Telling a 3-year old about the atmosphere
(A)
Hi--
My three year old daughter, Sarah, would like to know . Any help you can provide would be appreciated
Reply
Hmmm. Questions have come from all ages, and from all over the world. Some 8-year olds have asked quite profound ones--but Sarah is probably the youngest yet. I'll try to answer, but YOU are the one who has to make it clear to her.
First of all, it is a GOOD THING there is no air in outer space! Outer space is SO BIG, if our air could spread out over all of it, almost nothing would be left for us to breathe.
Luckily, air has WEIGHT. (You don't feel it, but the air in a room weighs more than Sarah).
Without weight, everything would FLOAT FREE, like astronauts in a spaceship (I hope Sarah has seen pictures). Weight is what keeps tables, chairs, people, cats, dogs, cars and so on standing on the ground, also keeps down the water in oceans and lakes. And it also keeps our air from floating away.
 
Weight tries to keep everything as LOW DOWN as possible. Water flows downhill. In the winter, sleds slide down a hill by themselves, but need to be pulled back. Weight also keeps air attached to the ground.
Weight is caused by GRAVITY--the pull which any piece of matter has on any other.
The pull is very weak: any two stones pull each other, but so weakly, you can never feel it. But the EARTH is such a BIG piece of rock, that it has enough pull to keep all of us, and all things around us, safely on the ground.
It even has enough pull to keep the MOON from running away. As well as all the satellites NASA and others have launched, which like little moons go around the Earth..
Please let me know if the above is good enough for a 3-year old!
Telling a 3-year old about the atmosphere
(b)
Thanks so much! This answer did seem to satisfy her (mostly, anyway). Now she just wants to know where the air came from in the first place Thanks again for such a thoughtful and speedy response--it was really helpful!
Reply
It is probably hopeless to try keep up with the questions of a 3 year old! On the long run, it may be easier to teach her to read (as we have done--see
http://www.phy6.org/outreach/misc/ilana.htm
and then let her find answers in the library.
Let me still try answer this one. Where did air come from? Hard to say. I guess it came with the Earth. There exist other "worlds" in space, a little like ours (some smaller, some bigger), called "planets." They look like stars in the night sky, though stars are different (stars stay in fixed groups, planets move among them). Some planets have "air", others (like the Moon, still another "world") came without it.
OUR air is different, however: we can breathe it! Air on other planets is too hot (if they are closer to the Sun), too freezing cold, if more far away. But in addition, hot or cold, we COULD NOT breathe it, we would choke. It would be the wrong kind.
For breathing, air needs to contain something we call OXYGEN. That is the part of the air which we breathe, the rest just goes in and out. Sick people in hospitals, who can't breathe well, sometimes get oxygen from special metal bottles.
Our planet at first had "bad air", too. It has oxygen only because we HAVE GREEN PLANTS. Trees, grass, green leaves everywhere, with the help of sunlight, change air we cannot breathe, and make oxygen out of parts of it. If our world stopped growing trees or grass, our air would slowly become bad. So we should thank plants for our good air!
Three-Color Vision
I have heard somewhere that three colors are not enough to reproduce ALL colors which the eye can see. Is that true?
(Adapted from question asked on Physhare, a list server of physics teachers)
Reply
It is true.
I have looked up this question in the writings of the true master, Richard Feynman, who dealt with color vision in chapters 35 and 36 of "The Feynman Lectures", volume 1. It is remarkable how far his interests have ranged, how few were the topics he did NOT look into. My own field of magnetospheric physics is unfortunately one of those few, and that only because his sister Joan (who is still with us) worked in it. When her brother showed interest, she told him to butt out, leave something for her to do. He did, and now we are stuck with all sorts of unresolved problems.
You should read Feynman's exposition, in particular chapter 35, which I tried to flesh out a bit here. Feynman is so much more articulate than I can ever be, and what I wrote below is anyway my own interpretation, not necessarily the same thing.
The important thing to do is to pose the proper question: what exactly do we want to claim? There is no question that the normal eye has 3 kinds of receptors, which we may designate (R,G.B) for (red,green,blue), although each is sensitive to a wide spectral range beyond the "pure" rainbow colors of red, green and blue.
When we look at some colored object, with some standard brightness (I choose here a standard brightness for all colors, so that only relative intensity matters), if the (R,G,B) sensors detect intensities (a,b,c), we will see SOME color, and that color will be characterized by the 3 numbers (a,b,c). The range of values for (a,b,c)--say, from 0 to 1 for each, after we have standardized intensities--covers ALL colors the eye can see. There exists nothing more.
The real question to ask is--is it always possible to "fake out" the color (a,b,c) by presenting the eye with a superposition of 3 colored sources, of three standard colors (U,V,W). These can be pure spectral colors, or not. (Red, Green, Blue) are a good choice, though, because each stimulates primarily one kind of receptor. Therefore, by twiddling with their intensities, we can make each type of receptor receive an EQUAL intensity, and the result registers in our mind as white. Let this be an assumed restriction on the choice of "primary" colors, requiring that we CAN get white; three different hues of green may never be able to do so.
Let the color U create response