ask mr. science - page 16: web pages,

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Last update 18 Apr 2003

Making web pages

Instead of doing science, I sometimes taught the 5th/6th graders how to make their own web pages, in plain HTML. I did this the whole 2000-2001 school year. HTML is as simple a 'real' computer language as exists anywhere. That is, you write code that gets interpreted by the machine (or the browser in this case). It is simple enough so that you can collect all the basic codes on one or two sheets.

Links to the class pages: 2000-2001 web projects
2003 web projects
HTML cribsheets: page 1, and page 2

18 April 2003

Are there more grains of sand on the beach than there are stars?

At first blush, this sounds like we have to compare two infinite numbers, and how could you do that? Actually, you can, but that is another story. The number of stars in the (observable) universe is large, but can be estimated. I did this before, and the number I got was about 7"100%" height=5 00000000000000000 or 10²¹ rounded up, give or take an order of magnitude.

So how about the sand? I don't live near a beach, so I got a handful of sand from the backyard. Now this does not look like beach sand at all - there are big chunks, and grains down to the size of dust. Beach sand is more uniform in size. So I poured the sand through a small tea sieve, and discarded the dust that fell through. Then I poured the sand through the big flour sieve, and discarded what remained in the sieve. What fell through were grains of fairly uniform size. This looked much better, and what's more, this was countable.

I measured out 1/8th teaspoon onto a sheet of paper. Still too many grains to count. Using the same technique as I used before with the children to count pumpkin seeds, I divided the sand in half, then quarters, and finally into 1/8ths. This was a countable 350 grains, give or take a few.

(more text)

Links to the class pages: 2000-2001 web projects
2003 web projects
HTML cribsheets: page 1, and page 2

18 April 2003

Rainbows

How do rainbows work?

1) The first step is to explain that waves can get bent as they are slowed down. I use waves on the beach to clarify this: the question is, why is it that waves always seem to roll straight towards the beach? After all, a little ways offshore the orientation of the waves is likely to be some random direction, unlikely to be parallel to the beach!

Here is a game I use to make the kids figure it out: On the floor, I mark with masking tape a line where waves slow down (red line in the diagram). This is where the water depth gets shallow, slowing waves down. I line up 2 kids side by side, and they are holding the ends of a broomstick (the blue line). The broomstick represents the crest of a wave. When the children walk forward, this is like a wave rolling across the ocean in the direction of my beach which is marked on the floor. The rules are this: when you are in the open ocean, you can take big steps, but as soon as you cross the line, you have to take very small steps. When they approach the beach straight on, nothing much happens, except that they slow down as they cross the line. However, when they approach the beach at an angle, the student who croses the line first will start making small steps before the other one does, and the broomstick gets swung around a bit and now is more parallel to the beach. If the purple dots denote footsteps, notice that the kid on the left takes one big step and 7 small ones, but the one on the right takes 4 big ones and only 3 small ones. If the water gets even shallower, and the waves are slowed even more, they get bent again and by the time they wash ashore, all waves are pretty much parallel to the beach, no matter how they started out.

2) The next step is to show that this same bending happens with light. For this I had a rectangular plastic container, filled with water to which I added a few drops of milk. When you shine your laser pointer onto the surface at a shallow angle, the beam gets refracted and dives into the water at a steeper angle.

3) The next step is to draw on the board the standard picture of the rays in a water drop that make the rainbow, see the second picture on this page. You can actually show this directly, with this simple setup: I hang a 2-liter bottle filled with water from the top of the blackboard (using a sharp hook filed from a piece of coathanger) Note the little cardboard standoff that holds the bottle away from the board. Across the room I set up a slide projector that shines light onto the bottle. (I found that if you cover the lens of the projector with some black paper with a vertical slit cut in it (1/4 - 1/2"), the rainbow colors are better defined). Now if you look at the bottle at exactly the right angle, namely 40 degrees to the left or the right, you can see the rainbow colors coming from the bottle, indicated by the green arrow in the picture on the left. You can hold a piece of white paper there and see the rainbow colors, as well as the fact that the inside of the rainbow is brighter than the outside. Just looking at the bottle and moving around makes the cause of this brightness effect plain: at angles to the inside of the rainbow, you can see all kinds of reflections of the slide projector light, and they are absent ar angles outside of the (primary) rainbow.

Links sites about rainbows: Beautiful rainbow pictures
Rainbow physics
More rainbow science
All the details

30 October 2003

Any suggestions, comments, greetings are greatly appreciated.
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