How can some animals see ultraviolet or infrared light?

I know that some animals like birds, bees, and fish can see ultraviolet and infrared light. Whether it to detect flowers that bare nectar, or the urine trails of prey. But what I don't understand is how they see these wave lengths. What is different about their eyes or brain that allows them to see different wavelengths than humans?

Yes, they have different photoreceptors as well as the circuitry to interpret the information from those photoreceptors. Humans have 3 types of cones which are tuned to respond best to red, green, or blue light, but it's not an on or off signal. A red or blue cone may still fire in greenish light, it just fires much less often. The brain takes that input and figures out the color based on the relative firing rates of the red/blue/green cones.

So for an animal that can see in UV or infrared, they need both the photoreceptors that respond to that wavelength of light, and the ability to integrate that response into the bigger picture of how all the photoreceptors are firing.

Short answer
In mammals dedicated UV cones have been found, as well as photoreceptors with secondary peak-sensitivity in the UV range. In fact, human blue cones are sensitive to near-UV.

In humans, the visible spectrum is generally accepted to range from 390 to 700 nm. Figure 1 shows the spectral sensitivities of the various photoreceptors in humans.

Fig. 1. Spectral sensitivity of the four receptor classes. Source: Wikibooks; Sensory Systems

In invertebrates sensitivity to near-UV is quite common. However, some rodents (mice, gerbils and gophers) also feature a peak in sensitivity at 359 - 511 nm. In these rodents and some marsupials sensitive to UV, it is thought to be attributable to a specific dedicated type of cone sensitive to near-UV (Jacobs et al., 1991; Winter et al, 2003). Near-UV is referred to as UV-A, encompassing 315 - 380 nm. Likewise, birds seem to feature a dedicated, fourth cone class to detect UV (Benett & Cuthill, 1994).

In the color-blind flower bat, UV sensitivity has been attributed to a photoreceptor with two peak sensitivities - one in the green range and the other around 365 nm and running down to 310 nm (Winter et al, 2003). Hence, UV sensitivity can be conferred by cones with broad sensitivities ranging into the UV range.

In fact, the aphakic human eye (eyes with the lens removed after cataract surgery are aphakic) has been shown to be sensitive to near-UV. As can be seen in Fig.1, the blue cones are actually pretty sensitive to near-UV. It appears that the lens absorbs much of the UV, rendering UV light useless to humans with healthy eyes (Griswold & Stark, 1992).

Infrared (IR) sensitivity in snakes is mediated by pit organs and not via the eyes. Basically they are heat sensors and likely don't mediate vision as such (Newman et al., 1982). I was not able to find any relevant information on IR vision, or how it should be mediated. In fact, I doubt IR is used for vision at all, i.e., to reconstruct the visual scene. IR vision is used more in the form of general heat detection.

- Benett & Cuthill, Vis Res (1994); 34(11): 1471-8
- Griswold & Stark, Vis Res (1992); 32(9):1739-43
- Jacobs et al., Nature (1991); 353: 655-6
- Newman et al., Sci Am (1982); 246(3): 116-27
- Winter et al, Nature (2003); 425: 612-4

Researchers find UV sensitivity in wide range of mammals

( —Biologists Ron Douglas and Glen Jeffery of City University and University College in the U.K. have upended the notion that few mammals are able to see in ultralight. In their paper published in Proceedings of the Royal Society B: Biological Sciences, the two describe how they examined the eyes of a myriad of donated dead animals and found that a wide variety of them had lenses which allowed UV light to pass through.

Humans are not able to see radiation lying in the ultraviolet range—the lenses in our eyes block out such UV light, preventing us from seeing what might be right there in front of us: patterns on flowers for example, or urine stains from a passing rodent. Scientists believe evolution has disabled UV sensitivity in our eyes in an attempt to improve our acuity. Up until now, it has been assumed that most other mammals have lenses similar to ours, preventing them from seeing UV light as well. In this new effort the research pair show that is not the case at all as many other mammals appear to have at least some ability to see UV light.

To learn more about how other animals see, the researchers asked for donations of dead animals from zoos, veterinarians, animal shelters etc. They received a huge variety, each of which had their eyes extracted. The researchers shone different lights through the lenses of each and measured what came out the other side. To their surprise they found a large percentage of the animals, all mammals, did not have UV blocking lenses which meant, at least theoretically, that they could see at least some UV light. The list included animals such as cats, dogs, okapis, ferrets and hedgehogs. This suggests that our pets can see things we don't, which might help explain their sometimes odd behavior.

