Our latest podcast in the Beyond Penguins and Polar Bears webzine has been posted.  This is a bimonthly webzine for elementary educators, to integrate polar science into their teaching.   This month’s webzine is on arctic peoples, and the podcast features a story on how light disappears and reappears in the arctic each year, that you can play right in your classroom.  Plus, suggestions on how to use this story with your elementary students.

The Boy Who Found the Light

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I’m not actually committing to posting a physics toy every Tuesday, but I’ll start small.

One of my favorite places to watch people back at the Exploratorium was the colored shadows exhibit.  This one’s always a winner.

Images from http://www.flickr.com/photos/soyunterrorista

This is an example of color addition.  Remember this from grade school? I only remember it because I had to teach it.  Color subtraction is what happens when you mix together pigments.  Red pigment absorbs all light but red (which is reflected to your eye).  Blue pigment absorbs all light but blue.  So mix red and blue and you’ve subtracted all colors, getting black.

Light’s weird, though.  You mix together all colors of light and you get white.  The primary colors of light are red, green, and blue.  You have receptors in your eyes for each of those colors.  If your eye senses both red and green light at the same place, your brain says “cyan” (sort of blue-green).  The really weird one is that red and green light together make yellow.  So, that’s why the shadows are colored.  The white light has all colors (R+G+B).  If you block just one of the lights, (say, the blue one) then you get (R+G+B) minus (B) which equals (R+G), or yellow.   Block the blue and the green lights and you get (R+G+B) minus (B) and minus (G), or Red.  Block all three, and you get a normally colored black shadow.

Of course, even if your receptors get the same amount of light as someone standing next to you, your brain might interpret that color differently, so people will often disagree if something is orange or yellow, for example.

Arbor Scientific has a version (Color Addition Spotlights) of this that you can buy for your classroom or, hey, if you’ve got a dorm room and some extra cash, wow, this would be a really cool party trick.  It’s actually not that expensive, considering.  But if it’s too much for you, they’ve got a Spectrum Demo kit that teaches some of the same stuff using your overhead projector (better spectrum than a wimpy little prism demo).

You can make this on the cheap from the Exploratorium’s Science Snacks website (which also has a good explanation of the science behind it) and a more detailed lesson on Paul Doherty’s teacher institute page.  And here’s a link to the Teacher’s Lab with some step by step science explanations, and how to use the science of light and color in the classroom.  And Science Buddies gives some detailed inquiry lessons using colored shadows, and a video of students doing the activity.

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Welcome to the first post at the new blog location!  Now that my webhome is established I can start posting more regularly.  Geek on!

Sebastien Martin of the Exploratorium has been working with what you can do with light traces — basically, tracing out the motion of something through space using light (say, by attaching an LED to the object and taking a time-lapse picture of it) for a while. He did some stuff with the science of baseball showing the path of a baseball through space, and has also posted some great pix on Flickr. Here is what he says about how to make these traces:

All the pictures were taken with a regular digital camera set to an exposure time of 0.5 to 3 seconds. The lines you see were created by LED lights attached to moving objects.

You can use any small light source to make the trace of a moving object visible (bicycle light, flash light, key light). Just attach the light to the object, and make sure the room is completely dark. Then take a long exposure picture of the moving light using your Digi Cam.

It’s also fun to go further and analyze the speed of the object! To do that, use a fast blinking LED light (such as the Inova pulsed LEDs you can buy at Target Stores for $7). The distance between the dots is a measure of speed.

In fact, Sebastien’s whole Flickr library is a source of amazingly creative science stuff.

(photo from Sebastien Martin)

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OK, I’ve been posting everybody else’s YouTube videos, so what about METube? After all, it’s all about me.

