There are two important properties that any waveform intrinsically has: amplitude and frequency. If you're looking at a waveform, the amplitude is how far from the mid line the height of the peak/low of the troughs are, and the frequency is how many peak-to-peak there are in a certain distance; frequency and wavelength are thus linked - the wavelength being the distance from one peak to the next peak, shorter wavelengths mean you can jam a higher number of them into the same distance. Shorter wavelength, higher frequency. I suppose you could also call amplitude waveheight, or wavewidth depending on the orientation of your head when you're looking at it
So there's a huge spectrum of electromagnetic radiation emanating from the sun and we see (with our eyes) just a tiny part of the spectrum. We see because our eyes have cells that can detect the amplitude and frequencies of some small sub set of the huge range of electromagnetic radiation that exists
In your eyes, you have some cells that are rod shaped, and some cells that are cone shaped, and hence colloquially get called "rods" and "cones". Rods don't need a lot of stimulation to register the presence of radiation so they do low amplitude stuff, but they don't discriminate frequency. Cones are responsible for detecting frequency but they need more of a kick to get them to respond. A single cone doesn't detect different frequencies; there are three different types of cone and each type is more sensitive to a range of frequencies than the other types.
Strictly (but simplistically) speaking, one cone type does radiation in the yellow frequencies, one does green the green frequencies and one does blue frequencies. In turn, and our brain maps the relative amounts of stimulation that each of them detect, to intensity and color.
If you look at an intense source of electromagnetic radiation in the blue part of the frequency spectrum (hereafter referred to as blue light), your blue cones are going "ooh, that's a lot of light" but the green and yellow-green cones don't have a lot to say at all - your brain turns that into "that's a blue light".
I picked on blue because it's easiest to explain - blue rods, blue light, blue stimulation, blue perception. Now we kinda need to talk about light composition
About light composition:
Red, Green and Blue are just terms of reference we've made up to help us describe and communicate with each other. You could hold up a ball that most people would say is red and teach a child that it's called green and they will really struggle to use traffic lights if anyone ever says "green means go". I mention this because what I see and what you see when we look at a red apple is quite possibly a very different thing, but if all we have ever known is "that's red" then we will both call it red even though what we are both seeing might be completely different
If all we have is red light, green light and blue light, we can make the others by mixing. Red and Green is Yellow, Green and Blue is Cyan, Blue and Red is Magenta. In reality there is a spectrum of these things, and how they are detected by the eye isn't so cut and dried either.
If blue light had a frequency of 5, and green was 3, then cyan light would be 4. Let's say the green cones are mostsensitive at green 3, but they can detect 2 and 4 (a little bit either side). For blue cones that detect 5 really well, they can do a bit of 4 and a bit of 6 too.
If blue 3 and green 5 light were shone into our eye, then the blue cones and the green cones detect it. Because blue and green cones are activating, we see it as cyan.
If cyan (4) light were shone in, blue and green cones would again activate because they have a range of frequencies they're sensitive to; the cyan light activates both of them in that overlap zone where they both weakly respond to cyan - we see it as cyan again even though the light entering is different to before. In practice there are many different combinations of different frequencies of beams of light that are detected in this varying map of intensity, and they get mapped to similar colors
I didn't talk about yellow cones much because they add a dimension of confusion. A yellow cone responds to what we might refer to as red, yellow or green light. It needs help from the other cones to determine what color it is seeing. If a yellow is activated, but green is not then the light source appears to be red. If yellow activates strongly and green activates weakly, it maps to yellow light, and if yellow activates weakly, green activates strongly, it maps as green
You talk about color blindness, and most commonly it refers to a deficiency of the green cone - if a person's green coneset is deficient they will have trouble telling red from green simply because the green cone set registers more closely to how the yellow cone set registers, and they have a reduced ability to detect greens as a result. Because detecting green light is vital for interpreting the information from the yellow cone set, but the deficient green cones behave more like yellow cones you end up with a situation of:
- "weak red light causes weak activation of yellow cone and incorrect activation of green when it shouldn't" and
- "weak green light causes weak activation of yellow cone and deficiently low activation of green cone when it should be more"
Bright reds and greens might not be as much of a problem; they wouldn't look very distinct but other cues might help a red-green color blind person discriminate. When intensity lowers, things become a problem because the amount of cone activation is so similar in different color situations.
