Blue Sky - Now you can explain why the sky is blue and the sunset is red!

    

Blue Sky

 

Now you can explain why the sky is blue and the sunset is red

When sunlight travels through the atmosphere, blue light scatters more than the other colors, leaving a dominant yellow-orange hue to the transmitted light. The scattered light makes the sky blue; the transmitted light makes the sunset reddish orange. 

 

• 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

 

(15 minutes or less)

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 beam shining through the liquid.

(15 minutes or more)

Look at the beam from the side of the tank and then from the end of the tank. You can also let 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 beam change from blue-white to yelloworange along the length of the beam.

If you want to look at a narrower beam of light, use a paper hole-punch to punch 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 a different frequency, and therefore wavelength, of light. The colors in the rainbow spectrum are arranged according to their frequency: violet, indigo, and blue light have a higher frequency than red, orange, and yellow light.

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


The shorter the wavelength of light, the more it is scattered by the atmosphere. Because it has 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 of red light. That is, if the electrons bound to air molecules are pushed, they will oscillate with a natural frequency that is even higher than the frequency of blue light. Blue light pushes on the electrons with a frequency that is closer to their natural resonant frequency than that of red light. This causes the blue light to be reradiated out in all directions, in a process called scattering. The red light that is not scattered continues on in its original direction. When you look 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 light takes a longer path through the atmosphere to your eyes than when the sun is directly overhead. By the time the light of the setting sun reaches your eyes, most of the blue light has 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 blue light 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 views it from the side. Notice that when the top person sees a bright beam, the side person will see a 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 the two polarizations are aligned, the beam will be bright; when they are at right angles, the beam will be dim.

Scattering polarizes light because light is a transverse wave. The direction of the transverse oscillation 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 all directions. horizontally, vertically, and all angles in between. Consider only the vertically polarized light passing through the tank. This light can scatter to the side and remain vertically polarized, but it cannot scatter upward! To retain the characteristic of a transverse wave after scattering, only the vertically polarized light can be scattered sideways, and only the horizontally polarized light can be scattered upward. This is shown in the drawing.

Read more »

Blind Spot - To see, or not to see!

Blind Spot

 

To see, or not to see

The eye's retina receives and reacts to incoming light and sends signals to the brain, allowing you to see. There is, however, a part of the retina that doesn't give you visual information. This is your eye's blind spot. 

 

• One 3 X 5 inch (8 x 13 cm) card or other stiff paper
• A meterstick

 

(5 minutes or less ) Mark a dot and a cross on a card as shown.

(5 minutes or more)

Hold the card at eye level about an arm's length away. Make sure that the cross is on the right.

Close your right eye and look directly at the cross with your left eye. Notice that you can also see the dot. Focus on the cross but be aware of the dot as you slowly bring the card toward your face. The dot will disappear, and then reappear, as you bring the card toward your face.

Now close your left eye and look directly at the dot with your right eye. This time the cross will disappear and reappear as you bring the card slowly toward your face.

Try the activity again, this time rotating the card so that the dot and cross are not directly across from each other. Are the results the same?
 

The optic nerve carries messages from your eye to your brain. This bundle of nerve fibers passes through one spot on the light sensitive lining, or retina, of your eye. In this spot, your eye's retina has no light receptors. When you hold the card so that the light from the dot falls on this spot, you cannot see the dot.

As a variation on this blind spot activity, draw a straight line across the card, from one edge to the other, through the center of the cross and the dot. Notice that when the dot disappears, the line appears to be continuous, without a gap where the dot used to be. Your brain automatically "fills in" the blind spot with a simple extrapolation of the image surrounding the blind spot. This is why you do not notice the blind spot in your day-to-day observations of the world.

Using a simple model for the eye, you can find the approximate size of the blind spot on the retina.

Mark a cross on the left edge of a 3 x 5 inch (8 x 13 cm) card. Hold the card 9.75 inches (25 cm) from your eye. (You will need to measure this distance; your distance from the card is important in determining the size of your blind spot.)

Close your left eye and look directly at the cross with your right eye. Move a pen on the card until the
point of the pen disappears in your blind spot. Mark the places where the penpoint disappears. Use the pen to trace the shape and size of your blind spot on the card. Measure the diameter of the blind spot on the card.

In our simple model, we are assuming that the eye behaves like a pinhole camera, with the pupil as the pinhole. In such a model, the pupil is 0.78 inches (2 cm) from the retina. Light travels in a straight line through the pupil to the retina. Similar triangles can then be used to calculate the size of the blind spot on your retina. The simple equation for this calculation is s/2 = d/D, where s is the diameter of the blind spot on your retina, d is the size of the blind spot on the card, and D is the distance from your eye to the card (in this case, 9.75 inches [25 cm]).

Read more »

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.

Read more »

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.

Read more »

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).

Read more »