That's not the whole story though—there is more to seeing UV light than what passes through the lens—prior research has shown that most mammals don't have visual pigments in the back of the eye that are sensitive to UV light, which suggests that even animals that allow UV to pass through their lines, still don't see UV light reflected back from the environment. But that might not be true either. More recent research has found that that other media in the eyes of some animals (such as the cornea) is sensitive to UV light as well, which might allow a type of sensitivity to UV light that isn't really understood. Clearly more research will need to be done to discover which mammals can see in UV and to what degree.

Although ultraviolet (UV) sensitivity is widespread among animals it is considered rare in mammals, being restricted to the few species that have a visual pigment maximally sensitive (λmax) below 400 nm. However, even animals without such a pigment will be UV-sensitive if they have ocular media that transmit these wavelengths, as all visual pigments absorb significant amounts of UV if the energy level is sufficient. Although it is known that lenses of diurnal sciurid rodents, tree shrews and primates prevent UV from reaching the retina, the degree of UV transmission by ocular media of most other mammals without a visual pigment with λmax in the UV is unknown. We examined lenses of 38 mammalian species from 25 families in nine orders and observed large diversity in the degree of short-wavelength transmission. All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity. Other mammals, however, such as hedgehogs, dogs, cats, ferrets and okapis had lenses transmitting significant amounts of UVA (315–400 nm), suggesting that they will be UV-sensitive even without a specific UV visual pigment.


Jaglavak, Prince of Insects

Animal Magnetism

Dogs' Dazzling Sense of Smell

With UV vision, birds see a much different world than we do. Feather patches that to us seem an unremarkable shade of blue scream brightly in UV. Birds can hunt their prey by following urine trails, which reflect UV light. But with the exception of some rodents and marsupials with a fourth type of cone, we didn’t see a way that mammals could access this extra dimension of color.

In a study published earlier this year in the Proceedings of the Royal Society B , scientists analyzed the eyeballs of 38 different mammalian species. What they found suggests that most mammals can, in fact, see UV light—including dogs, cats, ferrets, and reindeer. Unlike humans, these mammals have lenses that allow UV light though. Even though they lack the specialized UV-sensitive type of cone, the other three kinds of cones can combine to make up for it. (In fact, when people have the lens in their eyeballs removed—either through injury or surgery—they’re able to do the same trick, and report seeing UV light as something like a pale violet.)

Reindeers can see ultraviolet light.

This anatomical discovery could have major ecological impacts. Damian Carrington, writing for the Guardian:

“It was a big surprise but we now think the majority of animals can see UV light,” said Professor Glen Jeffery, a vision expert at University College London. “There is no reason why this phenomenon is not occurring around the world.”

[Dr Nicolas Tyler, an ecologist at UIT The Arctic University of Norway and another member of the research team] said the discovery has global significance: “The loss and fragmentation of habitat by infrastructure is the principle global threat to biodiversity – it is absolutely major. Roads have always got particular attention but this will push power lines right up the list of offenders.” The avoidance of power lines can interfere with migration routes, breeding grounds and grazing for both animals and birds.

One example is reindeer, whose habitat has become severely fragmented by growing infrastructure in the Arctic, including power lines. Up to now, though, exactly why reindeer avoided power lines was a mystery. But in the dark arctic winters, such light reflecting off the snow can be blinding.

Reindeer aren’t alone in their avoidance of power lines. We’ve long known that many animals avoid crossing high-voltage corridors, but blame was placed mostly on the change in habitat configuration—the mowed vegetation below the lines is often substantially different from the surroundings. That hypothesis is likely valid for some animals, but this new data on corona discharge suggests the problem is more complex than we originally thought.

Let the light shine in

In the film K-PAX, Kevin Spacey plays a psychiatric patient who claims to come from another planet. Although he appears human, he can see ultraviolet light. Does this prove his extraterrestrial origin or could it mean something else?

Professor Bill Stark, of the biology department of Saint Louis University, has carried out extensive research on ultraviolet vision in animals - and can see ultraviolet.

Light consists of electromagnetic waves. Visible light is measured in nanometres - billionths of a metre. Light with a wavelength of around 700nm is red, at 500nm it is green, 400nm is blue-violet, and anything below that is usually invisible. You can see this invisible light indirectly by fluorescence.