Your carrots picture isn't exactly accurate because it wouldn't look "that intensely green" - all the veg would look a more bland form of yellowy brown rather than being this green; it's the lack of help from the green components that rives everything toward being perceived as yellow/brown
People who are completely color blind are incredibly rare, and it's probably not quite like watching a black and white movie. Black and white movies are varying shades of white because white light makes all your cone sets activate. People who are monochromatic are more likely to see one color in varyiong intensity, so rather than being black white and grey, your movie would be more like a black and white tv with a faintly colored sheet of transparent plastic placed in front of it
In low light, the rods take over; they just register an intensity of light. At dusk and darker, everything starts to look the same because we can only see the presence or absence of light rather than its color
So that's "how we see". Next up to discuss is "how we see things"
We see because light enters our eye and tickles our rods and cones. We see things because visible light is coming off them and travelling into our eye, and tickling those rods and cones. Some things emit their own visible light; other things "emit" light because they're reflecting it from something else.
A vital consequence of the last sentence is that in order to reflect a frequency of light, the object has to receive it in the first place and the object has to be made of something that reflects it rather than absorbs it.
In terms of reflection, absorption and transmission in the real world:
Your eye pupils look black because they do a good job of capturing most of the light that enters them. Your goth friend's skin looks white because it's reflecting a lot of the visible light falling on it. Your other friend from Papua New Guinea has one of the darkest known skin tones because his skin absorbs a large amount of the incident light. Both your friends can go and get an X-ray and it works out because the X-ray waves are transmitted by/pass through your skin but not your bones (absorbed). All 3 friends can get a sun burnt by the UV present in sunlight, but not if they're in the shade, unless someone puts up a mirror reflecting the UV onto them again. Hiding behind a mirror might not do much good if you're close to a source of gamma radiation
The whole world is potentially being illuminated by a huge range of frequencies of electromagnatic radiation. If your eyes were sensitive to ~2.4GHz you'd see wifi routers and microwave ovens flashing like crazy. If you could see 500 - 600 Mhz, TV satellites in the sky would be flickering away. If you could do 900 or 1800 MHz GSM cell towers might look like the white walls of your living room when the TV is on at 2am..
As it is, we see what we see, and we don't see things outside that range. If we use some device to shift the frequency into a form that we can detect (point your cellphone camera at an infra red remote control, use a Geiger counter to turn radiation into an audible "flickering" then we can "see" more of the world around us. Without those things, we rely on our senses and what they do or don't pick up. If yellow light falls on a blue object, we see it as black; the object only looks blue because it is capable of reflecting blue light and it absorbs all others. Yellow light doesn't have a blue component, only pure yellow or some mix of red and green, so the object appears black; it can only reflect blue, and none of the light falling on it is blue.
Inks in your inkjet printer are cyan, magenta and yellow, because the paper can't emit light on its own - it has to rely on reflection. A paper painted red can only reflect red light. A paper painted green can only reflect green light. If inkjet in was red, green and blue we couldn't print yellow, because to get yellow, we need to reflect red and green. Simultanoeusly painting the paper with red and green ink would mean the red ink absorbed all the green light, the green in absorbed all the red light, and our yellow (or red+green_ light doesn't work. The inkjet printer will spray the paper with yellow ink and magenta ink, the common reflective component of these two being red, if you want red. Green is a spraying of cyan+yellow, as both these colors (in a reflective sense) can reflect a green component, and each of them filters out one of the other components (yellow filters out blue, cyan filters out red) leaving only the green (fro red,green,blue)
We don't see UV light; if something is visible under UV light on CSI, it's because the UV light is causing the object to emit a frequency of light that we can see. We don't see infrared either, but we can perceive some infrared radiation as heat because it feels warm on our skin. We could probably detect microwave radiation too; it will vibrate the molecules of water in our skin just like it heats the food in the oven. Don't stick your hand in a microwave oven, but do appreciate that after microwave ovens were invented, it was mooted possible to replace conventional heating systems with microwave systems that gently heated the humans in the house, using microwave radiation
If you're still awake, hopefully you know now how we see, and how we see things - so your questions are simpler to answer:
What determines whether colors you can't see are visible or not?
