I wanted to know the usefulness and practicality of carrying sunlight through fiber optic (I know there are commercial products which help getting sunlight from roof to dark corners of homes and there is extensive network of fiber optic around the globe, so it is not completely unheard of.) so that at least the costal areas of countries get sunlight through the night which is transmitted from another country in sunlight and saves energy consumption for, say road lights and other things over the years. For example the costal India can get sunlight transmitted from african countries. Here is the day-night map link for illustration http://www.timeanddate.com/worldclock/sunearth.html?iso=20160812T1632


4 Answers 4


To me it seems almost impossible at least with optic fiber.

First things you need to know about optic fiber is how it works and how it is meant to work. Signals are transmitted with optic fiber with lasers, which are focused light ray. Why aren't we using other kinds of ray ? Because the light of, let's say a lightbulb would just spread out too much, and can't transmit that much in a fiber.

I'll explain that with a short metaphor.

Well imagine you're in a cave with a very powerful torch. You can light up everything in front of you up to a very long distance. In fact, the only thing that really stopping you is a wall, so you can see everything in that big hallway you're in.

But, that hallway has a corner at one point, leading to another hallway on the right. And that hallway on the right lead to another hallway further on its left and it goes on like that.

You leave your torch on the ground, and walk to that junction. You see that the first hallway on the right is lighten too due to your torch. Indeed, a part of the light coming from your torch just bounce on the wall, and spread to the hallway.

Then you move on to the next junction and take the hallway on the left. You will notice it will be far darker than the previous one, which itself was darker than the main hallway you came from.

Each time the light of your torch bounce on the walls, a part of the light is not reflected in the good direction, and will be somehow lost. Now imagine your hallway is thousands of kilometers long. Also bear in mind that only the rays of light directly parallel to the hallway will go through it without bouncing before a junction. And now keep in mind you can't just draw a straight line from Africa to India. So you'll have a lot of junction with many different hallways, and your optic fiber will just be the same.

With a laser that problem is not the same, you know where you're sending your laser, and you can predict where it'll go since it's focused.Also less light will be lost with distance. In your case you're proposing to send sunlight through optic fibers to another country, thousands of kilometers away.

Definitely that's not something that could be achieved with optic fiber. On the other hand a system of mirrors on the ground and in the air could achieve that, in the same manner that the moon reflects a part of the sun light to the earth. Nonetheless, even if the moonlight can be strong enough to have quite a good visibility by night, it won't probably be enough to save that much energy.

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    $\begingroup$ The metaphor is highly misleading. Fiber optic cables can make curves just fine and the light actually bounces against the outside of the fibers almost as if it were a mirror. It's true that without a laser you're not going to get a coherent signal at long distances, but the question is not about data, only light. $\endgroup$
    – Cyrus
    Aug 12, 2016 at 12:04
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    $\begingroup$ Most fiber optic systems use LEDs now, only the long distance ones use lasers. If you put a big focusing mirror on top of a tower, you will have a focused beam stronger than a laser, even if you lose 90% that's aligned enough. It might melt the fibers though. I agree that it would be impossible in practice, but not for the reasons you state. $\endgroup$
    – Cyrus
    Aug 12, 2016 at 12:09
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    $\begingroup$ And ones seemingly have no right answer, which almost seem like attempts at crowdsourcing major plot points in people’s writing (and I thought there was a beta site with a similar theme for writing that which got closed for going against SE’s common goals). This seems like the former, and it seems like someone is getting punished for not providing a “Hard Science” quality answer in a site full of questions with no correct answer at all. $\endgroup$ Aug 12, 2016 at 18:12
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    $\begingroup$ It's one thing to have an answer that's opinion-based but factually accurate. It's another to have one that's provably false. $\endgroup$
    – Azuaron
    Aug 12, 2016 at 20:18
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    $\begingroup$ A Wikipedia article you might find useful: en.wikipedia.org/wiki/Total_internal_reflection $\endgroup$
    – Mark
    Aug 12, 2016 at 21:30


  • Optical fibers cost between 100 USD to 1 000 USD per kilometer. Multiply that with the distance and the number of fibers you need and you end up with a very large number.

  • One optical fible cannot carry more than a few Watt of light before it burns up. You need millions of optical fibers for your task.

  • The reason the fiber burns up is because it is not entirely transparent to the light; the light is absorbed by the fiber. You lose at least 50% per 10 kilometers. At 100 km you have lost 99.9% of the light.

The short version: it simply does not work. There will be no light coming out the other end.


For the budget spent on all that fiber optic, you could also launch a number of big mirrors into space that can light up areas at night. This has been tested by Russia, though the project was discontinued due to the cost.

Compared to digging trenches for fiber optics across most of the world (and maintaining them!) it may be a cheaper and easier option.


I tried to correct @Kaël's metaphor but it does not fit into comment margins.

Instead of a cave and a torch, imagine a corridor with inside fully covered with mirrors - and a person who can "bounce" of those mirrors as if he was a portion of light (as a matter of fact, we are looking from portion of light's perspective).

This person needs to reach corridor's other end, the exit.

Well, corridor is not straight - it is bent several times, so person can only see reflection of the exit. Many reflections, actually. The person can pick any of those and go straight for it - he'll eventually get there. The closer his path to straight line - the less time will it take.

However, there are complications.

First of all, walking is not simple. There is always some stray banana peels and alike to trip over and break your neck - or just lose your optimally-chosen path and resort to picking a new one which is likely to be longer.

This accounts for absorption. The longer our fellow's path is, the more he is likely to encounter something like this. And yes, zig-zagging for 100 km corresponds to a longer path than beelining for 100 m, even through the same corridor.

Then there's scattering. On a closer inspection, turns out that mirrors are not exactly perfect and our fellow can break his neck while bouncing from one of those too. Actually, it's more likely to happen if he's bouncing with a steeper angle and usually he's bouncing with very gentle angles.

In fact, we only see inside of the corridor as covered in mirrors precisely because it does not have steep enough angles where walls would not act as mirrors. Engineers who laid out the cable took care of that.

So, we need a messenger to deliver something to the other side.

We survey the path and choose breed of messengers that deals best with mirrors and banana peels of that particular path (we choose light's wavelength and maybe some other parameters too). Then we make a facility that trains messengers of that particular breed to act as one - and release a group of those guys in our corridor into way that looks good enough.

Actually, we'll weld the facility to corridor's entrance so messengers get spewed out in proper direction automatically.

This shall minimize losses and ensure that big enough group of messengers reaches other side.

Alternatively, we can use a facility that does not train them to act as one but produces significant enough quantity cheaply enough - that works too, for passing signals. That works too, up to a certain extent: when things get too crowded, other mechanics come into play and our carefully-engineered system stops working.

However, you propose passing energy instead of signals.

This will fail because you'll need orders of magnitude thicker cables so enormous armies of those fellows can be let in without it getting too crowded inside. That'll cost a lot.

After that, losses would come into play. Sure, you'l be able to detect if it's day or night on the other side, but you shouldn't expect significant power output on the other side if you're laying cables that are hours-of-earth-rotation long.

What you want can be achieved though - collect sunlight, transform into electricity and transfer that instead.


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