Here is my YouTube debut, talking about infrared light as part of a full-length webcast on climate change. This was totally fun, I left Paul D. back at the webcast studio and ran off-stage, across the museum to the infrared heat camera exhibit, where a museum person had kept visitors away from the exhibit for me. I was wearing headphones so I could hear what Paul was saying (and, confusingly, all the webcast film crew as well, off-air). There is, cleverly, a video feed from the heat camera to the webcast studio, so my image in infrared was piped directly to the webcast. But what was oddest, of course, is that I was just sitting at an exhibit in the middle of the museum, talking to nobody (there’s not even a camera in front of me), watching my image in the infrared while I try to explain about infrared (made a key mistake, can you catch it?).

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Yeah, yeah, I know, this is old news, but I finally got around to reading the articles about the fact that Scotch tape emits x-rays. I’ve known for a while that when you stick scotch tape to something and then peel it off, the scotch tape gets charged (negatively for those who care). This is a great way to make a cheap electroscope for your classroom (or just anytime you want to find out the charge on something). Just stick Scotch tape to a table, peel it off, and then hold it near some Charged Object. If the tape is repelled, then the Charged Object is negatively charged (since like charges repel). Try it, it’s cool.

So, anyway, when you peel the tape off the table, it gets negatively charged by ripping electrons off the table. This is, in effect, a current — electrons are flowing from the table to the tape. If you peel tape off a table in a dark room you’ll see light. From what I gather, as the electrons slow down when they hit the tape, they give off radiation (this would be Bremsstrahlung or “Braking” radiation). When you do this in a dark room, you’re seeing that radiation as visible light.

The new research shows that if you do it in a vacuum, instead of these visible photons (which are just a form of electromagnetic radiation with a relatively low energy), you get x-rays (electromagnetic radiation with high energy). The x-rays were strong enough to take a picture of one of the researchers’ finger.

The NY Times article on this says:

All of the experiments were conducted with Scotch tape, manufactured by 3M. The details of what is occurring on the molecular scale are not known, the scientists said, in part because the Scotch adhesive remains a trade secret.

Other brands of clear adhesive tapes also gave off X-rays, but with a different spectrum of energies. Duct tape did not produce any X-rays, Dr. Putterman said. Masking tape has not been tested.

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It can be hard to change your view of things. I was just talking about this with a friend last night — we get used to a certain model of the world in science, and it’s rather revolutionary to see the world in a different way. If you see something that doesn’t fit your view of how the world works, you can literally not see it. That’s what happened to Newton when he saw (or rather didn’t see) the evidence that light is really a wave. I just posted an episode of my Science Teaching Tips podcast where Exploratorium staff physicist Paul Doherty tells how to do the same experiment that Newton did back in the 1650’s, so you can see what he didn’t, and confirm the wave nature of light. Listen to the episode — Seeing the light.
Paul Doherty’s Web site

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A neat observation from one of the staff physicists at the Exploratorium:

Here is a little game to play with farsighted and nearsighted glasses. Ask all your students who wear glasses to put them on and stand up. Walk up to each of them, look into their eyes and you will be able to tell them if they are nearsighted or farsighted.

If they are farsighted (and therefore have convex lenses) you will see the contour of their cheeks move OUT when viewed through their glasses. If they are nearsighted (and therefore have concave lenses) you will see the contour of their cheeks move IN when viewed through their glasses. This is a nice opportunity for a ray diagram or two! Astigmatism, graded lenses and bifocals can make this more difficult, but it is fun to try. The stronger the prescription the better. Holding far and nearsighted glasses up to colored lights or shadows also produces discriminating effects.

This could be a great “nature of science” activity! Tell them you have mystical powers and can see the shape of their retina (or some such garbage) just by looking deeply into their eyes. (Of course, it won’t work with any students who wear contacts! Why not? Can they guess how you do it?)

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A teacher on a teachers’ listserv asked some fine questions about the nature of light. Here are her questions, and my answers.

1) If light is energy that is emitted by accelerating electric charges – often electrons in atoms – how do teachers explain the fact that light moves through a vacuum?

