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On our world we have any number of creatures that use:

as an alternative to their teeth and claws.

One thing that we don't really have though is creatures using light as a weapon, I don't mean using it as lures, bait, or for attracting a mate. I mean actual light used to damage the target.

It would probably make sense if it was aimed at the targets vision but something more dangerous would be even better.

So the question is, how could a creature (plants are acceptable) evolve to use light as a weapon. How would that weapon work and what would it look like?

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  • $\begingroup$ Do you include gamma/xray/uv rays as "light" or do you mean only visible light? $\endgroup$ – DoubleDouble Apr 21 '15 at 15:09
  • $\begingroup$ Visible light would be preferred but extending outside visible ranges would be fine if it leads to a better answer. $\endgroup$ – Tim B Apr 21 '15 at 15:11
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    $\begingroup$ Larry Niven's Known Space universe contains a genetically engineered plant called a Slaver Sunflower. Each individual plant has a small mirror that it uses to collect sunlight for photosynthesis, but the plants are capable of directing that light elsewhere. Whole fields of them can target flying things and incinerate them with their combined power $\endgroup$ – Mike Nichols Apr 21 '15 at 15:45
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    $\begingroup$ There's also Sentenced to Prism by Alan Dean Foster which involves an entirely different biology (most of the life is solar powered) and has some lazing entities. $\endgroup$ – user487 Apr 21 '15 at 19:48
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    $\begingroup$ Deep Sea fish use light as a lure to attract fish to their doom. Kind of a weapon... $\endgroup$ – Oldcat Apr 22 '15 at 18:07

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Lens Trees

Unlike standard Earth trees, Lens trees approach gathering light from a different perspective. Earth trees grow as many leafs and branches as possible, spreading out to gather light and turn it into food. Early Lens trees instead grew leaves of crystalline minerals, shaped into oval lenses, that gathered and concentrated light into lower areas of the tree where photosynthesis would occur. Structures would "move" by either hydrating or removing water, causing them to bend. This allowed the tree to focus and concentrate light throughout the entire day.

Over time, Lens trees evolved and adapted this light concentration method for other purposes. The first alternative use was likely to reduce competition - these Lens trees would use their concentrated light to burn out nearby competing plants, allowing themselves greater access to resources. This version of the plant quickly took over most of the planet that receives direct sunlight for most of the year.

Modern Lens Trees have taken this to incredible levels of sophistication. Many varieties of the trees have developed various mirrored surfaces, which they then use to direct concentrated light back up through other lenses that concentrate it further. This is used defensively, to blind or burn bird-type animals or insects that would otherwise attack the tree. The trees have also developed incredibly complex organs that appear similar in structure (if not in composition) to animal nervous tissue. These appear to allow the trees to use their control and detection of light as a form of radar, letting them pinpoint targets both in the air and on the ground and then attack with concentrated light. Finally, Lens trees will occasionally "bathe" themselves in this concentrated light to kill any parasites or unwanted guests.

Night and Clouds

Lens trees are more vulnerable without normal sources of light, and it's likely that they would have died out early in their history if not for a symbiotic relationship with another, bio-luminescent plant. This plant has grown so close that it appears to be part of the Lens tree - it was only through DNA testing that it was revealed to be a separate species. While this plant cannot sustain light, it can create bursts on request from the Lens tree, allowing it to defend itself for short periods even when the sun is not available.

