The World's most unanswered science questions:
Light

Everything to do with light and colour is covered on this page. Before diving straight into the questions, here's a quick table showing the special symbols and what they mean.

This symbol means that the question is difficult to find out in practise. However, through lateral thinking and common sense, an answer is possible.	This symbol means that the question is nigh-on impossible to verify by experiment alone. However, through lateral thinking and common sense, an answer is possible.
This symbol means that the question is delving into the theoretical realm and is once again difficult to test. The answer/s are possibly right - but not guaranteed!	The ultimate! Questions with this symbol push the boundaries of theoretical knowledge - and are nigh on impossible to verify by experiment. Any answers are based on our current understanding of the universe - and thus are subject to error.

Anchors:

Mirrors inside mirrors

Switching off the sun

The deletion of the atmosphere and ozone layer

The non-contoured moon

The Eclipse

Noise from the eye

Unusual light filters

The Magnified Sun

Luminous Liquid Light

The Humble Light Bulb

Dazzling light and silhouettes

Can the blind see in dreams?

The Perfect Microscope

Effects of Speed of light at 1 MPS

Microscopes, light waves and perceived color

Multiple Focus Lens

Street Lamps, Sky and Auras

Soft Shadows and Light Sources

Colorless Gases and Liquids

Communication and Monitor Display Technology

Blue and Yellow really green??

Where is Magenta in the Colour Spectrum?

Reverse Colour and the After image

The Mirror Timebomb

Tasting the Impossible

Extreme Wavelengths of Light

Deadly Rays, Microwaves and Lasers

Light Waves and Heat

Tons of light and no heat?? Tons of heat and no light??

How many unique pictures could you ever see?

The poor green light on monitors

Mirrors inside mirrors

Q: When two mirrors are directly facing each other, they reflect off each other infinitely producing a mirror inside mirror effect. If this was graphically slowed down, would you see a kind of build-up effect where each mirror is "drawn in"? After all, light only travels at a finite speed. In other words, is it true that while you're still looking at the multiple reflections, millions more are constantly being made due to light travelling at only a finite speed?

Answer: If you could slow time down you would see the mirrors popping into apparent existence further and further out. And yes, more are constantly being "made" while you watch, but as light travels so fast, in the first 10th of a second it has already "drawn" all the mirrors out to 9,300 miles (18,600 miles traversed in 10th of a sec but you are only looking at half of that because you're looking from the middle of the 2 mirrors). [M.E]

Answer: If you don't have access to two giant mirrors, or you don't have ability to slow down time, then one way you can see this effect is to try pointing your webcam at the monitor screen, or video camera connected live to a TV. With any luck, you should see images appearing one by one on the screen. Sometimes the effect can be beautifully dazzling, like a kaleidoscope (at least with my video camera). [editor]

For a further breakdown of this phenomena and its cool side effects, visit the Light and Colour Trivia page.

Switching off the sun

Q: If the sun suddenly went from the sky (switched off), would...
A: The place outside looks as though the sun is still there (the sky itself looks like daytime), but no visible sun was actually in the sky.
B: Everything blacks out except the sun itself - which eventually also disappears.
C: Everything immediately blacks out.
D: Everything blacks out after a period of time.

Answer 1: D is correct: Everything would look and feel normal (lighted sky and sun visible, and also warmth) for 8.3 minutes, after which the light and the warmth would abruptly go out, without fades. Light and "warmth" are all EM waves, and have a finite speed, and travel in finite-length "wave packets". There's no reason why there should be a gradual fade out...[V.E]

Answer 2: The above answer is basically true, but there's more to the story - since the sun is more than just a 'point' of light...
In actual fact, there would be a very short fade (after the 8 minutes) due to the vast size of the sun!
How big is the sun? Well, it's approximately 1,390,000 km in diameter and since the speed of light is roughly 299,792.458 km/s, this means that you really would see the sky 'fade' to pitch black over a period of... 4.63 seconds!
After a couple of seconds, you'd be able to see a cross section of the sun's core, and during the last few fractions of a second, you'd actually see the back of the sun! [editor]

Further amendments:

In fact, other variables need to be taken into consideration before determining exactly what would happen. Is the entire sun burning, or just its surface area? Is the sun transparent to itself? Can light from the back of the sun 'travel' through the rest? Would the sun take on a 'polo mint' appearance after 2 seconds - thanks to the internal structure of the core?

Q: Also, thanks to the loss of heat, the Earth's atmosphere would liquefy - and then turn solid! How long would this take?

The deletion of the atmosphere and ozone layer

Q: If the atmosphere was taken away, you would see just the sun and the black sky. How much darker (brighter?) would Earth be and what would everything look like?

