The sky offers a wide variety of stunning
optical effects. A source of inspiration for poets and songwriters
alike, these atmospheric phenomena include red sunsets, rainbows,
mirages, halos, glories, and coronas. These effects are the result
of the interaction of light from the sun or moon with the gases in
the atmosphere, clouds, ice crystals, smoke, dust and other airborne
particulates. Some of these phenomena can be seen almost every day;
others occur less frequently. In this issue of CoolStuff we will
examine examples of atmospheric optical phenomena and how they may
be demonstrated in the classroom.
The sky is the daily bread of the
- Ralph Waldo Emerson
She comes in colors everywhere;
She combs her hair
She's like a rainbow
Coming colors in the air
She comes in colors…
She's like a rainbow
- Mick Jagger / Keith Richards
Double Rainbow in Wrangell-St. Elias
National Park, Alaska
Over the Rainbow…
rainbow is a multicolored, circular band of light. The display of
colors is due to refraction and internal reflection occurring in
raindrops or other droplets of water.
Making Your Own
Rainbow I: Direct a fine spray from a garden hose in a direction
away from the sun. How far away do you estimate the rainbow to be? If
you do this experiment with a group of people, does everyone see the
same rainbow? Do you see your shadow? Where is it located in
relation to the rainbow? If you want to explore further, stand on a
ladder while producing your rainbow. Describe the rainbow you see
Making Your Own Rainbow II: In a darkened room, place a clear
(the clearer, the better) plastic box approximately three-quarters
full of water on the stage of an overhead projector. (Note: These
boxes are the type often used to store shoes.) Cover or remove the
projector’s top lens so that no light is projected into the room.
Arrange the water-filled box so that students can see both of the
rainbows formed (a rainbow is produced by each long side of the
box.) Examine the array of colors produced by the water-filled
plastic box. Are the rainbow colors in the same order as in a
Making Your Own
Rainbow III: Shine light from the bright flashlight or a slide projector
through a central hole in a piece of white cardboard. If a
water-filled flask is illuminated with the light passing through the
hole (see figure) a faint rainbow will appear on the cardboard. It
has the shape of a closed circle and its angular distance is about
42 degrees, with red on the outside, as in a naturally occurring
rainbow. You will need a completely dark room since the rainbow
formed is quite faint.
Discussions on rainbows and the optics of the sky always lead to
the topic of the electromagnetic spectrum. Click here for details on the
Demonstration Kit. Another great classroom tool is the Giant
Prism. Use it on your overhead projector to project a large
class-size rainbow! Click here for details on the
Hiroto Ashikaga; Tottori Technical High School, Syozan 111, Tottori
Making Your Own Rainbow IV: Tiny glass beads, such as those used
by your local highway department to make highway signs and street
markings highly reflective, may be used to produce rainbows like
those seen in the center photo below. The beads, behaving like
raindrops, work in concert to form a rainbow.
Most highway and public works departments will gladly give you a
container of glass beads. Once you have obtained the beads, cover a
piece of black foam core or poster board with a thin layer of spray
glue. Now sprinkle the glass beads over the black surface until the
surface is completely covered with beads. When a point source of
light, such as a Maglite with reflector removed, is used to
illuminate the beads, the beads will form a circular rainbow that
seems to hover above the cardboard.
Concept: Blue light interacting with molecules in the atmosphere
is absorbed and reradiated in all directions. Blue light is
scattered much more efficiently than light with longer wavelengths,
for example, red and green. As a result of scattering, the sky looks
blue no matter where we look. By contrast, to an observer on the
moon, the lunar sky appears black because there is no atmosphere to
During sunrise and sunset, the distance that light travels from the
Sun to an observer on Earth is at its greatest. This means that a
large amount of blue light and some green light is scattered. Since
white sunlight may be thought of as consisting primarily of blue,
green and red light, the blue/green deficient light that we see
coming directly from the sun appears red.
Blue Sky/Sunset Simulation I: One of the most frequently asked
questions is “why is the sky blue?” Using very simple equipment, you
can demonstrate and explain the phenomenon to your students. Add a
few drops of milk or a few grains of powdered milk to water in a
beaker or fish tank and stir. The milk particles serve as scatterers just as
air molecules do in the atmosphere. When light from a light bulb or
slide projector passes through the liquid, scattered blue light may
seen throughout the container.
