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?

Have fun.

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: http://lifesci.ucsb.edu/~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.