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  • $\begingroup$ This almost lends credibility to Durkon's blight .. here: giantitp.com/comics/oots0150.html lol $\endgroup$ – Ditto Apr 21 '15 at 18:21
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    $\begingroup$ Lens Trees are vulnerable to the Mirror-Mites, which use the same crystalline material as part of its exoskeleton to actually reflect most of the light away from its body. They can then survive for short spans of time while feeding on the tree and then retreat to some safe area afterward. Some light shows bring several Lens Trees and release a bunch of mirror mites and watch them battle for hours. $\endgroup$ – DoubleDouble Apr 21 '15 at 21:24
  • $\begingroup$ Concentrating light and photosynthesis in living things has a major concern - and that's the temperature increase in the target spot coagulating your proteins and boiling your water. So the development path "gathering light" by means of lenses has to be carefully designed. Second, real world photosynthesis peaks at intensities way below full sunshine so concentration yields no benefit (at least without photosynthesis developing completely different to what we know). $\endgroup$ – Ghanima Apr 22 '15 at 8:30
  • $\begingroup$ @Ghanima good point... perhaps it could be salvaged with something along the lines of a world where chloroplasts never developed but colony organisms like jellyfish containing pools of chloroplast-like bacteria did. Focusing the light into one area could make sense then. Roll on a billion years and you might have treelike organisms with hard outer layers, internal pools of chloroplast-things and jelly-like lenses instead of leaves to collect light for the internal colonies. Would probably need lots of water. $\endgroup$ – Murphy Apr 22 '15 at 11:37
  • $\begingroup$ I was randomly researching why trees are always green earlier today, so I happen to know that there are limits in the way photosynthesis works which limit how much energy a given leaf can absorb at once, going beyond that level does not provide much advantage in photosynthesis. using lenses to focus energy to one point would not significantly increase the energy that point is able to get from the sunlight, and the extra heat from that much energy would likely cause harm to the plant. Even ignoring the cost of growing 'lenses' it wouldn't be an adaptive approach. Still, it is a cool idea :) $\endgroup$ – dsollen Apr 22 '15 at 14:55
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The problem with how a creature could evolve to use light as a weapon is inherent to the issue presented - light does not make a very good weapon.

Even with all the technology that we currently have, light has several very important disadvantages:

  • poor efficiency of generating mechanisms when approaching damage-dealing energy levels
  • very poor energy transfer
  • is easily deflected or absorbed by even the most rudimentary armor (shiny or dark and matte, temperature-resistant material)
  • needs to be focused and directed or it dissipates
  • susceptible to environmental disturbance (dust, fog etc. blocks it)
  • telegraphs attacker's position when used

Thus in a fight light is usually used as a distraction, lure, stunning mechanism or for communication.

However, if we put all of these aside, how would that happen?

Light may be used to blind the enemy, which is an attack in its own right. It could be used as a defense mechanism, assuming the predator usually looks at his target, a sudden flash of light might blind or stun him. However, biological mechanism used to generate sufficient amount of light might be tricky to design. You could consider plants that have an amount of flash powder in a pod-like or hard shelled compartments used to discourage foraging animals.

A predator could be using external light sources, e.g. biological lenses to focus sun's rays on it's prey. While this would be a cool way to cook its meal in the process, it is rather complicated and unnecessary.

For a laser-like damaging light, forget it. Biological sources don't generate nearly enough energy to be converted to a laser-like beam. Also, coherent light emitted by lasers requires much better quality of optics than anything known to biology.

The question behind a question is in fact the issue with inefficiency and difficulty employing this mode of attack. Unless you would design your world using very strange, pro-light anti-claw characteristics, a good old rip & tear will trump shine & burn anytime. And since you ask about such weaponry as an effect of evolution, which assumes light would be somehow preferable as a weapon to other ones, it simply wouldn't happen.

