(File) 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.
MARCH 03, 2009April Holladay, HappyNews ColumnistQ: Can I touch a rainbow?
Azhar, Saudi Arabia
A: 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.
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.
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)
A: 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
A: 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
A: 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
A: 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
A: 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!