"Black light" used in discos is UV some surfaces absorb it and re-emit it in the visible spectrum, giving off a vivid glow. Washing powders contain fluorescent phosphors for this reason your clean shirt does not just reflect white light, it also has an added glow from the UV in sunlight. Thus it really does appear "whiter than white" in daylight. Just because you cannot see UV does not mean it has no effect on your eyes.

You can absorb large amounts of invisible UV without realising it. Exposure to high levels of ultraviolet - glare from snowfields or sunlamps - can cause snowblindness when the cornea (the clear part of the eye) is effectively sunburned. This inflammation can cause loss of vision and makes the eyes painfully sensitive to light. The effects usually only last a day or two, but with intense UV, there is a risk of permanent damage.

These harmful effects are reduced by the lens, which absorbs UV and prevents it entering the eye. When the lens becomes opaque due to cataracts, it may be surgically removed, and can be replaced with an artificial lens. Even with the lens removed (a condition known as aphakia) the patient can still see, as the lens is only responsible for about 30% of the eyes' focusing power.

However, aphakic patients report that the process has an unusual side effect: they can see ultraviolet light. It is not normally visible because the lens blocks it. Some artificial lenses are also transparent to UV with the same effect. The receptors in the eye for blue light can actually see ultraviolet better than blue. Military intelligence is said to have used this talent in the second world war, recruiting aphakic observers to watch the coastline for German U-boats signalling to agents on the shore with UV lamps.

However, the origin of the story has proven hard to track down. Ultraviolet vision was discovered in ants in 1882. It was thought to be confined to insects and some birds, but was later found in mice, lizards and other animals. Some flowers have distinctive patterns only visible in the ultraviolet, and some birds have colours in their plumage that are invisible to us but may be important in attracting a mate.

Other animals have more exotic reasons for seeing into the ultraviolet. Kestrels and other raptors can roam over a large area searching for food. From a great height, they need to identify likely hunting grounds. Rodents mark their runs with trails of urine that absorbs UV, and in 1995, Finnish researchers found that kestrels can see these trails. It seems the birds can spot areas criss-crossed by recent rodent trails and zero in on them. Smaller rodents such as voles urinate almost continuously, so a predator could simply follow a fresh trail to find prey.

It appears we are blind to wavelengths that are useful to animals, and we would expect an evolutionary reason. One suggestion is that without a UV-absorbing lens, there would be cumulative damage to the retina but aphakic patients do not seem to suffer seriously even after many years.

Another possibility is that cutting out UV gives us sharper vision. This is because a lens can only focus a limited range of colours at the same time. Increasing the range of wavelengths leads to a distortion called chromatic aberration, which will be familiar to people with cheap camera lenses.

The eye represents a compromise between clear focus and breadth of spectrum. What does ultraviolet look like? Prof Stark possesses UV vision because he is aphakic in one eye and, with Professor Karel Tan, has published research on the nearest visible equivalent. His conclusion is that it looks whitish blue or, for some wavelengths, a whitish violet.

This appears to be because the three types of colour receptor (red, green and blue) have similar sensitivity to ultraviolet, so it comes out as a mixture of all three - basically white, but slightly blue because the blue sensors are somewhat better at picking up UV. Our sensory system does not appear to be geared to revealing additional colours beyond the violet, though other animals will see things differently.

An illustration of how ultraviolet appears is provided by the Impressionist painter Claude Monet. Following cataract surgery in 1923, his colour palette changed significantly after the operation he painted water lilies with more blue than before. This may be because after lens removal he could see ultraviolet light, which would have given a blue cast to the world.

Birds, bees, biology professors and Impressionists may have the ability to see into the ultraviolet, but it is more likely to be a sign of cataract surgery than having come from another world.


The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells, which are lower-intensity light receptors) with different absorption spectra. This means that the organism may see wave-lengths beyond those of a typical human's vision, and may be able to distinguish between colors that, to a normal human, appear to be identical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species. [6]

Fish Edit

The goldfish (Carassius auratus auratus) [7] and zebrafish (Danio rerio) [8] are examples of tetrachromats, containing cone cells sensitive for red, green, blue and ultraviolet light.

Birds Edit

Some species of birds, such as the zebra finch and the Columbidae, use the ultraviolet wave-length 300–400 nm specific to tetrachromatic color vision as a tool during mate selection and foraging. [9] When selecting for mates, ultraviolet plumage and skin coloration show a high level of selection. [10] A typical bird eye will respond to wave-lengths from about 300 to 700 nm. In terms of frequency, this corresponds to a band in the vicinity of 430–1000 THz. Most birds have retinas with four spectral types of cone cell that are believed to mediate tetrachromatic color vision. Bird color vision is further improved by the filtering of pigmented oil droplets that are located in the photoreceptors. The oil droplets filter incident light before it reaches the visual pigment in outer segments of the photoreceptors.

