Depth Spinner - What happens when you get off the merry-go-round?

Depth Spinner

 

What happens when you get off the merry-go-round?

Your eyes's motion detectors are fatigued when you watch a rotating spiral. When you look away, the world seems to move toward or away from you. 

 

• Cardboard.
• Glue or tape.
• Pattern disk.
• Access to a copy machine.
• Rotator (record player, portable beater, variable-speed electric drill, hand drill, etc.).
• Optional: adhesive-backed Velcro™.
• Adult help.

(15 minutes or less)

Make a copy of the pattern disk provided as a separate page, here. Cut the pattern out and mount it on a circle of cardboard with glue or tape. Attach the disk at its center to a rotator. Old phonograph players that spin at 45 or 78 rpm are great for this. Adhesive-backed Velcro™ can be used to secure the disk to a variable-speed electric drill. The drill may also be reversed.

(5 minutes or more)

Start the spiral rotating and stare at its center for about 15 seconds.

Look away from the disk and stare at a wall or a nearby person. Notice that the wall or person seems to be expanding or contracting, like he or she is rushing toward you or away from you.

If you can, try rotating the spiral in the opposite direction. Now what happens when you look up from the spinning pattern?

Your visual system is sensitive to inward and outward motion. There are nerve cells in the visual cortex that fire more when objects move outward from the center of your field of view, and others that fire more when objects move inward. When you are looking at something that is standing still, the inward and outward channels are in balance with one another; they send equally strong signals to your brain. When you stare at this moving pattern, however, one detector channel gets tired. Then, when you stare at the wall, the detector that hasn't been working sends a stronger signal to your brain than the tired one.

If, for example, the spiral seemed to be moving away from you, the wall will seem to be moving toward you when you look up. If you rotate the spiral in the other direction, so that it seems to be moving toward you, the wall will then seem to be moving away when you look up.

You can make duplicates of the depth spinner. Make many copies of the pattern disk. Cut out the copies and paste them on small cardboard backings. You can spin your disk on a pencil point or a pin, or attach a disk to a pencil eraser with a pushpin.

Next time you are near a waterfall, try staring at one point of the waterfall for a minute. Then look at a rock or another stationary object to the side of the waterfall. The solid object will seem to flow upward. This apparent motion is due to the fatigue of the channels in your visual system that detect linear upward and downward motion.

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Cylindrical Mirror - This cylindrical mirror lets you see yourself as others see you.

 

 

Cylindrical Mirror

 

This cylindrical mirror lets you see yourself as others see you.

A flat mirror will always reflect an image that is right-side up and reversed right to left. A cylindrical mirror can produce images that are flipped upside down and images that are not reversed. The image you see in a cylindrical mirror depends on the orientation of the mirror and the distance between you and the mirror. 

 

• One 81/2 x 11 inch (22 x 28 cm) sheet of aluminized Mylar™.
• 1 transparent page protector (available in stationery stores).
• Construction paper or other stiff paper backing.

(15 minutes or less)

Put the stiff paper backing behind the Mylar™ and slide them both into the transparent page protector. Bend the Mylar™ to form a portion of a cylinder. When you bend the Mylar™, be sure that the long side is parallel to the axis of the cylinder.

Hold the cylindrical mirror so that its long axis is horizontal. Curve the Mylar™ slightly and look into the mirror. Position yourself so that you can clearly see a reflection of your face. Notice how the image changes when you move closer to or farther from the mirror. When you move far enough away from the mirror, your image will flip upside down.

Wink your right eye. Which eye does the image wink? The image may wink its left eye or its right eye, depending on how far your face is from the mirror. When you are close to the mirror and your image is right-side up, the image winks its left eye. When it is upside down, the image winks its right eye. (If you have trouble deciding which eye the image is winking, have someone stand beside the mirror and do what the image does - that is, wink the same eye as the image. Then ask your partner if he or she is winking the right or left eye. If the image is upside down, your partner will have to turn upside down, too. Your partner can bend over at the waist and look at you between the legs.)