Whether it is present; something has to be emitting it, something has to be transmitting or reflecting it - these two things alone determine its presence in your location - it has to be being generated there or arriving there.
Whether you can detect it is another question. You can't detect the radiation from your cellphone/network so you have to rely on your signal bars to tell you whether you have service or not. If you're near a tower (emitter) and nothing is blocking it(the free air is transmitting it) then its present (it's shining on you until you walk into that concrete bunker...)
So, when someone is red-green colorblind, the colors appear the same to them, like this
Yeess.. More like they have a diminished ability to tell certain different colors apart based on the normal metric you might apply. There are many variations of color-blindness
And if you're totally colorblind, then things presumably just appear like they would in a black-and-white movie
It's more likely to be that everything appears in a varying intensity of shade of a color (other than white)
However, this isn't how ultraviolet patterns seem to work. Compare how we see this flower to the version where ultraviolet is visible
There's no magic with UV; it's simply light/electromagnetic waveform just like anything else. The tips of that flower reflect UV, the main body of the flower doesn't reflect UV. UV light might not be falling on the flower, or it might. Your eyes can't detect it either way; that image has been made using technology (camera lens) that can detect UV and it was used in some context where there was UV light falling on the flower, being reflected and detected by the camera. It's been re-represented as visible color so you can appreciate it. Just like a Geiger counter makes a horrific noise to help you appreciate how well irradiated you're being
This time the flowers are purple, but the UV pattern is still invisible
They're different flowers, that might or might not have some part of their surface that reflects UV into your detector.. but you don't seem to have a detector with you so you're right - it's invisible
Shouldn't the UV pattern still be apparent on at least one of the flowers, just in a different color?
No. Not in "a different color" anyway. Whatever color UV is, it's not a color we see so we won't have evolved to have a name for it. Other than perhaps "UV". We can't see it, so we need a detector that can, and it might say "this part of the flower is reflecting 100% of the incident UV light, that part is reflecting only 80%.." etc, so a computer could generate an image using visible colors to describe the intensity of UV reflection different parts of the flower... This is in exactly the same way that a standard camera lens samples the intensity (and frequency) of light apparently-emanating from everything it can "see"
And on some other flowers, the UV does appear as a different color
If you're seeing it, it's not UV. Maybe it's violet or blue or some other electromagnetic radiation from a part of the frequency spectrum that is close to UV's frequency.. Like maybe the UV torch in your hand is also chucking out some visible light too. A true 100% UV light would emit no visible light. Just like you can't see the infrared LED in your TV remote flashing away either
Why is the UV invisible only sometimes?
It's invisible to you or I all the time
Does it have to do with the flower using iridescent structures to produce color, instead of a pigment?
It's perhaps not the right word; iridescence refers to a surface's ability to reflect incident light in such a way that it appears to have multi or varying colors depending on the angle. It will likely be like a prism and is causing a split or divergence of the different inbound light frequencies so that they appear at different angles and are no longer perceived as combined. A rainbow, prism or diamong ring might be similarly effective.
Fluorescence might be the word you're looking for; a substance that receives a higher energy radiation like UV or X-ray and begins glowing with a lower energy radiation emission like visible blue light. Kinda like microwaving something until it's so incredibly hot it's emitting infrared
Can this happen with red and green, as well?
Can UV appear as red or green? No; by definition it can't appear ans anything other than UV. If you could run it through some downsampling/frequency changing device so it went in as UV and emerged as red, then it wouldn't be UV any more. Remember that these are all just different speeds of oscillation of the same elementary particle, so there isn't any magical difference between them