Touching rainbows
Can I touch a rainbow?
Azhar, Saudi Arabia
We could touch rainbows, if they were physical objects. But
rainbows are not objects.
A rainbow is "a distorted image of the sun" whose
light many raindrops bend, reflect and scatter to our eyes, write
meteorologists
Raymond L. Lee, Jr.
and Alistair B. Fraser in The Rainbow Bridge.
Optically speaking, an image is "any collection of light rays that appears to
come from a more-or-less well-defined set of directions," Lee says. So an image
is made of light. The question remains: can we touch the image called a 'rainbow'.
The American Heritage Dictionary
defines touching: " To cause or permit a part of the body, especially the hand or fingers,
to come in contact with so as to feel: reached out and touched the smooth
stone."
But a rainbow is different from a
stone. In fact, we can no more touch a rainbow than we can touch a reflected nose in a
mirror. However, we can
touch the place on a mirror reflecting the nose. Likewise we can
touch water drops producing a rainbow.
But we can't touch the image. Find out why with a simple
experiment. Stand before a plane mirror side by side with a friend.
Touch the spot on the mirror where you see your nose. Keep your finger on
the spot. Have your friend touch the spot on the mirror where he
sees your nose. You each touched a different spot and saw a slightly
different view of the nose, because you view the nose along different lines of
sights. Neither of you touched the image. In fact, the image
is located behind the mirror, where it would seem to all observers that
reflected light from your nose originates.
In the animation, a yellow ball is in front of a plane mirror. The mirror
produces an image of the ball behind the mirror. The image location is where all
the reflected light rays intersect. Any person viewing the ball image must
sight at this image location.
Animation courtesy of The Physics Classroom, used with permission. All rights
reserved.
We can, however, touch an image location. Suppose you
stand a foot (0.3 m) in front of a mirror and see your nose, then the nose
image is one foot behind the mirror. You can touch that location, if you
can get behind the mirror. Even then, though, you couldn't touch the
image, because no light gets behind the mirror. "Light does not actually
pass through the [image] location on the other side of the mirror; it only
appears to an observer as though the light is coming from this location," says
physicist
Tom Henderson. The nose image at the image location is a virtual
image, not a real one. Real images are made of light.
Back to touching raindrops that make a rainbow: Suppose you turn
on a sprinkler and see a rainbow in the sunlit spray. You can "certainly
touch the spray" that generates the rainbow, Lee and Fraser write. By the way,
just as you and your friend saw different views of your nose, "each of your eyes
sees a slightly different rainbow," Lee emails.
However, unlike the nose image location, we can't touch the location of the
rainbow image. It is behind the rainbow (at the so-called
antisolar
point), much as the nose image is behind the mirror. But the antisolar
point is too far away to touch. Being an image of the sun, the rainbow
image location is at the same distance behind the raindrops as the sun is in
front — "effectively at infinity," Fraser says. It seems strange a rainbow
is as far away as the sun. But, try moving. The
rainbow moves with you, just as the sun does.
A rainbow looks nothing like the sun, so you might wonder if it's really an
image of the sun. A camera makes images that look like the objects they
represent. Similarly, binoculars, a magnifying glass and a slide
projector produce realistic looking images. But a rainbow is not a
man-made image. A
rainbow is like a mirage, whose "wild distortions are merely images formed by
the atmosphere behaving as a lens," says Fraser.
Lee explains how raindrops form the
sun's distorted image, a rainbow: Many raindrops, acting in concert, change direct sunlight so that
- reflection within the drops makes sunlight appear to come from the sky
opposite the sun (like reflection in a mirror makes a nose appear to be
behind the mirror)
- sunlight's spectral colors are revealed by its refraction on entering
and exiting the drops (like a prism bends light and reveals its colors)
- these drops' approximate spherical symmetry makes rainbow light appear
to come from a set of directions that encircles the antisolar point — that
point behind the rainbow, where the image exists.
Thus we see a bright, colorful circle of rainbow light — but it's merely a
greatly rearranged image of the sun — an image we cannot reach to touch.
Further Reading:
The Rainbow Bridge, Rainbows in Art, Myth, and Science by Raymond L. Lee and Alistair B. Fraser 2001.
University Park, PA: Pennsylvania State University Press.