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    $\begingroup$ Bio-lasers : nature.com/news/2011/110612/full/news.2011.365.html $\endgroup$ – Twelfth Apr 21 '15 at 17:48
  • $\begingroup$ Biological lenses to leverage external light sources is a cool idea. I like it $\endgroup$ – Ron Dahlgren Apr 21 '15 at 23:34
  • $\begingroup$ @Twelfth Nice link, very interesting. One problem though - they have only solved one requirement of laser light generation - a lasing medium. They still required external optical setup with mirrors and mentioned that the power of light generation is very weak - stronger than fluorescence, but still. Also, constructing a large beam out of many smaller ones is harder than one would think, also due to interference. Might be possible with optical crystals though. $\endgroup$ – eimyr Apr 22 '15 at 9:45
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    $\begingroup$ The problem I see is not so much that it isn't generally possible to find a lasing medium and such, but the fact that you would need very radical changes to an existing creature, which require several (possibly dozens) of generations. They are expensive but bear very little if any advantage in return. Evolution does not work that way. If there is no survival advantage, the mutation will not pass on. It's for the same reason that all bacteria on every normal person aren't antibiotic-resistant. It "costs" something to do that, and in absence of antibiotics, it's no survival advantage. $\endgroup$ – Damon Apr 22 '15 at 12:33
  • $\begingroup$ @Damin: That's actually slightly incorrect: if a mutation comes with a survival disadvantage, it becomes less likely to get passed on. If it doesn't change the fitness of an individual, then it's as likely to stay as to disappear. That's why most populations have thousands of latent mutations, passed over, that sometimes become valuable when the environment changes. E.g.: neanderthals had a different blood heamoglobin reaction to high altitude, which means nothing in most places - but got passed over to H.sapiens and preserved in Tibetan population, where it is very advantageous. $\endgroup$ – skolima Apr 22 '15 at 15:32
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The Reflecting-Oven-Jay

This is a small African predatory bird with a perfectly smooth set of wings with an area of ~100 cm2, so maybe the size of a pigeon. It hunts in large flocks, around 10,000 birds to a group (there are plenty of real birds that form flocks this size).

Using some basic multiplication, this flock can focus about 130 kW of sunlight. The flock decides to kill and eat an elephant, who has a metabolism of around 3000 Watts and already has to work to keep cool on a warm day (heat sink ears, spraying water on itself, etc). By spreading out and focusing sunlight on the elephant, the birds totally overwhelm the elephant's ability to cool itself, raising its body temperature by about a degree Celsius every 2 minutes (130 kW / 4 tons*specific heat of water), leading to slow heatstroke and death. Divided evenly, there's a lb of meat for every bird in the flock, a huge windfall, sustaining the group though any cloudy days until the next kill.

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    $\begingroup$ Very good: while lighting ships on fire is a myth, simply making an animal hotter, to overwhelm it when already near the end of its tolerance range), doesn't require a tight focus or a critical temperature. $\endgroup$ – JDługosz Apr 23 '15 at 15:44
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    $\begingroup$ It would be hard to "direct" the sunlight while in flight if its bouncing off their wings I would think. Considering the shape required for flight it would work best if they landed and used the underside of their wings. Imagines 10000 birds landing near an elephant and intentionally directing sunlight at it. $\endgroup$ – DoubleDouble Apr 23 '15 at 17:15
  • $\begingroup$ I considered having them land to attack, I'm not sure that the angles work but it certainly would be a sight to see! Perhaps they could hunt giraffe instead? :D $\endgroup$ – QuadmasterXLII Apr 24 '15 at 2:24
  • $\begingroup$ JDlugosz: I was inspired by how some bees kill invading wasps: swarm and then cuddle to death. $\endgroup$ – QuadmasterXLII Apr 24 '15 at 2:25
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This first part is sort of a wind up, skip to The Good Part for the good part.

As other posters have said before light isn't a very good weapon. But they haven't explained why that is. The reason it's not a very good weapon is because it's too diffuse.

Think of it this way, if you put a hose power (~750 watts) of power through a light bulb even if you opponent is say ~10 cm away from the bulb the energy flux hitting their body, even at its closest point is about a watt/cm^2 which would be pretty uncomfortable (it's about 4 times the intensity of sunlight), they would only end up absorbing only about 1/2 a horsepower over their entire body which is about 1/4 the intensity of sunlight.

Contrast this with spending one horsepower over one second to accelerate a human arm (~10kg) to ~28 miles per hour. Professional boxers punch at ~25 miles per hour and though they likely have the rest of their bodyweight behind that, they're not throwing one of those every second like we are here. Clearly kinetic attacks make more sense. Even if you could direct all that light into a 45 degree cone which would increase the power by about 8 times you're still not accomplishing much, especially when you realize, in order for your animal to generate that light, they have to be much closer to the light source than their opponent will be.

