With the sun still low on the horizon of a new semester, teachers look
into student faces still reddened from the summer break. Thus, it's apropos
to investigate the red appearance of the sun when it's low on the horizon.
Dawn begins with the eastern sky's red glow and then the sun loses its
exotic appearance to become a yellow-white object by noon. By sun-down,
the redness is back.
A
glass of milky water helps explain this phenomenon.
Exploration
phase. Divide the class into small groups. Ask a child in each
group to bring a strong flashlight to class. Have one student fill a clear
glass with water and another student use an eye dropper to add milk to
the water.
Turn
off the lights and pull the blinds to make the room as dark as possible.
Place the flashlight on a table so that the light shines through the water
and onto a sheet of white paper, which serves as a projection screen,
about 10 cm behind the glass (see Figure 1- description at end).
Every
time a drop of milk is added to the water, stir the solution and observe
the paper. Slowly, the paper becomes yellow-orange-red.
Concept
introduction. If possible, show photographs or a video of astronauts
outside the shuttle or walking on the moon, along with pictures of the
earth as seen from space. All images in these videos and/or photos are
surrounded by stark darkness. Space has no color. Lunar astronauts look
into the heavens that are without red sunsets, white clouds and a blue
sky.
White
light is composed of the colors red, orange, yellow, green, blue and violet.
Before this heterogeneous palette from the flashlight splashes onto the
paper as white light, some of the wavelengths disappear. The very diluted
milk solution looks red-orange because the other colors are somehow dispersed.
In
physics, the word scattering is used to describe what happens to light
waves when they collide with and are momentarily absorbed by the air's
component molecules (oxygen, nitrogen and water) and dust particles, and
then are randomly re-emitted in many directions.
While
their direction is random, there's a quantitative manner in which the
colors are dispersed. Shorter wavelengths (blue) are scattered about seven
times more easily than the longer red wavelengths; yellow wavelengths
are scattered about three times more easily than red.
The
following scale shows the relative ease at which different colors are
scattered: purple -- 10, blue -- 7, yellow -- 3, orange -- 2, red -- 1.
This is why, during the day, the sky appears to be blue most of the time.
But wait just a second! Purple is scattered more than blue, so why isn't
the sky purple? Easy. Because the retina is not as sensitive to that color.
When
the sun is low on the horizon, the longer wavelengths -- yellow, orange
and red -- penetrate the thick atmosphere while blue is seen in regions
perpendicular to the line of sight. This dispersal of specific wavelengths
is called selective scattering.
[Figure 2 shows that when the sun or moon is on the horizon, their light
travels through thicker air and more scattering and redness occurs.]
Waves
circle away from a stone dropped into a pond, and when they hit a cork,
it bobs up and down with the same frequency as the passing ripples. The
wave direction is slightly changed, too. This analogy explains how light
waves journey through the atmosphere and encounter its component molecules
and particulate matter. Following the collision, the light waves are sent
off with the same frequency (just like the cork), but in a different direction
(scattering).
This
reddening phenomenon is often visible four times a day: sunrise and sunset,
moonrise and moonset. Remember that when the moon shines, it's merely
reflecting the sun's light, so the earth's satellite also looks orange
as it apparently moves above the eastern horizon and again hours later
when it's low in the western sky.
Same
end result. The passage of sunlight and moonlight through the
atmosphere is analogous to the flashlight's battery powered wavelengths
passing through the milky water. But impurities are impurities, whether
they're dust particles and water vapor in the atmosphere or milk fat clouding
up a glass of water, so the end result is the same.
The
oil well fires during the 1991 Gulf War drove particulate matter high
into the air and also resulted in an intense reddening of the skies at
dusk and dawn An excellent reference with colorful diagrams can be found
at this Internet web site: http://av.yahoo.com/bin/query?p=optics+%2B+sc
attering&hc=O&hs=1
Figure
1: Shine a flashlight through milky water and see a sheet of
white paper change color. This experiment helps explain why the sun and
moon are orange-red when on the horizon.
Figure
2: Since the atmosphere is thicker when the sun is on the horizon,
there's more scattering and redness.
AUTHOR:
Michael Leyden
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