Image formation for plane mirrors by Tom Henderson, Physics Classroom
Tutorial
What causes rainbows, WeatherQuesting
Rainbows
by Alistair Fraser
Atmospheric Optics by Les
Cowley
Rainbows, by Rod Nave, Hyperphysics
(Answered March 9, 2009)
Readers' Answers
-
 Yes, you can catch a rainbow. While driving in a valley
in England I had the unique experience of driving through the end of a
rainbow.John Albinson,
Kingston, Ontario, Canada
- Yes! Since a rainbow is the reflection of light on tiny
water droplets, if you're close enough to touch the water that's reflecting
the light, then you are basically touching the rainbow. Try it with a garden
hose spray nozzle that can be adjusted to a fine mist. On a sunny afternoon,
spray it toward your shadow, and you should see a rainbow that, if the angles
are right, is close enough to touch.
Anthony Kerschen,
McDonough, Georgia, USA
- In short, you can touch someone else’s rainbow, but not
your own. A rainbow is light reflecting and refracting off water particles
in the air, such as rain or mist. The water particles and refracted light that
form the rainbow you see can be miles away and are too distant to touch.
However, it is possible to touch the water particles and
refracted light (if you agree that you can touch light) of a rainbow that
someone else is viewing. Imagine flying an open-cockpit plane through water
particles that are refracting light into a rainbow for someone viewing it from
a distant vantage point.
There is one instance where you can touch your own
rainbow, though, if you consider the refracted light from a garden hose spray
to be a rainbow. If you have ever tried this you may have discovered what I
did – that rainbows are ethereal and that waving your hand through the mist
has no effect on them and makes it seem as though they exist in some other
realm.
Janet Warner, Durham, North Carolina,
USA
- Yes, you can touch a rainbow. But simply not how most humans
typically observe a rainbow. I lived beside a lake in Arkansas, for 14
years. Every summer I rode a jet ski on that lake. Occasionally, the light
would be perfect, would interact with the spray behind the jet ski and produce
a small rainbow. Theoretically, I could have reached back, and touched any
part of it. So, yes, but on a smaller scale than most perceive.
Will McBride, Colorado, USA
- The rainbow is an optical phenomenon of refraction.
When sunlight hits a drop of water, it breaks down into colors at a certain
angle, and it is also seen only from a certain angle.
If you move from the position where you saw the rainbow to a different
position, you will see a different rainbow, reflected from different water
drops. And, if you get out of this angle altogether, the rainbow just
disappears. This can be easily experienced within reachable distance, when
seeing the rainbow produced by a nearby fountain-spray.
So, if you move from where your eyes touch the rainbow, to a position where
your hands could touch it, you will only be touching the air where it was, but
the rainbow will not be there for you see that you do! In other words, you can
never get hold of the rainbow!
Elisheva, Jerusalem
- While driving in Montana in 1999, I and a friend were fortunate enough to
find ourselves enveloped within a huge rainbow. Are we unique in this
experience, or has this experience been reported by anyone else?
Dave, East Peoria, Illinois
- I would like to expand on my account: Other accounts that I have
found offer no description of the experience other than having experienced
it. The inside view of a rainbow is remarkably in contrast with its brightly
organized outer counterpart. One might expect an explosion of color on the
inside. This is not the case. The inside of a rainbow is a very dull color
chaos. There are no organized bands of color of any kind.
The best way to describe it would be to pour different colored small piles
of sand onto a piece of paper in a group. Looking at the sand, one can
easily see individual colors. Next, stir the individual piles into one
larger pile. While you can still see the individual colored grains of sand,
everything has become random with no sense of organization. The reason for
this, of course, is the same reason used to explain the rainbow in the first
place - only being on the inside, you are still at a vantage point where the
colors are still visible, but not at the optimal angle where the colors are
the brightest.
Learning that the inside of a rainbow is nothing like one would expect,
maybe a few skeptics will convert and admit that it is possible.Dave, East Peoria, Illinois
- Reply: You are observant.
The inside of the arc of a rainbow is part of the rainbow, and is
brighter than the sky outside the arc. Raindrops bend and reflect
sunlight not merely to the outside arc, but also to the inside. As
you say, the scattered light inside just brightens the sky there, but does
not form bands of colors. Click
here
to read more about the inner rainbow circle.
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