The only way to really turn light into a weapon with substantial advantages over kinetic bombardment is with coherent (laser) light. Coherent light stays pretty well collimated over reasonable terrestrial distances so long as it doesn't turn the air into plasma. Until very recently there have been no biological lasers. There's probably some sort of evolutionary biology reason why this is, mostly because it's useless unless you can generate a whole lot of light (which nothing has evolved to do anyway) and have a big enough brain to aim it at a target.

This doesn't address dazzler (bright lights which are supposed to make things blind) type weapons, in fact, now that I mention it, I'm surprised there haven't been any animals to use this. That probably has to do with the theoretical maximum efficiency of bioluminescence or something... I don't know.

The Good Part:

Anyway, none of this really answers your question, it's more of an argument for why light weapons never evolved in the first place, but let's forget the evolution part. Lets think about the traits of an animal which would use laser weapons. It would probably live somewhere with a lot open space, and good line of sight, improving this it could be a flying animal. Typically most high powered lasers we have today are chemical lasers so the animal might be a big gas bag or something. I would imagine only carnivores would have a high enough calorie intake to generate laser pulses.

Hmmm... a big flying carnivorous gas bag. Where have I seen that before... Ahh yes. Allow me to present the Gas Dynamic Laser Dragon.

A Gas Dynamic Laser (or GDL for short) is a laser which is powered (or pumped as they say in the business) by the rapid expansion of gas rather than electric discharge like most other gas burning lasers. A GDL can be pumped by the combustion of of some gas who's products are the lasing medium which is then excited to produce light which is bounced around inside a tuned cavity. The cavity is just a space with a 100% reflective mirror on one side and a less than 100% reflective window on the other. The beam comes out the window.

Our Gas Dynamic Laser Dragon (GDLD) would, like most dragons be capable of combusting gases within itself, but this gas would be channeled downward through supersonic expansion nozzles made of bone (or preferably titanium) into a cavity which the dragon would have some muscular control over the shape of. On one end it would have a mirror and the other a window. The materials these are made of depend on the frequency of light laser produces which depends on the gas used. After the gas moves through the cavity it would leave the dragon as exhaust. If we put this nozzle/cavity/exaust system into the dragons head/neck it would be easy to aim and the beam could leave from the dragons mouth. Its esophagus could be used as the cavity if the mirrors slide into place when they're needed.

I don't know enough about GDLs to choose a good gas, but the explosively pumped GDL laser gases listed on the wikipedia page list two bio chemicals which, though at least one of them is toxic to humans, likely could be created in the dragons gut by some enzyme.

Okay, okay, lets be reasonable here, an animal like this probably wouldn't be able to fly, but still, a giant laser breathing lizard is about the coolest thing which could fight with light.

For perhaps a more reasonable, though still pushing it, example there are the sunflowers from Larry Niven's Known Space. The flowers or leaves of the sunflowers were reflective and grew into a parabolic dishes which could reflect sunlight into a bulb of chlorophyll. The plants also had a way of detecting non sunflower life and could, when cooperating with many other sunflowers, focus their light on the offending plant or animal to kill it, providing nutrients for the sunflowers.

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  • $\begingroup$ I want one of those! Given a high gain medium it might even work without a resonator in a single pass mode. More like amplified spontaneous emission (ASE) or superluminescence. This would simplify the internal setup of the dragon (no adjustment of resonator necessary) but could still output quite some optical "punch". $\endgroup$ – Ghanima Apr 22 '15 at 8:43
  • $\begingroup$ The exhaust would be on the other end, not the head. It would evolve a probosis to confine and aim it. $\endgroup$ – JDługosz Apr 23 '15 at 15:52
  • $\begingroup$ @Ghanima, originally I was thinking an explosively pumped gas dynamic dragon, but I figured that might be pushing it. $\endgroup$ – alessandro Apr 25 '15 at 6:07
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Directly, I don't think there is much that light can do other than to blind a creature. Whilst I suppose that constitutes damage, I don't think it would be enough to directly hurt either a predator or prey. Blinding another creature would then leave it very vulnerable to attacking with teeth/claws in order to finish it off.