The four cone types, and the specialization of pigmented oil droplets, give birds better color vision than that of humans . [11] [12] However, more recent research has suggested that tetrachromacy in birds only provides birds with a larger visual spectrum than that in humans (humans cannot see ultraviolet light, 300-400 nm), while the spectral resolution (the "sensitivity" to nuances) is similar. [13]

Insects Edit

Foraging insects can see wave-lengths that flowers reflect (ranging from 300 nm to 700 nm [14] [15] ). Pollination being a mutualistic relationship, foraging insects and some plants have coevolved, both increasing wave-length range: in perception (pollinators), in reflection and variation (flower colors). [6] Directional selection has led plants to display increasingly diverse amounts of color variations extending into the ultraviolet color scale, thus attracting higher levels of pollinators. [6]

Mammals Edit

Mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, [16] arguing against some of these obstacles however, the original publication's claims about plasticity in the optic nerve have also been disputed. [17]

Reindeer Edit

In areas where reindeer live, the sun remains very low in the sky for long periods. Some parts of the environment absorb ultraviolet light and therefore to UV-sensitive reindeer, strongly contrast with the UV-reflective snow. These include urine (indicating predators or competitors), lichens (a food source) and fur (as possessed by wolves, predators of reindeer). [18] Although reindeer do not possess a specific UV opsin, retinal responses to 330 nm have been recorded, mediated by other opsins. [19] It has been proposed that UV flashes on power lines are responsible for reindeer avoiding power lines because ". in darkness these animals see power lines not as dim, passive structures but, rather, as lines of flickering light stretching across the terrain." [20]

Humans Edit

Apes (including humans) and Old World monkeys normally have three types of cone cell and are therefore trichromats. However, at low light intensities, the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space [21] human rod cells' sensitivity is greatest at a bluish-green wave-length.

In humans, two cone cell pigment genes are present on the X chromosome: the classical type 2 opsin genes OPN1MW and OPN1MW2. People with two X chromosomes could possess multiple cone cell pigments, perhaps born as full tetrachromats who have four simultaneously functioning kinds of cone cell, each type with a specific pattern of responsiveness to different wave-lengths of light in the range of the visible spectrum. [22] One study suggested that 15% of the world's women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. [23] Another study suggests that as many as 50% of women and 8% of men may have four photopigments and corresponding increased chromatic discrimination compared to trichromats. [24] In 2010, after twenty years of study of women with four types of cones (non-functional tetrachromats), neuroscientist Dr. Gabriele Jordan identified a woman (subject cDa29) who could detect a greater variety of colors than trichromats could, corresponding with a functional tetrachromat (or true tetrachromat). [25] [26] [27] [28]

Variation in cone pigment genes is wide-spread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red/green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female new-world monkeys trichromatic vision. [29]

In humans, preliminary visual processing occurs in the neurons of the retina. It is not known how these nerves would respond to a new color channel, that is, whether they could handle it separately or just combine it in with an existing channel. Visual information leaves the eye by way of the optic nerve it is not known whether the optic nerve has the spare capacity to handle a new color channel. A variety of final image processing takes place in the brain it is not known how the various areas of the brain would respond if presented with a new color channel.

Humans cannot see ultraviolet light directly because the lens of the eye blocks most light in the wavelength range of 300–400 nm [ relevant? ] shorter wavelengths are blocked by the cornea. [30] The photoreceptor cells of the retina are sensitive to near ultraviolet light, and people lacking a lens (a condition known as aphakia) see near ultraviolet light (down to 300 nm) as whitish blue, or for some wavelengths, whitish violet, probably because all three types of cones are roughly equally sensitive to ultraviolet light however, blue cone cells are slightly more sensitive. [31]

Tetrachromacy may also enhance vision in dim lighting, or when looking at a screen. [27]

How can some animals see ultraviolet or infrared light? - Biology

I talked with my roommate (he is a biologist) last night about animals that can see light outside the visible spectrum. One thing that my roommate told me is that many of the animals that can see ultraviolet light are small animals, such as insects. Bigger animals seem to only see visible light. He wonders whether this has something to do with the size of the animals. As you may know, ultraviolet light has a wavelength that is shorter than visible light. My roommate thinks that small animals may be better suited to see smaller wavelengths of light because their eyes are smaller. We don't know if there is any truth to this, so you might want to do some research into this hypothesis. Interestingly, my roommate didn't know of any animals off the top of his head that can see infrared (as you mentioned). The one example that he knows of are monitor lizards, which can detect heat (possible by sensing infrared light). You might want to look into these animals for more information.