Shine light from a light bulb or slide projector through the liquid
and observe the color of the transmitted light. With much of the
blue light removed from the incident white light by scattering, only
the orange-red portion of the spectrum remains. When viewed head on
through the liquid, the transmitted light actually looks like a
you are using a slide projector and fish tank for the simulation,
you may wish to carefully rotate the tank as it is illuminated.
Allowing light to first pass through the narrow width, then through
the length of the tank, allows students to observe how the color of
the sun changes from a yellow-orange to an orange-red as it moves
from its noon day position to the horizon.
Blue Sky/Sunset Simulation II: A second method of demonstrating
why the sky is blue and the sunset red requires the use of two
common chemical substances: dilute sulfuric acid (H2SO4) and sodium
thiosulfate (Na2S2O3), hypo used in photography to fix developed
films. (Caution: be careful when handling the acid.) First mix three
teaspoons of thiosulfate with one liter of water. To this solution
add ten to twelve drops of acid. After a few seconds, the solution
will take on a bluish tint. With time the color will become more
intense, then fainter. After a few minutes the liquid will turn
These changes are due to the scattering of white light from tiny
grains of sulfur which gradually grow in size as the reaction
progresses. Initially, the grains are very small and serve as
scattering centers for short wavelengths of light, hence the blue
color. Eventually the particles become so large that they scatter
all wavelengths of visible light with equal intensity. This accounts
for the final milky appearance of the liquid.
that a cardboard mask blocks the light not passing through the
Scattering from particles whose dimensions are much less than the
wavelength of light is known as Rayleigh (pron. ray-lee) scattering. Rayleigh
scattering is responsible for the blue appearance of the Earth’s
sky. The non-preferential scattering by larger particles is known as
Mie (pron. me) Scattering and is responsible to the white color of clouds.
beautiful setting sun effect can be achieved by placing a beaker
containing the H2SO4 - Na2S2O3 solution on the stage of an overhead
projector (see image left). First mix three teaspoons of thiosulfate
with one liter of water. To this solution add ten to twelve drops of
acid. (Caution: be careful when handling the acid.)
mirror is used to project the light passing through the beaker onto
a screen. As the sulfur particles grow in size, the scattered blue
light will become more intense while the light reaching the screen
will change from white, to yellow, to orange and finally to a deep
The Color of Clouds
consist of water droplets and ice crystals that are significantly
larger than the wavelengths of visible light. Unlike the smaller gas
molecules that make up the Earth’s atmosphere, these larger
particles scatter all colors more or less equally.
Looking at a cloud, an
observer will, in most cases, receive all wavelengths of light and
perceive it as white. However, a cloud’s actual appearance is
governed by color of illuminating light, cloud thickness, shadowing by other clouds, age of the
cloud, and the brightness of surrounding sky and clouds. Thicker
clouds transmit little light and hence may appear darker. Larger
droplets in older clouds scatter less and absorb more light than
smaller drops and therefore appear darker.
The Whitest Cloud Around: What we identify as white is simply
the brightest gray in sight. A light gray cloud on a bright white
background will look much darker than the same cloud on a dark or
black background, in which case it might look white and bright. To
demonstrate this, obtain a variety of paper samples, each of which
appears to be white in isolation. Place them side by side, or cut
them so that they can be nested on top of one another, for
comparison. Usually only one will be perceived as white; the other
samples will appear gray by comparison, as it is with clouds.
Concept: A halo is an optical phenomenon due to reflection and
refraction of sunlight or moonlight in atmospheric hexagonal
ice-crystals. Halos appear as bright rings around the sun or moon.
Although they are more common in cold weather, halo-producing cirrus
clouds can be present in warm weather. Colored halos are formed by
refraction in the crystal; white halos are produced mainly by
reflection. (see below left)
Coronae are circular bands of color centered about the sun or moon.
(see above right)
They are produced by the diffraction of light by tiny water droplets
in clouds or small ice crystals. While circular coronae are somewhat
rare, partial coronae are not.
Download! Double-Slit Diffraction
With just a Laser Pointer and a Laser Printer
each of your students can now generate their own double-slit
patterns -- and it's FREE!