But it would require a lot of energy to produce such a significant amount of light, so I'm not sure whether it would be useful or not, in terms of the creature surviving or eating the blinded creature to restore the energy used for the light show.

It could always be indirectly damaged however. As indicated in the question here the light sources could be used in a way that would frighten predators/prey, putting them into a state of submission.

Extending tentacles or antennae with lights on the end and flaring them up in the darkness would make the creature appear to be bigger than it actually is, achieving this effect.

Once the animal that has been attacked is in a frightened state, it would likely be much easier to escape or attack them as they are disoriented.

An animal creating light could always make it flash very quickly in order to induce seizures or hypnosis in other animals, making them even more vulnerable to attack (like how a stoat dances to hypnotize a rabbit before it attacks) but it would need a very specific predator or prey to succumb to it.

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The obvious answer to this question would be to blind or to stun - not unlike the current worlds flashbang.

But let's take a different approach. Lets look at ants for example. What if the light could be focused into a point, and, if your creature were big enough, basically act as a giant magnifying glass and cook it's victims to death. Roasted prey.... tasty.

So how would it come to be? Imagine a world where the entire surface of the planet was unstable and completely random. Balance on the surface would possibly require more than one tail, to start off with. Perhaps, 9 for example since it's a nice number. Maybe it's a really sunny world, and there are lots of creatures that are adverse to light that hide in pockets of shade in the nooks and crannies. It all started out with your creature trying to get those little buggers out of the holes. Eventually, one of them mutated a lens in its tail. This lens could be used to redirect the sunlight into the shadows so it could get a better look at it's prey or whether it's there or not.

Eventually, it grew more lenses in order to see more places or get more light into the shadows.

Finally, it discovers that it could use all the lenses together (many magnifying glasses) on a single point in order to burn it's prey, or cook it alive if it were trapped. Voila! Light as a weapon!

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Blindness

A sudden, bright flash of light can cause temporary blindness. Some sources claim that only if there are Ultraviolet or Infrared rays could you possibly get permanently blinded, while others claim that a long enough duration of a bright enough light causes heating of the retina if the energy cannot be dispersed quickly enough, and therefore permanent damage.

However, perhaps on another planet where there is so little light that even a moderate light on Earth is "blinding"; light might be much more damaging to the other creature's eyes since they are not used to having to disperse so much light.


Long-term Aggression

Your creature could always be giving off a wavelength that causes radiation, which could damage the other creatures and plants in the vicinity, but I get the feeling you are looking for an immediate defense.


Immediate Physical Damage

Theoretically, something such as a laser or a series of incredibly strong pulses could "fry" the enemy, but the energy required to do so would be immense, probably much more than any single creature could generate. It is much easier for the heat from a reaction to harm something than to focus so much light that the heat generated from the light harms them.

It would make more sense if you could redirect an already extremely strong source of light, yet the magnifying glass vs. ant effect only works on a sunny day and ants are very small. It doesn't take a ton of energy to heat their low amount of mass, yet consider how big the lens is compared to them. On Earth, a creature that redirects light would be ineffective whenever there's a cloud, not to mention the animals that might eat it at night. There's also the problem of having to create whatever biological material would allow a creature to have a "lens" in order to redirect and focus the light - it would likely be expensive resource-wise for the creature.

That being said, maybe this creature lives on a planet which receives a ton of light due to many or close-by suns. Maybe any day-living creature has to develop a certain degree of reflective-ness(not a real word) just to survive the daylight. A large plant or creature provides shade and reflexively can withdraw that shade in certain areas to kill a threat or when it decides it is hungry. Maybe there is another certain creature which it provides shade too that is able to protect it at night, if such a world has a night.

