Clearly, the phenomenon is real, as a rainbow can be observed by multiple people, can be photographed, the physics of how a rainbow is “formed” are well understood, etc.
However, when looking at a rainbow, changing your position changes the position of the rainbow, so even if you are standing right next to me you will not be seeing exactly the same rainbow as I am observing. When seen from a moving vehicle, the rainbow appears to move with you.
So is a rainbow an observer-dependent reality or does it exist independently of observation?
When you shift your position slightly, everything you see moves to a different angle in your field of view, and a rainbow is no exception. It’s just that, if you use the shift in angle to find a distance by parallax, you find that the rainbow is 93 million miles behind you.
I don’t see how this is different than any other physical object. It is just light being transmitted through the eye and interpreted into a specific signal in the brain. Position and time matter but that is true for a distant tree on a plain or the moon as well for all your examples.
Technically what you’re saying is true about anything observable. When two people stand next to each other and look at a box, they are seeing different shapes and angles. Your brain just “knows” it’s a box.
A rainbow is just what you are seeing through the lens of airborne water - white light split up into the spectrum. Like looking at a magnifying glass, images you see in it seem to move as you move ie “following you”. Same sort of thing, though IANAPhysicist.
The position of the “water lense” is key and your physical relativity to that. The photons of light always exist whether they are refracted or not. So the light is real, but the position for the phenomenon to occur is as elusive as wind and water.
While a rainbow “experience” requires an observer present to see it, the water lensing rainbow experience does not exist separately from the physical presence of the water lensing phenomenon. IMO if the opportunity for experiencing the rainbow existed you have grant that the rainbow existed even if it was not experienced.
Very small water droplets, suspended in the air or gently falling, will reflect sunlight back toward an observer at about a 42° angle to the line from the sun through the observer, forming a rainbow.
In reality, a wide swath of area is bathed in multi-hued light due the to the refraction and reflection of light passing through the water droplets. But, since white light is just what our eyes perceive when all the colors of the spectrum are combined, we do not notice anything particularly unusual. If you happen to be standing in such an area, however, and look at the ring of points 42° from the antisolar point (it will be below the horizon during daylight hours) you will only perceive the red light being reflected into your eye. At an angle of about 40° from the antisolar point, your eye will only perceive violet light rays. In between, the rest of the colors of the spectrum will appear. The antisolar point is unique to each observer.
The apparition called a “Rainbow” is just a visual phenomenon, visible to a person looking in a particular direction with the necessary physical requirements, i.e., water droplets in the air, low sun angle, relatively dark sky beyond and so forth. Other people nearby will also see a rainbow, and, as you move, the rainbow will move with you, because at each point, you are essentially looking at a different rainbow! The rainbow does not have a locus in Three-Space, independent of any observer. In a very real sense, the entire volume of air occupied by the suspended water droplets and illuminated by the sunlight is the rainbow. Precisely where you seem to see it is merely a function of your position with respect to this block of atmosphere.
For those who didn’t understand DHMO’s post: It’s not that the rainbow moves with you, but there are infinite rainbows, and when you move you’re looking at different ones. Thus (to give the OP a direct answer) all those rainbows do exist even before anyone looked at them.
Rainbows are everywhere, all the time. Anytime there is a spray of visible water particles in the sun (like a lawn sprinkler or a huge fountain in a lake). There are visible rainbows around certain kinds of outdoor lights on a damp night, too. Rainbows aren’t really a “thing.” They’re just a trick of the light. Light cannot be interpreted without a living being’s presence, or a lens set up by a living being. So yes, they “exist” in many places, insofar as a phenomenon that has no physical manifestation can be said to exist (there are lots of water sprays on cloudy days that wouldn’t cause the formation of rainbows, for example, but the same spray on a sunny day would). Many of those manifestations will never be seen by anybody. There’s no reason to think that IF someone were there that they wouldn’t see a rainbow.
Rainbows are holograms, but holograms based on geometrical optics (lenses) rather than employing diffraction. Actually a rainbow is a very distorted image of the sun. People on a planet orbiting a binary star would see two overlapped rainbows.
So, is the “three-D image” seen in any hologram really there, since each separate eye of each observer sees a different shape, and there is no physical object located at the image points?
