Saturday, July 26, 2014

The Fan Cart

The classic physics problem, the action-reaction pairs in Newton’s Third Law can be explored from one of the projects I have made at The Exploratorium Summer Institute Teacher Training Program.
Let us ask a question to ourselves: “If a sailboat is stuck because there is no wind, is it possible to set up a fan on deck and blow wind into the sail to make the boat move?” The answer to this question can be solved by constructing a “Fan Cart” using simple materials, e.g. a cart, a motor, 4 CDs, a few drinking straws, a fan, a sail, straight round sticks, Velcro fasteners, a pair of small batteries and a battery case.
Make the fan cart look like the one in the pictures or you can design your own. 

Now notice the following observations:
1. Attach the sail and then attach the fan to the cart with Velcro so that it will blow air towards the sail when it is running. Turn on the fan, and observe what happens.
2. Leave the sail in place, but remove the fan assembly and turn it around (or leave the fan assembly in place and reverse the electrical connections to the motor), so that the fan will blow air away from the sail when it is running. Turn on the fan, and observe what happens.
3. Remove the fan assembly, and hold it in your hand while it blows air towards the sail. Observe what happens.
4. Replace the fan assembly so that it will blow air towards the sail when it is running, but then remove the whole sail assembly. Turn on the fan, and observe what happens.
5. Return to the original assembly, with the fan and sail both attached to the cart, and the fan blowing air towards the sail. Now insert a stiff piece of paper between the fan and the sail, and observe what happens.

What's going on?
Here is a summary of the first result from the situations above:
1. Cart doesn't move.
The behavior of the cart is a classic example of Newton's Third Law: For every action, there is an equal and opposite reaction.
In case 1, the fan pushes the air forward, and the air pushes the fan backward. A crucial thing to keep in mind is that the action and reaction forces - often called an action-reaction pair - do not act on the same object. If this was all that was happening, the cart would move backwards; the fan would be pushed backward, and since it's attached to the cart, the cart would be pushed backwards also.

Try to identify the action-reaction pairs in cases 2, 3, 4 and 5 and use them to predict why the cart behaves as it does.

Thursday, July 24, 2014

We can’t believe all that we see

Without a boundary, it's hard to distinguish different shades of gray. Sometimes we can't believe all that we see. Two slightly different shades of the same color may look different if there is a sharp boundary between them. But if the boundary is obscured, the two shades may be indistinguishable.
To try this experiment we can use the image provided below. Attach the white thread tail above the boundary between the two pieces, so that it hangs down and covers the boundary.
The tail like thread is used to obscure the boundary between two gray areas. We see one uniform gray area when the tail is in place, and two different gray areas when the tail is removed. But I have never seen the truth before the experiment. The truth in both gray areas is they are really identical in grades from light gray at one edge to dark gray at the other. In general, our brain ignores slight gradations in gray shades.
If we try this activity with our friends, most of them will see a uniformly gray piece of paper with a rope hanging down the middle.
What is going on?
Actually, the two rectangles are exactly the same. At the right edge both rectangles are light gray. Both become darker toward the left. Where the rectangles meet, the dark part of one rectangle contrasts sharply with the light part of the other, so you see a distinct edge. When the edge is covered, however, the two regions look the same uniform shade of gray.
It is difficult to distinguish between different shades of gray or shades of the same color if there is no sharp edge between them. If there is an edge between the two shades, the difference is obvious.
Your eye-brain system, however, condenses the information it obtains from more than a hundred million light-detecting rods and cones in the retina in order to send the information over a million neurons to your brain. Your eye-brain system enhances the ratio of reflected light at edges. If one region of the retina is stimulated by light, lateral connections turn down the sensitivity of adjacent regions. This is called lateral inhibition. Conversely, if one region is in the dark, the sensitivity of adjacent regions is increased. This means that a dark region next to a light region looks even darker, and vice versa. As a result, your visual system is most sensitive to changes in brightness and color.
When the thread tail is absent and the normal boundary is visible, lateral inhibition enhances the contrast between the two shades of gray. The bright side appears brighter and the dark side darker. When the tail is in place, the boundary between the two different grays is spread apart across the retina so that it no longer falls on adjacent regions. Lateral inhibition then does not help us distinguish between the different shades, and the eye-brain system judges them to be the same.