I guess that I've written quite a bit and not really answered your question. Basically, the reason for the different ability to see different wavelengths of light has had to do with evolution. Suppose an insect species could only see visible light, but some mutation allowed its children to see ultraviolet light. If this ability helped the children to reproduce (maybe by allowing it to better find edible flowers), then pretty soon you would have a new species which had the ability to see ultraviolet light.

The mutation that I spoke about would involve some change in the cells in the eyes. There are certain cells in the eyes which work as light detectors. If some genetic mutation occured which changed these cells, allowing them to see UV light, then you have the first step to a new species. You not only need a mutation, but there also must be some advantage to being able to see UV light, if this ability is to be passes on to enough offspring so that the ability persists. Humans cannot see UV light because either (1) there was no mutation (unlikely) or (2) the ability to see UV light provided no big advantage to the "mutants" who had this ability.

For an introduction to the physics of sight you might want to look at "The Feynman Lectures on Physics, Vol I" Chapter 36. This book is written for college students and is, in general, very difficult to read for high school student, but this chapter is an exception. It is over 25 years old, but I think most of the science is still valid.

Almost 4 billion years of evolutionary history is encapsulated in the nervous system of homo sapiens and indeed many other animals and plants. Having sensors sensitive to say Xray's would be a terrible waste for a creature living at the bottom of an ocean of air. Similarly, photsynthetic systems that make fuel from star light are optimized to work at about 450 to 500 nm because this is the dominant radition wavelength from the Sun. Perhaps on some planet orbiting a slightly brighter star, life forms would be less sensitive to IR and red light and more into the violet and maybe even UV part of the spectrum.
In short, the best way to understand the nervous system and the "dectectors us primates walk around with ( and I dont mean sony walkmans) is by putting the question into the evolutionary context. a most EFFICIENT prism for separating the wheat from the chaff. lifeforms represent a countercurrent in the inexorable march towards disorder and entropy. such systems have learned to be ideally suited to their environment. Snakes who hunt for rodents at night have far better IR sensors than humans. they need them, we dont !! so that part of the brain is better developed. same with sense of smell. more important for other organisms for their SURVIVAL.

Our eyes are a complex product of both evolution and biology. The Sun puts out maximum energy in the band of the electromagnetic spectrum that we call white, "visible" light. Natural selection during evolution has maximized our ability to use this kind of electromagnetic energy by selecting for a certain mixture of physiology (rods and cones) that was generally successful for people in the past (with very different lifestyles from ours today). Judging by the results (our eyes today), we can guess that at some point in the past sharp, binocular color vision was more valuable for an omnivorous biped (that's us) than, say, black-and-white vision across a wider part of the spectrum. Here's a question for you: Humans are generally awake during the day, and our eyes are optimized for visible sunlight. Nocturnal animals are awake at night when there is no sunlight and only occasional light from the Moon (often casting just black/white shadows). If I told you that at night, one of the most abundant kinds of energy is thermal radiation given off from cooling objects (like plants, rocks, people, worms, etc.), what kind of eyes should a nocturnal animal have?

The answer has nothing to do with the brain, but rather with the back of the eye, where the light is detected by "rod" and "cone" cells. Each cell can only see certain colors of light, and humans seem to only have developed cells that can only see the "visible" part of the spectrum. Many deepwater fishes can't see red light because only blue and green light penetrate to their depth. so their idea of a visible spectrum is green, blue, and purple.

Which raises the question: Why do you suppose that humans, and the primates from which we developed, adapted to see red, orange, yellow, green, blue, and purple light better than infrared? Do you suppose apes prefer to hunt at night or during the day?

The difference is not so much in the brain as it is in the eye. The cells in the retina (inner cladding of the eyeball) are sensitive to a certain range of wavelengths of the electromagnetic spectrum. This wavelength is a property of the light which is related to the color of the light, and to whether the light is visible, infrared or ultraviolet. The process of light detection occurs as follows: when light arrives to the eye, it's absorbed by some molecules that are present in these cells in your retina. These molecules then undergo some changes, and the result is an excitation of the optical nerves, that connect the eye to a part of the brain which is on the back of your head, where it is processed. The portion of the electromagnetic spectrum that we can see depends not on what the brain can process, but to which wavelengths (colors) of light the cells in your retina are sensitive to, and this in turn depends on which light-absorbing molecules are present in these cells.