Produced by irregularly-sized droplets, these coronal fragments
appear as wisps of iridescent pastel colors in clouds. (see right)
Cool Coronae: To produce a corona, simply breathe on a cool
piece of glass. More often than not, a corona will be seen by
looking at a light source through the water droplets that condense
on the glass. If you wear eyeglasses, simply exhale on one of the
lenses. When you look at a light source through the lens you will
see a corona whose colors change with time. Since the colors
produced depend on droplet size, the colors change as the droplets
get smaller and finally disappear.
You may not even have to breathe on glass to observe coronae. You
may see them through a fogged windshield or on steamed up glass in
Iridescent Cloud in a Bottle: Iridescent coronae are often
produced by the water droplets that make up thin clouds. So to
produce coronae it would seem that all you need to do is make a
cloud. Using a gallon jar, a rubber glove, some water and a match,
you can do just that. First cover the bottom of the jar with a thin
layer of water. Drop a lit match into the jar. Quickly place the
fingers of the glove inside the jar and stretch the open end of the
glove over the mouth of the jar. Put your fingers the glove and pull
the glove outside the jar. Presto! You should see a wispy cloud
inside the jar.
observe a corona, shine light from a bright source such as a slide
projector or flashlight through the jar. Initially smoke particles
will scatter all wavelengths of light producing a white cloud. As
the smoke disappears, leaving smaller droplets, pastel colors will
be seen at certain viewing angles. You’ve just observed your first
corona in a bottle!
The figure above shows a rare
atmospheric optical phenomenon known as a circumhorizontal arc.
Caused by the refraction of light through the ice crystals in cirrus
clouds, it occurs only when the sun is high in the sky, at least 58°
above the horizon.
Reminiscent of a rainbow, the circumhorizontal arc is produced only
when the ice crystals making up cirrus clouds are shaped like thick
plates. Furthermore, these plates must have their faces parallel to
the ground. The chances of having all these conditions satisfied are
low, hence the infrequent observation of this amazing optical
Other Cool Sky Stuff
the photo shown here, the Aurora Australis is seen over the National Science Foundation’s Amundsen-Scott Pole
Northern and Southern Lights, or more formally Aurora Borealis and
Aurora Australis respectively, are produced when charged particles
from the Sun pass through the Earth’s upper atmosphere. The
high-speed particles energize gas molecules which in turn emit the
ephemeral colored lights we associate with the Aurora.
image is courtesy of UK photographer Rich Lacey. While spending time
in Northern Canada Rich had to opportunity to capture the best
Aurora photos we've seen. You can see more of his images and order
prints for your class on his web site at
Often seen in very cold weather, light
pillars seem to be beaming up from terrestrial light sources such as
street lamps. Many initially mistake light pillars to be
searchlights. Light pillars result from the reflection of light from
hexagonally-shaped, plate-like crystals. These crystals fall with
their flat surfaces in a horizontal orientation. The flat surfaces
serve as mirrors, reflecting the sun’s light downward.
sun pillar is a vertical shaft of light extending upward or downward
from the sun. Like light pillars, they are produced when sunlight
reflects off the surfaces of plate-like ice crystals. Sun pillars
are usually seen at sunrise or sunset when the sun is low on the
Page Web page full of links to great science sites.
You'll want to bookmark this page and maybe send us a few
of your favorites.
CoolStuff Archives A complete
list of all the CoolStuff Newsletters. Find student
activities for specific topics.
The Heat is On...
Believe it or not, thermal physics, the study of
the transfer and transformation of energy, touches virtually
every aspect of your students' lives. In the next issue of
CoolStuff you will learn how you can engage and excite your
students through demonstrations illustrating the concepts of
heat and temperature, the thermal properties of matter and
the principles of thermodynamics.
You have received this email as a customer of Arbor Scientific, because you
have opted into the newsletter mail list, or because you have requested
information from Arbor Scientific. We would love to hear from you, and find
out what you think of our newsletter "Cool Stuff". You can click on the link
below to leave your comments, receive messages tailored to your interests,
and update your profile at
send this email to a friend just click on the link
and include the email address in the body of the message:
Email addresses will not be shared with third parties.