Wednesday, July 23, 2014

A Simple Oscilloscope

By humming, singing, or talking one can create a variety of cool laser light patterns. This device will allow one to see sound as vibrations or pressure waves. It is called ‘’Vocal Visualizer or Simple Sound Oscilloscope’’ and is one of my first projects at the Exploratorium Summer Institute Teacher Training Program.
I will share how to make this simple device and what is the science behind the working of the device.
We have to cut the pipes into different sizes and arrange it as shown in the picture. Arrange the elbow and “T’’ joints and insert according to the image. Attach the vibration chamber which is made out of drain pipe, balloon and small mirror. Insert the laser into the central single pipe as shown.  Carefully point the laser at the mirror attached to the membrane.
Aim the device on the wall, screen, floor or other reflective surface. Hold the device close to the mouth and hum, sing or just make some weird noises.   
As we make noise, changing the pitch (frequency) and volume (amplitude) we will see a different kind of patterns created on the wall.
What is going on?
When we make sounds, we cause air molecules to vibrate. These vibrating molecules strike one another and hit the rubber membrane. The membrane vibrates, which causes the mirror to wiggle. The laser light bounces off this wiggling mirror, tracing out various shapes and patterns that we can see.
Different amplitude and frequency of sounds coming from your mouth in turn causes different shapes and patterns.
Some shapes look chaotic, others more regular and repeating. Various frequencies will cause the rubber membrane to dance around in resonant vibration modes, in effect creating fluctuating waves. This will be fun for students to learn and play.

Tuesday, July 22, 2014

Colored Shadows: Not all shadows are black

A prism breaks white sunlight up, spreading its component colors out into a spectrum of light visible to the human eye stretching from red through yellow, green and blue to violet. Scientists analyzing these colors find that they have a wave nature, and that one given wavelength of light is perceived as one color when viewed by a person. However, there are colors which do not occur in the spectrum, such as magenta. These colors can only be created when two different wavelengths hit the same spot on the retina at the same time. Without human perception there is no color magenta. Indeed, there is no white either. To understand the colors we must understand the human retina.
The retina of the human eye has three receptors for colored light: one type of receptor is most sensitive to red light, one to green light, and one to blue light. With these three color receptors we are able to perceive more than a million different shades of color.
When a red light, a blue light, and a green light are all shining on the screen, the screen looks white because these three colored lights stimulate all three color receptors on your retinas approximately equally, giving us the sensation of white.
With these three lights you can make shadows of seven different colors: blue, red, green, black, cyan (blue-green), magenta (a mixture of blue and red), and yellow (a mixture of red and green).

When red, R, green, G, and blue, B light shine onto the retina in roughly equal amounts, then humans perceive white, W. So we can say that W = R+G+B.

When red and green light shine on the screen, humans perceive yellow. So Y = R+G. Now yellow is also a color of the spectrum, which means that yellow is the color humans perceive when the retina is illuminated by a single wavelength of light. The single wavelength for yellow is between the wavelengths for red and green, and the yellow causes both the red and green cones to fire nerve impulses. The electrical signal sent to the brain when the eye is illuminated by one wavelength of yellow is similar to the signal sent to the brain by the combination of two wavelengths R+G.

Cyan, C, is a color of the spectrum. The wavelength of cyan light is midway between the wavelengths of blue and green. The crayon that used to be called blue-green is now called cyan, C. Cyan can also be created by adding blue light to green light. C = B+G.
When we mix blue and red light, our eye perceives the color magenta, M. Magenta is not a color of the spectrum: no single wavelength of light can produce the color sensation called magenta. M = R+B.

Blind Spot in our Eye

The eye's retina receives and reacts to incoming light and sends signals to the brain, allowing us to see. There is, however, a part of the retina that doesn't give us visual information. This is our eye's blind spot. We can experiment the blind spot from the following activity.
We need One 8 x 13 cm card. Mark a dot and a cross on a card as shown.
We have to hold the card at eye level about an arm's length away. Close right eye and look directly at the cross with the left eye. We will notice the dot. But focus on the cross and be aware of the dot as we slowly bring the card toward our face. The dot will disappear, and then reappear, as we bring the card toward our face.
Now close left eye and look directly at the dot with your right eye. This time the cross will disappear and reappear as we bring the card slowly toward our face.
What is going on? Our optic nerves carry messages from our eye to the brain. This bundle of nerve fibers passes through one spot on the light sensitive lining, or retina, of our eyes. In this spot, our eye's retina has no light receptors. When we hold the card so that the light from the dot falls on this spot, we cannot see the dot.
A question remains why we don’t notice the blind spot in our day to day observation of the world? Now as a variation on this blind spot activity, we can draw a straight line across the card from one edge to the other, through the center of the cross and the dot. We will notice that when the dot disappears, the line appears to be continuous, without a gap where the dot used to be. So, our brain seems to automatically "fill 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.

