stay at the surface even when the squeezing stops.
The key to this behavior is to carefully adjust the diver initially, so that it barely floats. As the diver sinks, the pressure outside the diver increases slightly with the water's depth. This increase is in addition to the increase in pressure you cause by squeezing the bottle. When the diver reaches the bottom and you stop squeezing, the pressure resulting from the increase in depth remains and continues to compress the air bubble a little. If the diver has been carefully balanced, this small compression of the bubble will be enough to keep the diver submerged. The process reverses when you squeeze the narrow sides to raise the diver.

Since ships float, their weight must be equal to the buoyant force of the water. The weight of a ship is therefore called its displacement.

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