Randall Munroe posted an interesting  in which he claims that you can't use moonlight to light a fire, because it is impossible for reflected light to raise the temperature of an object higher than that of the reflecting surface, which of course is obviously false.  I continued thinking about the problem, and read a couple of   on the topic, and I'm not asking (as those questions do) whether it is possible, as it clearly is, but how you could do it.  So, here is the scenario: You are in an open plane with no obstructions (trees, clouds, buildings etc) between you and the sky. The moon is full. You can magically suspend a lens of any size and shape anywhere between you and the moon (but any sunlight that hits it mysteriously vanishes - moonlight only!). You are attempting to set fire to a piece of paper.  How big would the lens need to be to achieve this?
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You can start fire by focusing the sunlight using the magnifying glass.   I searched the web whether you can do the same using moonlight. And found  and  - the first two in Google search results.  What I found is the thermodynamics argument: you cannot heat anything to a higher temperature using black body radiation than the black body itself, and Moon isn't hot enough.  It may be true, but my gut feelings protest... The larger your aperture is, the more light you collect, also you have better focus because the airy disk is smaller. So if you have a really huge lens with a really short focus (to keep Moon's picture small), or in the extreme case you build a Dyson-sphere around the Moon (letting a small hole to the let the sunlight enter), and focusing all reflected light into a point it should be more than enough to ingnite a piece of paper isn't it?   I'm confused. So can you start fires using the Moon?
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I need this for an animation. I tried using a smoke simulator, but the flames rise upwards and that's not what I was aiming for. I want the flames to maintain the vorticity but still stick close to the surface. Can anyone help me out here? Update: With the previous answer's help (along with a particle system) I was able to achieve this, however, i want the flames to have a more sharper look to it. Something more akin to the plasma bolt from 'Sorcerer's Apprentice'
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I know this sounds like (and probably is) a stupid question, but I can't figure it out.  As far as I know, the crescent shape of the moon is when the moon is on the sunny side of the Earth, but that wouldn't explain how it can be seen from the night.  Horrible, not-to-scale and probably completely wrong diagram of what I think :    How is it that we can see clear to the other side of the Earth?
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I wanted to know that if FIRE is Matter or Energy. I know that both are inter-convertible. but that doesn't mean there is interconversion taking place just like nuclear fusion or fission. If it is matter, then its has to occupy space and have mass. I really doubt that fire has mass. If it is energy, then i don't get it. Please enlighten me from my ignorance.
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In discussing , I came to realize that the real question was whether Darkness blocks vision or merely creates "darkness".    Per the wording of the spell:     Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration. The darkness spreads around corners. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. (emphasis mine)   Normal darkness is defined in the game as creating a heavily obscured area and the only description added to the darkness is that it is magical, which (to me) just means that it is created by magic and is subject to the magical rules.    Darkvision is defined in the game as basically being able to see in darkness as if they were seeing in dim light. So, the text "darkvision can’t see through this darkness" merely means that it affects darkvision in the same way it affects normal vision. (See earlier versions of the spell below, which had similar wordings.)    I see nothing there that implies it is a barrier to vision, just an active  and utter absence of illumination.    Previous editions have varying descriptions.  and  give accurate accountings of the various versions.   The 1e Wizard version seems to actually imply a sphere of opaque blackness that even blocks infra/ultravision, while the 1e Cleric version is a reversal of the Light spell and creates totally normal darkness, with no block to special visions.   The 2e version is similar to the 1e Cleric spell. It creates an area of darkness "equal to an unlit interior room".   The 3.0e version comes closest to the 5e version. It "causes an object   to radiate darkness out to a 20-foot radius. Not even creatures who can normally see in the dark (such as with darkvision) can see in an area shrouded in magical darkness." Still no reference to any sort of opacity and the wording makes it clear that darkvision is being treated specifically here.   3.5e Darkness is similar, although instead of darkness, it creates "shadowy illumination". This affects darkvision as well as normal vision.   4e doesn't have a Darkness spell, but rather a Cleric Utility called Veil of Darkness, which creates "a zone that is heavily obscured and blocks line of sight." So, finally, a reference to opacity, in "blocks line of sight", although I question if opacity was the intent.
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My friend asked me to create a photo like this:    I noticed that its shadowless and my first question is whether it was setup to be shadowless, or was it made shadowless in post? How many lights are involved here? I think there's one for the background, and one above the product pointing something like 30 degrees down, and then some reflectors to the side. Just guessing.   I did some Googling and came across . How could that be shadowless? Looks like the product is a few feet above the ground, which is a white seamless background, and the camera and all the lighting are all pointing down. How could there be no shadow on the ground?  Thanks!
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This week I've read two news stories about two different camera lenses that can magnify things that are very far away.  The first is the , which is a point-and-shoot camera that costs $600, has an 83x optical superzoom lens ("the same field of view as a 2,000mm lens on a full-frame camera"), and lets you see the craters of the moon.  The second is the , which weighs 36 pounds, costs $180,000, and can take portraits of people a half mile away.  A non-photographer might ask, "Why would somebody spend their life savings on a gigantic telescope of a lens when you could take far away pictures with a handheld camera for 1/300th the price?"  Well, I am basically a non-photographer, and I would like to ask that question. What does the big one give you that the little one doesn't provide? I understand that small changes in lenses can make a big difference in size and price, but this seems to be of a different scale entirely.   What is going on with the optics of the lenses that makes the small one able to be so small and the big one have to be so...big?   I'm trying to get a basic intuition for what makes lenses different, and I realize I don't understand enough about how lenses work to even know how to compare these apples and watermelons.