Now orient the cylindrical mirror so that its long axis is vertical. Notice how the image changes when you move closer to and farther from the mirror. Wink your right eye and notice how the image in the mirror responds. When you are close, the image will wink its left eye. When you are far away, it will wink its right eye.

You see the world because light gets into your eyes. You see these words, for example, because light reflecting from this page enters your eyes and makes an image on your retina.
When you make a visual picture of the world, you assume that the light entering your eyes has traveled in a straight line to reach you. But mirrors and other shiny objects change the path of the light, bouncing it back in an organized fashion. When you look into a mirror, you see your image because light reflecting from your face bounces off the mirror and back into your eyes. Your eyes and brain assume that the light has traveled in a straight line to reach your eyes, so you see an image of your face out there in front of or behind the mirror.


What you see in a mirror depends on how the light bounces off the mirror and into your eyes. When light hits a mirror, it bounces off in the same way that a ball would bounce off the mirror. If you threw a ball straight at a flat mirror, it would bounce straight back. If the mirror was curved so that the ball struck the surface at an angle, it would bounce away at an angle.

When you look into an ordinary flat mirror, the image of your face is right side up: Your hair is on top of your head and your chin is underneath. To reach your eyes, the light from your hair hits the mirror at a slight angle and then bounces into your eyes from above - which is why you see your hair on top and your image as right side up.

When you look into a cylindrical mirror with the axis of the mirror horizontal and with your face a foot or more away from the mirror, your image is upside down. That's because the light from your hair bounces off the curved mirror and comes to your eyes from below.

       
To make sense of the angle at which the light is entering your eyes, your eyes and brain must see the image of your face as upside down and a little bit in front of the mirror.

As everyone knows, a flat mirror reverses your right side and your left side. How does it do that? Suppose you are standing face to face with another person. If your right ear points toward the east, his or her left ear will point toward the east. Now, instead of facing another person, suppose you are facing a flat mirror with your right ear pointing to the east. The light from your right ear will bounce off the flat mirror and enter your eyes from the east. Even though your east ear is the east ear of the image, your right ear has become the left ear of the image! (Yes, this is a little mind-boggling at first reading. But once you get it, it will seem simple.)

Now look into the cylindrical mirror with its axis vertical. Stand at least a foot away from the mirror. Once again, place your right ear so that it points to the east. Light from your right ear bounces off the curved mirror and enters your eyes from the west. Light from your right ear appears to come from the right ear of the image. In this cylindrical mirror, you see yourself as others see you. You see the image of your face just a little bit in front of the mirror.

Here's a classic tricky question: "If a flat mirror reverses right and left, why doesn't it reverse up and down?"

The answer is that a flat mirror actually reverses in and out. That is, light that travels "in" to the mirror is bounced back "out" of the mirror. This reversal does not change up into down, but it does change right into left. Consider the outline of the hand below. Is it a right hand or a left hand? You cannot tell which hand it is unless you know whether the palm of the hand is facing "in" to the page or "out" of the page. So right and left depend on in and out.


This hand is either left or right, depending on which way the palm is facing.

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Curie Point - When a piece of iron gets too hot, it is no longer attracted to a magnet

 

Curie Point

 

When a piece of iron gets too hot, it is no longer attracted to a magnet. 

 

A piece of iron will ordinarily be attracted to a magnet, but when you heat the iron to a high enough temperature (called the Curie point), it loses its ability to be magnetized. Heat energy scrambles the iron atoms so that they can't line up and create a magnetic field. Here is a simple demonstration of this effect. 

 

• A small magnet. (Radio Shack's disk magnets work fine.)
• A stand to hold the magnet pendulum and wire. (The stand can be easily made from Tinkertoys™ or pieces of wood.)
• One 6-volt lantern battery (or other 6-volt power supply).
• 2 electrical lead wires with alligator clips at both ends (available at Radio Shack).
• One 3-inch (8 cm) length of thin iron wire, obtainable by separating one strand from braided picture-hanging wire.
• String, about 1 foot (30 cm) long.
• Adult help.