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Instead of the organism generating light as a weapon, you might consider an environment filled with dangerously high levels of radiation (e.g. a planet close to its sun). In such an environment, plant life could evolve to live off of this radiation and absorb it, creating shade below in which more sensitive organisms could survive. These plants could defend themselves from the peskier of these shadow dwellers by developing mechanisms to selectively allow radiation through the canopy whenever the plant's roots/stem/etc. are touched or damaged, irradiating the stimulating creature below.

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There is a great example in an old "Blue Planet" BBC episode, "Into the Deep"

(http://www.dnatube.com/video/4767/Animals-in-The-Deep-Ocean-and--Bioluminescence-Part-2) [starts at 5:15].

Here, a shrimp with bioluminescent ink sprays it onto the face of an attacker. Not only does it confuse the attacker and allow the shrimp to escape, but it sticks to the attacker's face, lighting it up and making it a target its own predators. While the light doesn't do direct painful damage, it does create a sort of blinding, and certainly does direct damage to the safety of the attacked fish. Pretty sweet.

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Sharks with laser beams attached to their heads!

Really your options other than blinding light, would be either a laser beam of some kind and that takes a lot of juice to produce one with enough power to harm, or we have to move one to fire.

A short burst of light can be created with fairly small energy input, look at LED technology, if the animal can have some capacitors to store the energy to a quick release they could use it to blind and confuse a predator, kind of how zebra stripes work but much more distracting.

Predators with this ability would be pack hunters and would use it to scare and herd their prey, likely to an ambush.

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Killing a creature with light would be too hard, so how about blinding prey? Such a creature would blind then pounce. It would be fast, and it would be small, because it takes less light to disorient. It would gain the energy to make light by eating. Because of its style of attack and its method, it would rest for most of the day or more. The light would be created through short, intense flashes of bioluminescence.

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Light as a killing weapon is not going to work. Justification for my blanket statement: Do you know of any human hunter who uses a laser instead of a gun?

Light as a stunning weapon is already known to work. To wit (always wanted to say that): it is illegal to hunt deer at night in conjunction with a spotlight, because it's not sporting, since it freezes the deer and allows the hunter to pick them off at leisure with his rifle.

Even better (and more do-able, biologically) would be a flash-blinding weapon, used by night hunters. This would be especially effective if the terrain were inherently dangerous (e.g., cliffs, jagged rocks, plants with poisoned thorns). In that case the hunter only has to flash-blind the prey and scare it into running away blind. The prey is likely to kill itself, so the hunter didn't have to be fast/strong enough to kill the prey.

Seems like this has a chance of evolving, from a bioluminescent mating lure, to a prey lure, to a (sexually dimorphic) display of virility, to a flash-confuser, to the flash-blinder. The hunter could actually get less deadly as the light power increased, and its favored habitat might change.

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You Can think about the a large panel of mirrors or lenses with variable reflecting/refracting index that can focus a large amount of Light at one place on variable distances. This can cause a huge damage on any creature when exposed to that focused energy.


Here is the Idea. Imagine a creature who are having some part in there body filled with some transparent liquid and they can vary the concentration of that liquid and can also give a different shapes( More similar to lanses i.e. concave or convex).

Now Working Principle of Lense is as

"A lens produces its focusing effect because light travels more slowly in the lens than in the surrounding air, so that refraction, an abrupt bending, of a light beam occurs both where the beam enters the lens and where it emerges from the lens into the air."

It also depends upon the material used for making the lense. For example refractive index of the water is 1.33 and for glass it is 1.62.More will be the refractive index of the material it will more bend the light. In other words you can say it is the bending capability of light.

In optics the refractive index or index of refraction n of an optical medium is a dimensionless number that describes how light, or any other radiation, propagates through that medium.

The point of discussion of refractive index is directly related to the concentraion of liquid that the creature will have. As creature is able to do modification in shape and Concentration of the liquid that creature will be able to bend light according to the choice and by this he can also focus it at whatever distance he want.

