Paper Plate Education
Activity: Rainbow Finder
Abstract: In this activity,
rainbow seekers make an inexpensive device which
illustrates that the primary rainbow is seen about 42 degrees up from the
antisolar point, or up about four fists stacked above the top of the shadow of
the viewers head.
A typical rainbow ray is
created from the dual refraction, singular reflection, and dispersion of
sunlight as it encounters a spherical raindrop.
The red color appears in a circlea portion of which is seen as the
rainbow arcroughly 42 degrees from the antisolar point, as suggested by Rene
Descartes in 1637.
The height of a rainbow
above the horizon depends specifically on the altitude of the sun.
When the sun is low, a rainbow is high.
When the sun is high, a rainbow is low, perhaps even below the horizon.
To find a rainbow, an observer
should place her back to the sun. The
spot where the shadow of her head is cast is the antisolar point.
Forty two degrees above the head shadow is where a rainbow would appear.
The observer can stack her fists, held at arms length, atop one
another four times with the bottom fist appearing to rest on the head shadow.
Above the fourth fist will be the rainbow.
The Paper Plate Rainbow Finder
is a device that reinforces Descartes finding and eliminates confusion about
where to look. Required materials
are a paper plate, a protractor, and a pencil.
From the left edge of a paper
plate, draw a line across the plate yet below the center point, with an arrow on
the line where it terminates on the right edge of the plate.
Label the line To Head Shadow. From the same starting point on the
left, draw a line 42 degrees up from the first, also ending with an arrow.
Label the second line To Rainbow.
Between the arrow heads on the
crinkled perimeter of the plate, draw four clenched fists atop one another.
Above the top fist draw a rainbow.
Below the bottom line, draw a
persons shadow so that the tip of the shadows head touches the bottom
line. Above the bottom line, on top
of the shadows head, draw the shadows of four clenched fists atop one
another. The combined height of
these smaller shadow fists are roughly the height of one fist drawn on the
Between the two lines, at the
point of origin, draw an eyeball within the angle.
Your Paper Plate Rainbow
Finder is ready for use. When water
droplets from rain (or from a lawn sprinkler) are present on a sunny day, the
observer places her back to the sun. The
observer holds the paper plate so the drawn eyeball is near her eye.
Turn the plate so the bottom line points to the top of the shadow of the
observers head. The observer
then simply looks along the top
line of sight to locate the rainbow.
A secondary bow, caused by two
internal reflections in addition to the two refractions and dispersion,
sometimes appears about 10 degrees above the primary rainbow and may also be
drawn on the paper plate. Remember
that its colors are inverted from the primary rainbows colors.
See http://www.unidata.ucar.edu/staff/blynds/rnbw.html for more rainbow information.
Contributed by Chuck Bueter.
GLPA Proceedings, 1998, pp. 110-111.
Copyright ©2012 Chuck Bueter. All rights reserved.