Two more interesting pieces of information about vision are the following:

+Not all animals can see "in color". In fact, in the retina or the human eye there are two types of cells one detects the intensity of light, allowing us to see "in black and white", and the other one is responsible for making us distinguish between different colors. The animal species that don't have the second type of cells are therefore color-blind. As an interesting anecdote, bulls are color-blind, so the fact that, in a bull-fight, the bull is attracted by the red cape, or in general, that bulls are attracted by red-colored objects, is not true. What they are attracted by is movement, and it's the movement that the bullfighter gives to the cape what makes the bull go for it, not its color.
+The property of vision that does depend on the brain process is, however, the threedimensionality. Most animals see only in two dimensions, but humans see in 3-D. This is possible because of the slightly different angle with which both your eyes see objects the brain then processes these differences allowing us to perceive sensations such as depth, distance, volume, and so on. This property is used on the 3-D books, in which apparently meaningless spots on a page take volume and "grow" in front of your eyes to give you a full sensation of three dimensions. The spots are distributed around the page in such a way that, when looked at from the right distance, the brain produces this sensation of volume and depth.

Hi inquirers. Your questions show that you know some important things about the system. For one thing, you know that in order for us to "see" something, our eyes have to pick up the information and send it to our brain. Then the brain itself has to make sense of the message. In this case, the reason we can only see part of the spectrum is because we don't have all possible sensors in our eyes.

We don't "see" infrared, but we feel it as heat. Some snakes, like pythons, have special organs to sense heat. (Why do you think they have them? Does the type of prey they eat matter in whether they can use them?)

We also do not have ultraviolet receptors. Bees have them, so flowers that use bees as pollinators often have markings that bees can see and we can't. (Why should flowers "advertise" to bees?)

So why don't we have all of the possible sensors? For one thing, there are many tradeoffs in building something if your resources are limited. If you go to your favorite restaurant and only have a little money, you have to order only the most important food and skip the less important things. This is an example of making a tradeoff. Night vision (which requires receptors called rods) is important to cats, so they give up color vision (which uses receptors called cones). Having no color receptors allows them to have more night vision receptors.

Animals that had every possible sensor would be very expensive for their parents to produce. Since energy and nutrients are almost always in short supply, they might not be able to make any offspring at all. They certainly couldn't make as many as a parent that only gave each offspring the essentials. Over time, then, the offspring with all the extras would disappear, and the ones with the essentials would be more common. Of course, parents don't really "choose". The map for their offspring is encoded in their genes.

Why do we have the receptors we do have instead of having great night vision, visual UV receptors, and infrared receptors?

Basically vision (or more generally stated light perception) in any organism is accomplished via one or more compounds that have evolved to detect light. The visual compound in human eyes is called opsin (sometimes also called rhodopsin for rods). These compounds, also generally called pigments work such that when light strikes opsin it causes a physical change in the shape of the compound which works to activate opsin. Activated opsin causes a whole sequence of events to occur known as second messenger events. The eventual result is that there is a change in the flow of ions across the photoreceptor cell membranes and this signals the cell that light has been perceived. Opsins in humans are specifically designed to detect light of specific wavelenghts. Rhodopsin (the opsin responsible for dim light vision) has a maximum sensitivity at 510nm which is blue-green light. Humans also have cone vision or color vision. We have 3 different opsins to see red, blue and green light. The "blue" opsi n
is very specifically designed to have a max sensitivity to light of 455nm, the "green" opsin is very specifically designed to have a max sensitivity to light of 530nm, and the "red" opsin is very specifically designed to have a max sensitivity to light of 625nm. The max sensitivity means that only light of that wavelength or close to it has the energy necessary to cause that opsin to change its physical structure and thus induce the cell that houses the opsin to "detect the light". So it's all in the compound that initially absorbs the light energy. It doesn't actually have anything to do with differences in the brains of different organisms. Some deepsea fish can see far red/infrared light. This is because they have a compound like our opsins that physically change their structure when light of that long wavelength strikes it.[There is a good website about this see:

biolum/organism/dragon.html] The difference does not lie in their visual processing centers in their brains. There are
certain shrimp which are sensitive to UV radiation, and again it is due to the presence of a certain compound in the shrimps eyes (specifically in the retina) that allows them to be sensitive to this part of the electromagnetic spectrum. If a scientist wants to find out what part of the electromagnetic spectrum that a particular organisms is sensitive to, they would take the retina from that organisms eye and run a pigment analysis. Pigment analysis is done by shining light of different wavelengths onto the retina sample and looking for wavelengths that are absorbed by the retina versus wavelengths that pass through without being absorbed. The wavelengths that are absorbed will tell the scientist which wavelengths the organism sees. What wavelengths do plants "see"? What compounds do they use to do this?