(15 minutes or less)

Make a stand from Tinkertoys™ or other wood as shown in the diagrams. Suspend the magnet from the top of the stand with a string. Make a pendulum at least 4 inches (10 cm) long. Stretch the iron wire between two posts so that, at its closest, the wire is 1 inch (2.5 cm) from the magnet.

(15 minutes or more) Touch the magnet to the iron wire. It 
should magnetically attract and stick to the wire.

Connect the clip leads to the terminals of the lantern battery. Connect one clip lead to one side of the iron wire, and touch the other clip lead to the iron wire on the opposite side of the magnet. Current will flow through the iron wire, causing the wire to heat up. (CAUTION: The wire will get hot!) As the iron heats up and begins to glow, the magnet will fall away from the wire. Take a clip lead away from the iron wire. Let the iron wire cool. When the iron wire is cool, notice that the magnet will stick to it once again.

If the wire does not heat up enough to glow red, move the clip leads closer together. 
 

The iron wire is made of atoms that act like tiny magnets, each one having a north and south pole of its own. These iron atoms usually point in all different directions, so the iron has no net magnetic field. But when you hold a magnet up to the iron, the magnet makes the iron atoms line up. These lined-up atomic magnets turn the iron into a magnet. The iron is then attracted to the original magnet.

High temperatures can disturb this process of magnetization. Thermal energy makes the iron atoms jiggle back and forth, disturbing their magnetic alignment. When the vibration of the atoms becomes too great, the atomic magnets do not line up as well, and the iron loses its magnetism. The temperature at which this occurs is called the Curie point

Inside the earth, there is a core of molten iron. This iron is at a temperature above the Curie point and therefore can't be magnetized. Yet the earth is magnetized, with a north and a south magnetic pole. The magnetic field of the earth comes from an electromagnet, that is, from electrical currents flowing inside the liquid metal core.

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Critical Angle - Why your phone calls don't leak out of optical fibers.

  

Critical Angle

 

Why your phone calls don't leak out of optical fibers.

A transparent material such as glass or water can actually reflect light better than any mirror. All you have to do is look at it from the proper angle. 

 

• A light source  with a well-defined beam. A laser is best, if one is available. Otherwise, you can use a Mini-Maglite� flashlight focused to make a beam, or a slide projector with its beam narrowed. (To narrow the beam of a slide projector, cut an index card the same size as a slide, and then make a hole in the middle of it with a paper holepunch. Put it in the projector so the light only goes through the hole.)
• A rectangular aquarium filled with water.
• A few drops of milk (or some powdered milk) to add to the aquarium water to make the beam visible.

(15 minutes or less)

Fill the aquarium with water. Then add the milk a drop at a time, stirring after each drop, until you can see the light beam pass through the water. If you use powdered milk, add a pinch at a time.

(15 minutes or more)

Direct the light beam upward through the water so that it hits the surface of the water from underneath. You can shine the beam into the water through the transparent bottom of the aquarium, or in through the side wall. (With the Mini-Maglite�, you can seal the light in a watertight plastic bag and place the light right in the water.) The beam will be more visible if you can dim the room lights.

Point the beam so that it hits the surface of the water at just about a right angle. In the aquarium, you may be able to see both the reflected beam, which bounces back into the water, and the refracted beam, which comes out of the water and into the air. (Dust in the air helps you see the refracted beam. You can add chalk dust to the air. You can also search for the beam and track it with a piece of paper.) Notice that most of the beam leaves the water and only a faint beam is reflected back down into the water.

Slowly change the angle at which the beam of light hits the surface of the water. Notice that the beam reflected into the water grows brighter as the beam transmitted into the air becomes dimmer. Also notice that the transmitted beam is bent, or refracted. 