Answer: The lit parts would be much brighter (and hotter) but the dark parts would be much darker (and colder). The atmosphere diffuses the light out, spreading it around (it does the same with heat). [M.E]

Shouldn't the moon crescent look something like A or at least B...? Nope - it's actually mostly like C...

The non-contoured moon

Q: When there is a half moon, you are seeing a circle cut in half. Shouldn't there be a smooth black to white (silver) gradient rather than this instant "white-one-half black-the-other" effect?

Answer 1: I think it works like shadows: it's almost a "binary type" transition, with a hardly visible transition contour, due to refraction effects. [V.E]

Answer 2: Is this perhaps because the surface of the moon is so rough and pitted - so light is reflected in a random direction? [A.R]

Answer 3: As a point of interest, if you look closely with binoculars you can see that it is not as sharp as it first appears. [M.E]

The Eclipse

Q: Why is an eclipse more harmful to look at than just the sun?

Answer 1: It is hard to look at the bright sun because your brain is wired to prevent you damaging yourself, but the eclipsed sun is not so bright and it is easier to defeat your internal programming. Unfortunately the bright bits are still producing plenty of strong ultraviolet light and will damage the retina just as effectively. Another added problem is that as the sun is not as bright, your pupils open up to let more light in. [M.E]

Answer 2: The sun is always harmful to look at, especially during the hottest hours of the day, with no clouds or other obstacles between it and your eyes. Even looking at the sun behind clouds is still harmful enough. The eclipse phenomenon DOES block some of the sun's radiation, but unless it's a full eclipse (which can occur only in a very limited region of the globe for a limited amount of time), the sun still has its full-bright appearance (and high radiation power and luminosity, even at an 95% eclipse level).
In other words, the (now crescent shaped) sun looks as bright as usual, and is as harmful as usual to the eyes.
Then why do we hear all those warnings about not looking at the sun without protection "during eclipses"? Plain simple, because we are more PRONE to look at the sun during an (expected) eclipse, in the hopes of actually seeing a portion of the sun "put out"! Whole minutes against split seconds, and you realize that eye damage probabilities increase a lot during these phenomena! [V.E]

Answer 3: As an added point of interest, I /did/ have a quick few 'glimpses' (without special glasses or filters) when the sun was about 98% blocked out during the 2001 eclipse. Much to my surprise (and in confirmation of answer 2), I could barely see any difference - and it looked as though the sun was fully there! After another momentary glimpse, I found that if I squinted a bit - then I could just make out that the sun was mostly hidden behind the moon's view. [editor]

Noise from the eye

You've probably seen this before whether you realise it or not. It's actually the strange geometric noise you see if you shut your eyes very tight for a minute.

Q: If you close your eyes, there is a very faint but noticeable flickering 'random light mess'. Is this simply 'damage' that has accumulated over the years, or is it some kind of brain activity? Can people who are blind also see this?

Answer 1: I think it's low-level 'noise' - much like the background hiss from tape-recorders, etc. [A.R]

Answer 2: (Daring an) Answer: Some of this "random light mess" is surely quasi-permanent damage to the eyes, due to high intensity light sources or other such causes. Another potential source is probably brain activity, more properly NOISE SIGNAL (yes, human neurons aren't immune to "noise", after all - they're based on electrical activity!). Normally, the S/N ratio of the "human CCD" is very high, but like real ones, this ratio drops significantly with low luminosity (in fact, solid-state CCDs are known for suffering from high noise at low luminosities). In fact it's easier to see this mess when closing your eyes or simply when it's too dark. [V.E]

Answer 3: I'm not entirely sure if people who are blind can see this, but I have heard that there are some blind people who don't see a thing - not even the colour 'black'... (kinda hard to imagine I know...) [editor]

Q: Also, if you keep your eyes shut very tightly for about a minute or so, various geometric patterns start appearing and 'evolving' (see picture). How does this happen?

Answer:

Unusual light filters

Q: Is there a kind of light filtering device which filters out the entire range of 'invisible' light (infra-red and ultraviolet) whilst retaining the full visible spectrum? Also, same question, but filtering out only the full visible light instead.

Answer:

Would one see A or B ?

The Magnified Sun

Q: Why is looking through a telescope at the sun more harmful than usual? Isn't it just enlarging the size of the sun - not the brightness? See Diagram for clearer explanation of question.

Caution: Never ever look at the sun through binoculars or a telescope, as it can easily blind someone permanently for even an instant of viewing.

Answer:

Q: Is there a limit to how powerful a single telescope can get - given a certain size?
Is the size of telescope proportional to its zooming accuracy?

Answer:

Luminous Liquid Light

Q: There are certain types of liquids which produce their own light independently, or when mixed:
Can all the primary colors be obtained from these liquids or just white light?