The "visible portion" of the spectrum provides sharp boundaries for objects, so we can tell how large the object is, where it is, what shape it is, and see specific details: such as the eyes and teeth and head position of a person or an animal. No other portion of the spectrum provides sharp details. Suppose, on the contrary, our eyes could see only infrared: All shapes would appear " fuzzy" or " wavy" without definite boundaries, and without specific information about the details of the object. Suppose our eyes could only see x-rays: we couldn't see some portions of objects at all: For example we could see the bone of an arm but not the whole arm, etc etc. I could extend this discussion to any other part of the electromagnetic spectrum:

So, on the evolution scale, it was advantageous for humans to see distinct boundaries and specific details in sharp focus rather than in fuzzy or wavy form or not all, for the "fight or flight", for a meal or a tool or a weapon. If we could NOT see the specifics of those objects, we might not survive. So our eyes "needed" to see the specific details, and the only spectrum-segment that provides such details is the segment that we actually evolved to be able to see.

1 Answer 1

Stars are not lasers. Stars emit energy based on their surface temperature, in rough accordance with expected blackbody emissions from an object of that temperature. You can see on the graph above the emission magnitude (in log scale on the y-axis) for different wavelengths (x-axis). These wavelengths are a small pieces of the overall spectrum, but you can see how all stars emit at all frequencies.

Also note that the white and blue giant stars (in the 10000 K temperature range) emit more light at all wavelengths, often by orders of magnitude, than cooler F and G stars like our sun (with its surface temp of 5777 K).

In your example, a 'UV' star would in the 10000 K range, with lots of emitted energy in the 'UV' range as you can see on the graph. An 'IR' star would be cool M-type red dwarf like the 3000 K line, where peak radiance is in the IR range.

The important thing to note, is that if a planet received radiation from both stars, there would still be a great deal of visible light that the planet received. Both visible spectrum 'eyes' and photosynthesis would be viable on such a planet.

Its also important to note that the graph is on a log scale. There is no IR peak in combined magnitude the hotter 10000 K star will be so much more powerful as to overwhelm the lesser star. There is no reason to specifically develop sensors for IR, since there will be more energy in the visible spectrum, and even more still in UV.

Overall, despite having two stars with different spectral peaks outside the visible range, you can still use the standard visible light range on your planet.

Want ultraviolet vision? You're going to need smaller eyes.

Unlike humans and most other mammals, birds can see ultraviolet light. But not all avian eyes are created equal, according to new research. Apparently, birds with smaller eyes are better able to perceive the UV spectrum.

Many animals can see ultraviolet light, including some amphibians, reptiles and fish. Even a few select mammals can see UV light, such as rats , bats and, surprisingly, reindeer . In general, this ability helps animals attract mates, find food and detect predators.

Reindeer are the only mammals that can see ultraviolet light

There's a reason that certain wavelengths are known as "visible" light, because those are the…

In the 1970s, scientists first learned that birds also see ultraviolet light, when they discovered UV-sensitive photoreceptors in the retina of certain birds. "[The researchers] found those receptors and saw certain behavioral responses to ultraviolet light," explained Olle Lind, a biologist at Lund University in Sweden. Today, scientists don't even need to look at an animal's eyes to see if it has the photoreceptors necessary to see UV light — they can figure it out by looking at the genome .

But possessing UV-sensitive photoreceptors is only half the equation. "If you want to see something, you have to get the light through the eye and to the retina," Lind told io9. That is to say, even if you have the right receptors, you will only see UV light if your ocular media — which consists of the cornea, aqueous humour, lens and vitreous humour — is transparent to the light. Your UV vision will be dampened if parts of your eyes are absorbing or scattering some of the UV light before it gets to your retina.

UV Light Detection

The secret behind the feline vision "superpower" is ultraviolet light (UV) detection. A new paper, published in the Proceedings of the Royal Society B, found that cats, dogs and certain other animals see this form of light that is usually invisible to humans.