I’m not sure how teachers explain it, but the accelerating electric charges *emit* light. The light then self propagates. Sort of like how a ballplayers arm throws a ball, but once moving, the ball no longer needs the thrower’s arm to keep moving after the initial toss. Except in the case of light, the ball (photon) keeps itself going, and doesn’t stop moving.

And an additional comment from Paul Doherty:

Take two balloons, hang them from the ceiling of the room with strings so they are about waist high and touching each other. Then, rub them with wool.

They move apart repelling each other.

We say that each balloon creates an electric field that exerts a repulsive force on the other balloon which has the same electric charge. The electric field is a straight line between the centers of the balloons.

If you put the balloons in a vacuum they will still repel. The electric field has no problem going through a vacuum.

Move one balloon and the other will move in response. By moving one balloon you change the electric field direction.

The change in the electric field actually propagates down the electric field line as a wave.

To move the balloon to the side you must accelerate it to the side. And this acceleration makes a kink in the electric field line that propagates along the electric field line and is the electric part of an electromagnetic wave.

I find that kids have an easier time with this model, for some reason they accept the existence of elecric fields in a vacuum better than the existence of electromagnetic waves in a vacuum.

And another teacher weighs in with another way to teach this, with a nice advertisement for the simulations created by our education group here at U. Colorado:

The PhET group at U. Colorado has a neat applet that demonstrates this idea very well.

In the applet, you can grab an electron in an antenna and wiggle it up and down. The screen displays a line of force and the the resulting electric field. The behavior of another electron in an another antenna is displayed on the other side of the screen. It is easy to see how the two electrons interact with each other.

If you have trouble moving the electron in the first antenna smoothly, you can set the applet so that it oscillates the electron. It is really easy to see how the other electron’s motion is effected by the first electron. The applet is in Java, so you will need to have it installed on your computer, but you probably already have it.

As an aside, I can’t say enough about the PhET collection of applets. They are really cool and my students find them very helpful

The teacher’s questions continue:

2) What propagates the light/electromagnetic radiation (photons) from the sun to earth through space?

The simple answer — nothing. That is, nothing outside of the electromagnetic wave propagates it, it propagates itself. It does this by electromagnetic induction. Say you shake electric charges, as you mention above. That creates an electric wave (which is an electric field that changes over distance). But what does a changing electric field make? A changing magnetic field (by electromagnetic induction). And what does a changing magnetic field make? A changing electric field. So, the electric and magnetic fields swap energy between each other, as one grows the other diminishes. It’s like the electric wave throws energy to the magnetic wave, which then throws it back, as the two of them run forward. I picture it like two people running and throwing a ball back and forth, but that is an incomplete analogy. They keep each other going. The energy doesn’t diminish so it keeps going.

It can do this even in a vacuum, since nothing is “shaking” — it’s just an electric and magnetic field feeding each other.

3) Light moving through atoms is easier to grasp then vibrating electric charges self propagating … anyway do photon’s self propogate and do the photons or vibrating electric charges move sort of up & down and forward?

Vibrating electric charges creating the electric wave can move up and down (or some other more complicated movement). That creates the electromagnetic wave, which is just photons. The photons do self propagate (since “photons” is just another way of saying “moving electromagnetic wave”). The photons themselves don’t move up and down. Rather, the magnitude of the electric and magnetic fields increase and decrease as the wave moves along. (How rapidly they increase/decrease gives us the color of light, and of course it always moves at speed c).

Another teacher asked:

4) I don’t understand light, photons, light’s momentum, and the bending
of light. Are photons “real”? Do photons have measurable mass when
they are moving? (You told me once that photons have no rest mass).

And Paul Doherty answered:

Photons are “Real” in the sense that they do carry measurable energy and momentum from one place to another.

Mass in relativity is a tricky concept and photons are relativistic.

Do you want inertial mass? When an atom emits a photon the atom does recoil.