Answer: As far as I know - only a pale greenish blue light can be produced by these, but I could be wrong. [M.E]

Q B: If this liquid light was compressed, then would the light become stronger e.g. 5x compression = 5x light intensity.

Answer:

Unlike gasses, most liquids are almost completely incompressible. If you put the liquid under very high pressure, you might speed up the reaction, but you might stop the reaction altogether by changing the chemistry. The best way to find out is to do the experiment and see, but you would be unlikely to get much extra light from the liquid. [M.E]

QC: If the liquid was heated to become liquid-light vapour, would the air around become luminous?

Answer: In this case the chemicals would almost certainly break down. If you wanted to get a luminous mist, your best bet would be to spray it as tiny droplets like an aerosol spray, but I don't think it would stay luminous for long that way... however, again the only way to find out would be to do the experiment and see. [M.E]

Q D: If the liquid was frozen, what effects would this have on the light?

Answer: I would guess that it would stop because the chemical reaction wouldn't be able to proceed while the component parts are frozen in place instead of mixing freely... but again, the only way to know for sure would be to try it. [M.E]

Q E: What does the liquid taste like? Is it toxic?

Answer:

Q F: How long does the light last for?

Answer: About an hour I think... [M.E]

Q G: How (if?) can you 'recharge' the light?

Answer: Only by adding more of the two liquids to react together... unless you find some other material that acts differently. Lots of things produce light under various circumstances. Do you have any sugar cubes? Mint flavored Lifesavers will do. Go into a dark room and wait for your eyes to dark-adapt. Get close to a mirror, pop a Lifesaver or sugar cube in your mouth and crunch down on it. It is called triboluminescence. Get a couple of quartz pebbles (ones smoothed by river erosion are best). In the dark as before, watch the stones carefully as you scratch one across the other's surface. Next time you are at a party where someone has an ultraviolet light, look at people's teeth, they will appear unnaturally white. That is fluorescence. Fluorescent tubes are called that because the mercury vapor inside them emits ultraviolet light. It radiates out and hits the powder coating the inside of the glass, which absorbs the ultraviolet and re-emits lower energy light in the visible spectrum. (Incidentally if you ever get to see the powder on the inside of a broken tube, be careful - because I think it belongs to a class of very toxic compounds called PCBs that once absorbed, the body has great difficulty getting rid of. PCBs can cause terrible problems with your joints and make you very sick over a long time.) [M.E]

Answer 2 (to all liquid light questions)
"Chemical light" usually gives out greenish or bluish light, like most natural fluorescence phenomena... but I don't know how these substances' properties change with compression/expansion (liquids can't be compressed that easily, though (almost impossible)). I think that there would be a maximum in the emission with a particular CONCENTRATION value of the reagents, and decreasing for non-optimum values. Other possible parameters would be external temperature and light: In fact phosphorescent substances are "suppressed" by strong external lighting, and "recharge", by absorbing radiation instead of emitting it, and vice-versa in the dark.
Temperature...hmm, depends on the substance. Most chemical reactions accelerate with higher temperatures, while there are some which may also take advantage of low external temperatures!
Also, these liquids are usually toxic - and last for approximately 6-8 hours. You may have seen these "emergency lights", which are a plastic/rubber bar of transparent material with a yellowish/greenish substance inside - and is somehow stiff. To "activate" it, you must bend the bar, making some internal diaphragm break, the substances reagents mix and they start glowing for about 8 hours. Can also be used underwater!
Unfortunately, the chemical reactions involved are usually irreversible, and thus you can't recharge them (much like a match that has burned all the way - unlike phosphorescent objects, which however last much less even if fully "charged" and glow much weaker). If the reagents are perfectly mixed, you can't even stop the reaction, once started.
Not only that, but the substances involved aren't eternal: in fact, those bars have an "expiration date" on them (usually 2-3 years). [V.E]

The Humble Light Bulb

Q: Light bulbs have not changed much since Edison first introduced them many years ago. They all still have one thing in common though - they run out sooner or later. If they exist, how much more expensive are the infinitely lasting light bulbs? If car lights and LEDs can last so long, then surely they can do the same for the household bulbs?

Answer 1: You can now get light bulbs lasting up to 20 times longer than normal household bulbs. But I think they're duller than normal - and obviously more expensive. [editor]

Answer 2: Car light bulbs: They are mostly incandescent/halogen, working at 12 V, but they are not eternal. They may be a little more "rugged" than household bulbs, but come think of it, you surely use them much less than a household bulb. It's just more difficult to see one of them burned (it's easier to see a broken one, by the way, or change the car before you even burn a single one) yet it does happen, sometimes. [V.E]

Q: The light coming from a coloured light bulb is (mostly) white and therefore requires a coloured glass bulb filter to show any colour. Can coloured light come from a transparent glass bulb? Also, what wavelengths of light do light bulbs emit? Is it 'coincidentally' just the RGB waves that are needed to make white light perhaps?