"There are many examples of things that reflect UV, which UV sensitive animals could see that humans can't," co-author Ronald Douglas told Discovery News. "Examples are patterns on flowers that indicate where nectar is, urine trails that lead to prey, and reindeer could see polar bears as snow reflects UV, but white fur does not."

A reindeer, a cat and a dog could therefore probably see a white-furred animal, such as a bunny, hopping through a snow blizzard, while most people would just see a blur of all white.

Douglas, a professor of biology at City University London specializing in the visual system, and co-author Glen Jeffery, a professor of neuroscience at University College London, determined that cats, dogs, rodents, hedgehogs, bats, ferrets and okapis all detect substantial levels of UV.

"It has been known for nearly a hundred years that many invertebrates, such as bees , see UV," Douglas said, adding that birds, fish, and some reptiles and amphibians were added to the list in more recent decades.

This is how sunscreen looks to birds and bees

That's how you look if you are pale and take care of your skin. These are two pictures of the same

"However," he added, "it was assumed that most mammals do not see UV because they have no visual pigment maximally sensitive in the UV and (instead possess) lenses like those of man, that prevent UV reaching the retina."

Cats and Dogs May See in Ultraviolet

A house cat's bizarre antics may be more than just feline folly. The kitty may be seeing things that human eyes can't.

Unlike humans, many animals see in ultraviolet, and a study now suggests that cats, dogs and other mammals can, too. Knowing these animals see things invisible to humans could shed some light on the animals' behavior, the researchers say.

"Nobody ever thought these animals could see in ultraviolet, but in fact, they do," said study leader Ron Douglas, a biologist at City University London, in England.

Light is made up of a spectrum of colors. Visible light (that humans can see) spans from red to violet, and beyond the visible lie ultraviolet wavelengths. Many animals are known to have UV-vision, including insects (such as bees), birds, fish, some amphibians and reptiles, and a handful of mammals (such as some mice, rats, moles, marsupials and bats). [Images: See the World Through Cats' Eyes]

Seeing in ultraviolet

The lens of the human eye blocks ultraviolet light, but in animals with UV-transparent lenses, ultraviolet light reaches the retina, which converts the light into nerve signals that travel to the brain where the visual system perceives them.

Even in animals whose retinas aren't very sensitive to UV light, some of the light is still absorbed. (In fact, humans who have had their eye lenses removed, such as in cataract surgery, without being replaced by ultraviolet-blocking lenses report being able to see in the ultraviolet.)

In this study, the researchers obtained eyes from a smorgasbord of mammals &mdash everything from hedgehogs to red pandas to macaque monkeys &mdash who had died or were killed, donated by zoos, veterinarians, slaughterhouses and science labs. The scientists measured how much light got through the lens of each animal's eye to its retina.

The team found that many of the animals, including hedgehogs, dogs, cats, ferrets and okapis (relatives of giraffes that live in the central African rainforest), have lenses that allow some ultraviolet light through, suggesting these animals may see in the ultraviolet.

This begs the question, what purpose does ultraviolet vision serve?

"The question is only being asked because humans can't see it," Douglas told Live Science, adding that nobody asks why humans see other colors.

Nevertheless, ultraviolet vision does serve several purposes. Bees and other insects use it to see colors or patterns on plants that can direct them to nectar. Rodents use it to follow urine trails. And reindeer may use ultraviolet light to see polar bears, which, in visible light, blend in with the snow.

Why block UV?

The better question, Douglas said, is why human eyes block out ultraviolet light. One possibility is that ultraviolet light damages the retina, just as it damages the skin over time. But many long-lived animals that are active during the day, such as reindeer, have ultraviolet vision, and "their eyes don't fall apart," Douglas said. [What If Humans Had Eagle Vision?]

A more likely explanation for why human eyes filter out ultraviolet light is to improve visual acuity. Skiers wear yellow goggles that block UV light specifically for this reason. The researchers looked at the animals that blocked the most ultraviolet light, and found these were the same animals with the highest-resolution vision.

Humans are good at seeing detail, because they have a high density of color-sensitive cells, or cones, in their retinas, which produce high-quality images with just a small amount of light. By contrast, nocturnal animals have eyes that let in as much light as possible, including ultraviolet light, though it may not serve any special purpose.

Ultimately, knowing that many animals have ultraviolet vision could provide a deeper understanding of why they behave the way they do. Or maybe your cat really is just crazy.