And in those internally-silvered flying-saucer mirror devices with the hole on top? The ones where you drop some coins inside, and the 3D image of the coin appears floating in the upper hole? Are the illusory coins still there even if nobody is watching?
This is where I’m seeing the ambiguity. Obviously water droplets exist, and obviously light exists, but it would seem that a rainbow = water droplets + light + observer. Because a rainbow is really just an optical illusion, the observer seems to be an essential part of the equation, which would indicate, to me, that a rainbow does not in fact exist if no one is there to see it.
Perhaps this really is just a philosophical issue.
I’m afraid this is all incorrect. Rainbows aren’t holograms, and the real rainbow effects most certainly are due to diffraction – geometrical optics cannot correctly describe the color or intensity observed in a rainbow, nor why the relative widths of the bands and how they vary with droplet size.(If geometrical optics were the full explanation, rainbows would look exactly the same for all sizes of drops; they don’t) Nor can geometrical optics explain the supernumerarary bows.
I have to take issue with DCnDC’s description of rainbows as an “optical illusion”. That term is generally used to describe a misperception of an object, usually due to some way our brain processes the input from our eyes – seeing straight lines as curved, or seeing dark patches where none exist, or color in a black and white field. a rainbow is none of these – a camera and monitor will see the same rainbow a human eye sees at the same location, no brain input needed, and no misperception present. I suspect that this is simply a terminology issue, but my use is in line with the more common use.
The rays making up the rainbow, with their separated colors, are certainly present even if no human being, animal, or machine vision device is placed there to capture them. By my lights, that means a rainbow exists even without an observer to see it. Any arguments about this are, as DCnDc suspects and I agree, philosophical.
I also think a tree falling in a forest with no one around makes a sound, for what that’s worth.
I realize this thread is dead, but just to clarify a point for future readers:
The difference between viewing a physical item (e.g. a tree on a distant hill) and viewing a rainbow is that the actual matter (the molecules) that is being perceived is quite different.
Assuming viewers are looking at the same side of that tree, it’s the SAME molecules of the tree that light reflects off to impinge on the viewers’ retinas; in the rainbow, each viewer is experiencing the same perception of the rainbow, but it’s due to light passing thru DIFFERENT water molecules in the air which are aligned at the exact angle needed for the rainbow to be seen by each viewer. They’re each looking at different molecules of water vapor, not the same ones.
Of course, we don’t see the entire water vapor cloud, just a part of it. That’s not the case for a physical object: seeing is generally not as angle-dependent.
So in fact the image of a rainbow IS being formed at many different locations, although someone may not be standing in the exact location required to perceive it.
Nitpick: You and I, standing next to each other, might be observing some of the same droplets of water, but it may be that, say, some of the droplets that are in the right position to be refracting blue light to your eyes are simultaneously refracting green light to mine.
It is different from viewing a tangible physical object, but even when we’re looking at the same tree, we’re doing so by intercepting different rays/photons from one another (and it’s geometrically impossible for two people to be direcltly looking at exactly the same view of anything simultaneously, but that’s also a nitpick)
Rainbows share at least one property with holograms, in that they are examples of what’s called a light field.
Actually, that’s a bit wrong. There’s really only one light field, and it’s everywhere and always around us. It encodes not just the color of a point in space, but the transport of light in every possible direction for every point in space. It is, at the least, a 7-dimensional function: three spatial dimensions, one for time, two more to encode direction, and one for frequency.
Most light fields are fairly regular, though, and so we don’t think much about them. The field produced by a (point sized) incandescent light bulb is a simple function with a fairly even spectral distribution radiating equally in all directions. The light field for, say, a photograph is also quite regular–every point stores the picture (from a different angle) but the picture is always the same.
The light fields for rainbows and holograms are quite distinct, though, and this is why they have such unique properties. The hologram not only gives each point its own picture, but it gives each one a different picture–one with the correct perspective for that point. A rainbow works in the opposite direction, with each point seeing virtually the same rainbow, with almost zero dependence on position.
And even if you stay in one position, you’re still seeing a multitude of different rainbows, because the drops are falling. At any given moment, there are drops in the right place to give you, say, the red band, but wait a moment, and those drops fall out of position, to be replaced by others.