Experiment of the Week - #191 Pretty Pebbles
This week's experiment grew from a question sent to me by Miranda
Hughes. She and her children had been picking up pretty stones at the
beach but noticed that when the stones were dry, they became dull and
lost most of their color. She thought she knew the reason (She was
correct.) and thought that it would make a good experiment of the week.
To explore this, you will need:
* aluminum foil
* clear, plastic wrap
First, lets examine why the surface of the dry rock looks dull. Why are
some things shiny? Look around you and find several things that have a
shiny surface. You should find that they all have a very smooth
surface. Cut two pieces of aluminum foil, each about a foot square. Lay
one of them on a flat surface, being careful not to bend or crumple it.
Crumple the other into a loose ball and then unfold it. Place it beside
the smooth sheet. Notice the difference between the two.
Take both sheets into a dark room. Place them on a flat surface. Shine
a bright flashlight onto the smooth sheet. It acts much like a mirror,
reflecting the light. When you shine the flashlight on the crumpled
sheet, you should notice a difference. Instead of reflecting a strong
beam of light, the crumpled sheet spreads the light out in all
directions. The more crumpled the surface is, the more it spreads or
diffuses the light.
Turn on the lights. Take the crumpled sheet and crumple it again and
again. Each time you crumple it and unfold it, the surface will diffuse
more and more of the light. Place the crumpled sheet beside smooth
sheet. Even though they are both made of the same material, the
crumpled sheet looks lighter, because no matter which angle you look at
it from, some of the scattered light is coming to your eye.
The impact of scattering of light by rough surfaces is even more
dramatic with clear materials. Look at a sheet of clear, plastic wrap.
It is shiny and clear. No surprise there. Now, crumple the sheet into a
ball. Is it still clear? No. Instead, it looks white. All of the colors
of light are scattered in all directions. Blending all of the colors of
light gives you white. (I can hear the screams of people saying that
mixing all the colors gives you black. That happens when you mix
paints, which soak up the light, but not with light.)
You can see the same thing with other clear substances that are very
rough, or that are in small pieces. Sand, sugar, salt and snow are all
made of substances that are clear. If you look at them under a
microscope, they are still clear. Their white color comes from the
scattering of light.
Now that we know why the rocks look dull and whitish when dry, why do
they look so pretty when they are wet? The water fills up all of the
rough spaces to decrease the amount of scattering. To see this, fill a
bowl with water. Take a sheet of plastic wrap, put it under water and
then crumple it into a wad. Compare this with one that has been
crumpled in the air. The one that is underwater is much more clear.
Light moving from the plastic into water scatters less than light
moving from plastic into the air. Less scattering means more of the
light passes on through and it looks more clear. Filling the rough
spots on a stone with water is much like filling the spaces in the wad
of plastic wrap. More of the color comes through. To see those colors
when the rocks are dry, you either have to polish the rock in a rock
tumbler to smooth its surface, or you have to fill the rough surface
with something clear, such as clear fingernail polish or lacquer. You
can also do like Miranda and store your rocks in a container of water,
so they always look pretty without changing them.
to Krampf Index
Including permission from Robert
Krampf to post his
experiments on my web site