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.

Read more »

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.

Read more »

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.

Read more »

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.

Read more »

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.

Read more »