Sunday, July 20, 2014

Mimosa - The Sensitive Plant

I was introduced to this plant for the first time in 2013 by two of my students when we walked along the road at the Chokyi Gyatso Institute in Dewathang, Eastern Bhutan. I was surprised to see the behavior of the plant and how it reacted with a gentle touch on it. It closed up and contracted its leaves. The students asked me what the name of the plant was in English and why the leaves are closed up. I neither knew the name nor the scientific reasons behind the closure of the leaves. I said, “sorry and I don’t know.’’ This plant grows abundantly in and around Chokyi Gyatso Institute.  When we saw this plant, I was warned by those two students that ‘’we will accumulate bad karma if we play with the plant.’’ I asked them, why? And they said, they were told by their parents that the process of closing up is very difficult and hard work for the plant.
Recently, when I saw the same plant displayed for an exhibition at the Exploratorium in San Francisco, I was very excited to learn the scientific reason behind the behavior of this plant. I took time to read the label explanation and the plant is called mimosa.
Now, when I compare the reasons stated by my students and the scientific explanations that ‘’the opening and closing of the leaves take a lot of energy from the mimosa plants’’, it make sense to me logically what my students have explained to me.
The mimosa plants can respond differently to harmful versus non harmful touches through regulation of energy. Because of this, mimosas have evolved the ability to habituate to stimuli that aren’t harmful. This means the plant is learning in many ways i.e. which types of touches will hurt it and which won’t.
What is actually happening with the mimosa when it closes up? The mimosas hold up their stems and leaves from the inside, using balloon like sacs filled with water. Our touch activates tiny receptors on the surface of the leaf, which sends a signal to drain the water from the sacs and it closes up. If the plant doesn’t respond, it may be over stimulated and we can try and play with another plant. In the wild, these plants close up during rain, heavy wind or when touched by an animal.
Now, I have a gift of an answer to my students for their then gift of question to me and we will together explore and experiment the mimosas in Dewathang.  

Friday, July 18, 2014

The Sound Cups

The sound cup activity is an interactive, collaborative and fun activity to pair up and introduce students or large group of students, and experiment with sounds through careful listening and observation.
To make students work in pairs, we will need to make 2 identical sets of a single type of sound cup, e.g. make 2 pairs of cups containing straws, 2 pairs of cups containing a penny, 2 pairs of cups that contain a pencil, and so forth. If we want students to work in groups of 4, we will need to make 4 identical sets of a single type of cup, e.g. make 4 pairs of cups containing straws, etc.
We have to do this activity in silence and pass out one sound cup to each student. Ask students to listen carefully to their cups, and then to find the person who has a cup that sounds the same. Have students listen to each other’s cups, and to sit with their sound cup partner when they think they have a match. If the pair of students do not know each other so well, ask them to introduce each other.
This activity can be replicated with students as a part of an experiment with sounds, providing an opportunity to have hands on activity, and improve on their listening and observation skills. This can be done by making the students work together to try to build an identical sound cup using the materials provided in the class. Ask students to put the items such as paper, paper-clip, gravels, sand, pen, eraser, grains, etc. in one cup, then place another cup upside down on top of it and tape the cups together. Make another cup exactly the same way before you start to make a different type of sound cup. Encourage them to talk as they try to make a sound cup that is identical to their own. They may not open their original cups as they try to make a matching sound cup.
Once students have finished building and taping their new sound cup together, have each group present their work. Each group should demonstrate the sound of the original cups, talk about what they noticed and how they decided to make the sound cup that matched their original pair.
The science behind this simple and fun activity is that the plastic cup makes wonderful resonant chambers inside, allowing simple objects to sound very strange. Only through careful observations will students be able to make accurate matching sound cups.
From this simple activity students can learn and venture into the world of investigation and experimentation in fun and exciting ways.