He can focus the light on some other animal or something for doing harm or for killing it.

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  • $\begingroup$ Welcome to Worldbuilding SO. Your answer is a tad bit on the short side. Maybe you would like to elaborate a bit? $\endgroup$ – Burki Apr 22 '15 at 12:04
  • $\begingroup$ Hi I have elaborated the Logic. Please let me know if you have any ques. $\endgroup$ – Navneet Nandan Jha Apr 23 '15 at 10:29
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IIRC sunlight actually has been used as a weapon at least once. The antic warriors focused their copper shields on the mast of a galley and caused a fire.

In your imaginary world, you can give this light-reflecting weapon to either social insects or to some big animal that predates social insects. The latter would have to possess some king of a large mirror, that would be focused onto the insect nest. This is not really meaningless even on Earth: the bees escaping from a fire cannot bite because they load themselves with honey, as much as they can take.

Hunting moving animals with reflected sunlight would be a bit more tricky, but still possible.

How evolution could lead to this? A reptile with big ears (or wings) could use the reflected heat to soften or melt bee wax. After developing the mirrors, the animal could use them on different targets, e.g. blinding and later burning a mouse before eating it. Such evolution is possible, say, if the wax melting creature lives on a peninsula, then the peninsula becomes an island, big predators disappear, the creature becomes a predator itself, and then the island once more becomes a peninsula and the creature goes to the continent.

One more use of the reflected sunlight frying could be to neutralize the poison of some prey or plant, but such animal would be very much specialized on a single food source.

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As stated it's very hard to imagine this being viable, but the best option to me would be confusion and misdirection.

Imagine an area with unusual light patterns, possible in a dense forest where light filters through trees in unusual patterns, or in the ocean. A creature that could slightly light up various part of their body may be able to create odd light patterns that would confuse those looking at them. In effect they could make something like an active camouflage effect where constant shifting and flashing lights confuse a predator on the exact shape, distance, and movement of the creature it's chasing by making it hard to process the constantly changing light patterns.

Realistically the expense of producing these lights is high enough that it's hard to think of it as adaptive, but it's less implausible then many of the other options. The key would be to make an enviroment with limited light in unusual light patterns, and when which is pretty congested so that fleeing a predator was less about raw speed and more about being able to duck around foliage and through areas the predator has trouble perusing etc. Then when being chased you turn on the light show to make the predator have a harder time following your movements so he can't detect that your shifting your weight to dive for a whole in the brush until after you have done it and his inertia took you past him.

Keep in mind in this case I'm not talking about high light show, were talking about very small shifts in light pattern. Putting out light is expensive, the less light you need to use the better because it costs less. With this approach excessive light wouldn't help, it's intentionally making constant slight shifts in shade and lighting that works because it's subtle enough to make it feel like the changes could be due to odd light filtering down on the creature. The effect works through subtlety and randomness, not by overwhelming the opponent with expensive energy.

Still...I don't really believe it could evolve. I'm doing my best to give a valid answer but my real answer would be that no creature would evolve using light as a primary weapon.

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Others have put up some good ideas for how to generate light.
As eimyr pointed out, tooth and claw is going to be a lot more effective than shining a bright light on them, though using it to blind/stun isn't a bad idea.

Here's a possible why. If their primary predator was photo sensitive, then a light attack would do more damage than it would to you or I. This would be a bit like a vampire in the sunlight, or the underground monsters in Pitch Black.

There is a rare genetic condition in humans that makes them more sensitive to light. If the predator had a severe case then being able to make light would be useful.

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Well, consider "Tapetum lucidum" a alternative. http://en.wikipedia.org/wiki/Tapetum_lucidum Imagine a animal with very big eyes walking on the sun. It preys on big animals that does not get it like a menace because it is smaller. It get close and close to the point it is facing the big prey. So it manage to look to the face of the prey and... full open its eyes, making the prey blind by a flash of light. Just a moment and the predator cuts the prey's troath with it's talons. Maybe beasts like this one could had lived along dinosaurs.

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