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Let me show a figure first about the shape of flame for a candle without any external electric field in the dark night:        Now consider the following physical configuration: If you put this lighted candle into the parallel plate capacitor, in which the electric field produced by it can be turned up or turn down arbitrarily.  So my question is: what's the shape of the flame of the candle when we keep the electric field stationary and how is it changed as we increased the field or decrease the field yielded by the parallel plate electrical capacitor?
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I have seen many folks doing this  concluding that shadow from point A on the Moon can travel to point B on the Moon faster than light.  What I fail to understand here (and I am sure I am wrong), is nothing can travel faster than light. I also read about Relativity where Einstein stated that every event happening for you depends on how fast the light gets from where the event is happening to you.  This is my particular problem with this Moon shadow experiment. You cannot make a shadow travel faster than light, because in order for the "disappearance" of light to get to the Moon, it has to travel at light speed.  Here is one way to understand my point. Consider we shine a flashlight on Moon, like the guy in the linked video did, we move the finger across the face of the flashlight, before we moved the finger it was casting the shadow at Point A on Moon, after we finish moving our finger, the shadow is at Point B on the Moon.  When the finger reaches the end of it's moving length, the light waves that have left the flashlight before the finger reached there will have to hit Point B and it will take them a second and a half to get there, and another second and half for us to see that shadow on point B, which makes it 3 seconds + finger moving time for us to actually see the shadow at point B. That means shadow traveled (say) across the diameter of the Moon in 3 seconds.   So. about 3000 Km in ~3 seconds make it 1000 Km per seconds which is very tiny compared to the speed of light. How can these experiments conclude that Shadow could be made to travel faster than light? What is the obvious clue that I am missing here?
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I have been recently wondering, if I take a powerful enough energy source (photon) and I have an perfect mirror exactly in front of it and assume an "emitter" shot the light towards the mirror.   As perfect mirrors absorb no energy of ANY kind from photons, should this mean that the perfect mirrors would never move due to transfer of momentum of the light?
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The question is the well-known      If there are so many/infinite stars, in every direction, why is it dark at night? Where goes all the light we don't see?    Discussing with a friend we found different answers.   The first (my friend's answer) is that given the fact that in a finite amount of time (in this case, the age of the universe), light can travel only a certain, finite, distance, we cannot see, if they actually exist, stars farther than such distance (about 13 billions of years, I guess). Then he states (or better, I got him notice he implicitly does this)  that within this radius there aren't enough stars to fill the night sky, and that's why we have darkness without sun's light.   The second answer (mine, or anyway the one I heard from a minutephysics YouTube video) is that despite of the big size of the observable universe, cosmological expansion red shifts most of the light out of the visible radiation range, so that the majority of light can't be seen by our eyes at night, while if we use, for example, an infrared camera, we should see a brighter sky, and lenghtening more and more the wavelength at which we observe the sky, we come to the uniform background radiation.  So my question is, what is the right answer? Is it one of the above-mentioned or even a third one?  EDIT: I've seen both answer may be acceptable, but what is the main contributing phenomenon?
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I spent some time studying about temperatures and color of objects. It turns out that as we heat something it turns to red, then yellowish white and if we heat it more it turns to bluish-white.  Like we can say a blue star is hotter than a red star. But why isn't it the same with flames?  Blue flame isn't always hotter than a red flame. It's just the chemistry of it all.   I mean I know the electrons jump from one orbit to another and gets into excited state when heated.  But the questions in my mind are:   If one object appears blue and another red, that doesn't always mean that the blue object is hotter than red. Is that correct? If yes then how is it so? I am sorry but I'm bit confused over it. With the "Color Temperature" concept on Wikipedia. They say 1,700 K to match a flame. But 15,000–27,000 K for a clear blue poleward sky.   This confuses me. The sky appears blue, does that mean its hot? But it isn't right? It's colder. I am not sure if I'm able to frame it properly. It's a bit confusing to me.  Is this something like a glowing object vs reflecting object? The sun is white but Earth is blue. But sun is hotter than the earth. It's the sun radiating light but earth is just scattering it.  But the surface temperature of sun is 5778K while the Wikipedia states our sky is about 15000K-27000... I know there's something I'm missing here. I'm hoping someone can tell me what is it.
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Why can't you see a reflected image on a piece of paper? Say you put a pen in front of the paper, even when light rays are coming from other sources, hitting the pen, reflecting back, and hitting the paper, there is no reflection.   What's wrong with the following "ray diagram" and why such even don't happen and the image of the pen don't form on the paper (right side is a paper)?      When then can you see the image of a torch when you shine it on the paper?   When you put a convex lens in front of the pen, why you can now see the image of the pen on the paper?
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I have read about a strange fact that fire has no shadow. I am wondering why.  Is it because of the fact that the constituent particles of fire have tremendous amount of speeds ?
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I recently saw this scene while on vacation:    What I would like is for the moon not to be completely blown out, but to also get reflections on the water.  If I expose for the reflections, the moon blows, which is to be expected, since the moon is in daylight.  If I try to create a composite shot combining an exposure to just the moon and this shot, I have problems with the halo around the moon;  the daylight exposure doesn't have it, but the nighttime one does, and that goes past my meager photoshop/lightroom skillset.  Is there a way to get this shot without postproduction, but entirely in-camera?
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I see pictures of the moon that looks like it is very close to the earth; I am wondering whether they are real or is there a trick to show the moon that big?  Here is an example:
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A  is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at about 6000K. The target will then heat up to 6000K and start to radiate as a blackbody, reaching thermal equilibrium with the sun.   The question is: is there any way to surpass this temperature, perhaps by filtering the light to make it look like a BB spectrum at higher temp, then concentrating it back on the target?