Experiment until you find the angle at which the transmitted beam completely disappears. At this angle, called the critical angle, all of the light is reflected back into the water.  
 

In general, when a beam of light (the incident beam) hits the interface between two transparent materials, such as air and water, part of the beam is reflected and part of it continues through the interface and on into the other material. The light beam is bent, or refracted, as it passes from one material into the next. 

When the angles marked are greater than 49°, light is totally reflected from a water-air surface.	When the angles marked are less than 49°, some light leaves the water.

The farther the beam is from perpendicular when it hits the surface, the more strongly it is bent. If the light is moving from a material with a low speed of light into a material with a higher speed of light (for example, from water into air), the bending is toward the surface. At some angle, the bending will be so strong that the refracted beam will be directed right along the surface; that is, none of it will get out into the air. 

Beyond that angle (the critical angle), all the light is reflected back into the water, so the reflected beam is as bright as the incident beam. This phenomenon is called total internal reflection, because very nearly 100% of the beam is reflected, which is better than the very best mirror surfaces. 

The critical angle for water is measured between the beam and a line perpendicular to the surface, and is 49 degrees. 

Total internal reflection helps transmit telephone messages along optical fibers. Any light that is not aligned parallel to the axis of the fiber hits the wall of the fiber and is reflected (totally!) back inward,since the angle of incidence with which the light hits the wall is much larger than the critical angle. This helps prevent the signal from weakening too rapidly over long distances, or from leaking out when the fiber goes around a curve. This demonstration can also be done by replacing the aquarium and water with a small transparent plastic block, which can be bought at a local plastics supply store. Such blocks are also available as part of the Blackboard Optics� set made by Klinger Scientific.

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Coupled Resonant Pendulums - Take advantage of resonance

Coupled Resonant Pendulums

 

Take advantage of resonance

By taking advantage of resonance, you can cause two pendulums to swing in identical cycles. 

 

• Tape
• A drinking straw
• Scissors
• Four pennies
• Two paper clips
• String (thin)
• Two pencils

Tape the two pencils to the edge of a table as shown in picture above. Cut two strings of equal length (20 to 30 centimeters works well) and tie a paper clip to each end. Tie the other end of each string to the end of a pencil and adjust the knots so that you have two pendulums of equal length. Attach two pennies to each paper clip. With the scissors, shorten the drinking straw to about 15 centimeters, cut small slits along the sides of the straw, and use the straw segment to link the two pendulums together (see the picture above).

Pull one pendulum toward you a short distance and let go. Notice that after a few swings, the second pendulum will begin to oscillate, or swing back and forth, with the same frequency as the first pendulum. With each swing, the second pendulum will increase its amplitude, or the height of its swing. Eventually, the pendulums will swing in unison - the second pendulum will swing in resonance with the first one.

Every pendulum has a natural vibration cycle that depends only on its length. For example, a weight tied to the end of a 25-centimeter-long string will complete one swing "to-and-fro" in about 1 second. The two pendulums in this activity have the same natural frequency because you made them equal in length.

When you start the first pendulum oscillating, it makes the attached drinking straw twist back and forth with the same frequency. Each time the first pendulum completes a swing cycle, the twisting straw gives the second pendulum a tiny shove - like a parent pushing a child on a swing. Because the straw is pushing with the same rhythm as the natural frequency of the second pendulum, the weight swings progressively higher and higher with each tiny push.

I was stopped at a traffic light recently when a loose door panel on my car began to rattle loudly. What was making it vibrate so energetically even though the car was at a complete stop?

Like swinging weights on a pendulum, my door panel has a natural vibration frequency. The pistons, which were moving up and down in my idling engine, matched the resonant frequency of my door panel. Metal between the engine and the car door, like the drinking straw in the pendulum experiment, transmitted the pushes and pulls that eventually got the loose panel to shake violently. Each tiny motion of the car body made the loose door panel vibrate harder and harder - until finally the amplitude of the vibrations were large enough to get my attention.

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