The photon has momentum, the Mercury spacecraft Mariner 10 lost its fuel due to a stuck valve, scientists used the force exerted by solar photons bouncing off and being absorbed by the solar panels to propel the spacecraft and change its orbit, so indeed photons have momentum.

Do you want gravitational mass, the photon does fall under gravity, and photons do exert gravity. When a photon goes straight up against gravity it loses energy and so shifts its wavelength to the red, when it goes straight down in a gravity field it blue shifts.

Relativistic mass, E = mc^2 photons have energy, step out in the sunlight and feel the energy in the photons, so m = E/c^2

But there is a definition in relativity of a type of mass called rest mass, electrons have it 9 x 10 ^-31 kg. It is the mass when the electron is at rest. Photons in a vacuum always move at the speed of light, they are never at rest so what could the rest mass mean? We get around that by defining it as 0. Only objects with zero rest mass travel at the speed of light.

Newton predicted light would fall under gravity, Einstein did too, but Einsteins prediction was just twice Newton’s. During solar eclipses the bending of starlight has been measured and confirms Einstein’s prediction.

And here’s a very nice post from Built on Facts about the fact that light can push stuff around (ie., it has momentum), which is how solar sails work.

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This photo was posted by a teacher who took her own light walk… Notice how all the light patches on the ground are round. That’s because the spaces in the leaves in the trees — though they’re not round — act like pinholes. The round spots are images of the sun. This is true — it’s not just that the light “blurs out” around the edges of the spaces between the leaves. Check out the light walk link above for more information about the weird tricks that you can play with light. It will make you rethink what light — and shadow — is. You can see more of her pix at her Picasa light walk site.

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Sorry for the long delay in posting (not that it matters — I see my stats — most of you are off reading my old posts about how water goes around drains or whether polar bear fur is fiber optic). I’ve been on vacation back in my old haunts in the SF Bay Area, and thought that I would have lots of time to post, but I was too busy enjoying myself.

While I was back, I stopped through my old alma mater — the Exploratorium — and watched Paul Doherty teaching about light. You might know the old trick of using a diffraction grating to see the rainbow. You put a diffraction grating over a bright light (an overhead projector works great) and you see white light projected on the wall. Next you block off most of the light except for a narrow slit (you can cut a manilla folder to do this). You’ll see a rainbow (blue, green, red) projected on the wall on either side of the slit. What’s going on? Light bends around the tiny slits in the diffraction grating (red bends more), making infinite numbers of overlapping rainbows, which we see as white light. The slit blocks out all the rainbows that are there except for one, so we can see a clear blue/green/red pattern. (Think about that a moment, it’s a subtle point, and important).

The tiny grooves in a CD act like a diffraction grating too, that’s why they look rainbow colored.

However, it gets really interesting. Now, take away the “slit” so we just see white light again. Put an “antislit” in front of the grating… basically, a long thin strip of paper the same size and shape as the “slit” was. Instead of letting in a narrow strip of light, we’re blocking all but a narrow strip of light.

Instead of a rainbow to either side of the antislit, we now see the *complement* of the rainbow — yellow, magenta, cyan. Why is *that*?

Think about it.

WIthout the “antislit” there, you have white light, an infinite number of overlapping rainbows.

When you put the antislit there, you have blocked a “slit” — blocking the rainbow pattern that you saw with the slit there.

So what you see is “white minus blue” which is yellow, plus “white minus green” which is magenta, and “white minus red” which is cyan.

This is similar to Bob Mlller’s wonderful light walk, in which white light outside is made of an infinite number of images of the sun. When we look at the light projected through a pinhole (even if it’s not round) we see a round image — one image of the sun. If we look at the light that goes around an anti-pinhole (like a piece of paper, even triangular) you see a round shadow… the opposite of an image of the sun!

Here is the antislit activity from Paul Doherty’s website. As he puts it, “The anti-slit removes one wavelength at a time from white light. Thus we see the spectrum of subtractive colors”

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