Answer: I'm not sure about non-visible light, but I do know that most common household light bulbs emit intensities closer to the red end of the spectrum than the blue. This is confirmed with a simple experiment: compare the monitor's white to the reflected shade of a sheet of paper. Incredibly, it turns out that the average 100-watt bulb consists of about 49% red, 33% green and 18% blue - giving a very 'creamy' colour. [editor]
For a further breakdown of how poor light bulbs are, visit the

Light and Colour Trivia page.

Q: Whilst we're on the subject of light bulbs, how many would it take to get the brightness that the sun gives out? 10,000 watt perhaps? Rooms would look great this bright :)

Answer: Incredible - there is an answer to that :-)
On a winter day, the average illumination level for exteriors is 10,000 lux (1 lux=1 lumen/m^2). So if you want to light a 5*5 room with 3 meter high walls, so that the total surface is 2*5*5 (ceiling, floor) + 4*5*3 (walls) =50+60=110 m^2. You would need a 1,100,000 lumen light flux, which is definitively overkill! About 734 standard 100W light bulbs (at 1500 lumen each) would be required, with a 73,400 W total power consumption!
Too bad that more than 65,000 W would be pure radiation heat, so it would be pure suicide to sit in such a closed space with such a big heat source (imagine, a centralized heating for a house or even a small residential building is a mere 20 KW!).
With more efficient lamps, such as compact fluorescent, you would need about 917 21W lamps, with 1200 lumens each. The power consumption would lower to 'just' 19,250 Watt, and of those, just 9625 (50%) would become heat.
It's still far from ideal but a lot better already. When designing heating/cooling plants for interiors, the presence of numerous and powerful light sources is taken into account, as well as any kind of power-intensive machinery, and if possible incandescent light is replaced with fluorescent, especially with REALLY big places, such as auditoriums, university halls, warehouses etc.

But, attention, I mentioned a WINTER day, at 12:00 !
On a summer day, at 12:00, the illumination level is of 100,000 lux!
Tenfold the luminosity, tenfold the power and the heat!
Now, sure for a closed space you don't need an equally luminous ceiling and some walls will have windows which need no illumination etc. and so you can lower on those figures, but you understand that even the best household (and industrial use) illumination shrieks in front of the power and efficiency of the sun, and you will always have to compromise.
Even if you take out the ceiling's and half the walls' surface in the previous example, it's still too unpractical, at least with the incandescent solution, while the fluorescent one seems feasible already, at least for an important application such as a nuclear lab or something, but not for a normal household.
In some applications it's possible to see daylight like illumination, for example sports fields - but it's an open air application, and you need to illuminate "just the pitch", not the crowd or the sky. Yet, we're talking about Megawatt-class halogen or arc projectors. [V.E]

Dazzling light and silhouettes

Q: When you are dazzled by a bright light source and what you see becomes a dark 'silhouette', is that what you are 'meant' to see - or is it some kind of fault with the eye or brain?

Answer: It works like a digital camera with automatic gain control; the impulses from the camera are scaled so that the largest value represents white. When a very bright light is present, the impulses from objects with normal illumination are insignificant in comparison to the 'white-level' so appear to be silhouetted. [A.R]

This is what I suspected. Strictly speaking, the dark 'silhouette' is actually the same color - it's just an optical illusion with the eye that makes it look darker. [editor]

Can the blind see in dreams?

Q: Can people who are blind at birth see sights in their dreams?

Answer 1: No, though they still hear in their dreams. They have no experience in what 'seeing something' would feel like, so they have nothing to relate to when dreaming either.
If someone goes blind after the age of 6 though, then they're still able to see in their dreams for many years afterwards.

There is a twist to this story though. Around 75-80% of people who are blind from birth have been able to see during a 'Near Death Experience'. It's truly the only time they've been able to 'see' something, so there could even be a paranormal connection. See here for more information regarding the

NDE phenomenon. [editor]

The Perfect Microscope

Q: How close is technology to this kind of microscope:
First stage: Monitor display. Real-time zoom in or out operation with 4-way real-time scrolling. The speed of scroll would be proportional to zoom level.
Second stage: Same as above but with picture displayed in 3 dimensions with auto-focus and near to infinite zoom potential. On top of ordinary zooming, there would also be an option to /enlarge/ the screen's picture. Other options such as moving and rotating the object and/or lens could be added. Perhaps a variable focus setting enabling everything or a defined distance (or anything in between) to be displayed.

Answer:

Effects of Speed of light at 1 MPS