where light goes

[mab]

When light comes out of source and has traveled some distance and then if the source is shutout instantly the all become dark.

My question is “where light goes which had traveled some distance”?

* Does this light changes to heat energy?

* What about light traveling through space in that case would the light keep traveling even if the source is shutout?

*When the star dies sometimes its light reaches after many thousands years. Does this light reach the earth at the time when that particular star is completely dark?

[Severian]

When light comes out of source and has traveled some distance and then if the source is shutout instantly the all become dark.

My question is “where light goes which had traveled some distance”?

* Does this light changes to heat energy?

 

When it hits an object' date=' light will usually be absorbed by the material but some wavelengths will be reflected (which wavelengths are reflected determines what colour you see the object as). The reflected light will move off (again at the speed of light) until it hits something else, where it is absorbed or reflected. This goes on until the amount of light left is undetectable.

 

The reason that it goes dark 'instantly' when you turn off the light, is that light travels so fast that the time taken for the 'last' photons which leave the light bulb to bounce about the walls being absorbed is so small that you can't distinguish it.

 

* What about light traveling through space in that case would the light keep traveling even if the source is shutout?

*When the star dies sometimes its light reaches after many thousands years. Does this light reach the earth at the time when that particular star is completely dark?

 

Yes, in fact many of the stars you see at night are no longer there. They are so far away that it takes lights years and years to travel to us, and in the mean time the star may have died.

[mab]

It is surprising for me that light travels through space while the source (star) is finished or turned dark.

I always thought, “ Light is there, until the source of light is active”.

I have two more questions,

· Why in space “light is not absorbed or finished before reaching earth”?

· If we place a box with reflecting mirrors inside (with low capacity to absorb light ) and

Torch inside the box. Then place this box in space, will the light remain even after the torch is switched off.

And thanks for information you have given.

[swansont]

It is surprising for me that light travels through space while the source (star) is finished or turned dark.

I always thought' date=' “ Light is there, until the source of light is active”.

I have two more questions,

· Why in space “light is not absorbed or finished before reaching earth”?

· If we place a box with reflecting mirrors inside (with low capacity to absorb light ) and

Torch inside the box. Then place this box in space, will the light remain even after the torch is switched off.

And thanks for information you have given.[/quote']

 

Light isn't absorbed if there is nothing to absorb it. Space is a pretty decent vacuum. A little of the light will get absorbed or scattered - what we see is what's left over.

 

Mirrors don't have 100% reflectivity. Eventually the light gets absorbed. Since c is large, this will happen pretty quickly.

[Gilded]

Is it even physically possible for matter to have 100% reflectivity? What about 100% absorption (without heavy gravity like black holes or such)?

[5614]

yes, something like black holes absorb everything. a few months ago, there was a news article [in the newspaper] about how scientists had made the darkest black ever [on earth] but that black holes which absorb everything would probably be darker still.

[Gilded]

I think there was a black paint in Guinness World Records that absorbs light amazingly well, I think it was somewhere around 99.9%. It would be nice to paint a house with that stuff.

[Pangloss]

There used to be a great quandry in physics called "Olbers' Paradox". The general idea of it was that if space is infinitely big, and the number of stars in it is infinite, then why is the sky not permanently lit with uniformly bright, white light?

 

But of course since then we've discovered that space is not infinitely big, stars are grouped into galaxies (which greatly condensces the area of "coverage", obscuring stars behind other stars), and there aren't an infinite number of them either. There's also lots of matter out there that's not lit, but which still absorbs light when hit (so it can block stuff behind it from being seen). So in a practical sense, all we can actually see in the night sky are stars in our own galaxy and a handful of other galaxies just barely bright enough to see with the naked eye.

 

But I've always thought that the paradox is interesting, and it's a fun question to ask an astronomy novice and see if they can puzzle it out.

[Severian]

But of course since then we've discovered that space is not infinitely big' date=' stars are grouped into galaxies (which greatly condensces the area of "coverage", obscuring stars behind other stars), and there aren't an infinite number of them either. [/quote']

 

That's not the reason at all. Space is infinitely big in the sense that we could keep travelling in a straight line forever without coming to the 'edge' of space. The reason why we don't see stars in every direction is because the further the light has come, the longer ago it must have been emitted. Since the universe is 'only' 14 billion years old or so, the furthest away stars that we see light from can't be any further away than 14 billion light years (actually they must be a wee bit less than this because the stars took some time to form).

[Woxor]

When light comes out of source and has traveled some distance and then if the source is shutout instantly the all become dark.

My question is “where light goes which had traveled some distance”?

* Does this light changes to heat energy?

* What about light traveling through space in that case would the light keep traveling even if the source is shutout?

*When the star dies sometimes its light reaches after many thousands years. Does this light reach the earth at the time when that particular star is completely dark?

I think this is a point where it is helpful to think in terms of light consisting of particles rather than waves (though it is not necessarily one or the other). Light is radiation emitted by the star' date=' and one can imagine that this radiation comes in tiny packets (called "photons") which, in some respects, behave just like other particles. They are ejected from the star and travel away from it at the speed of light. When the star dies, it stops emitting the photons, but that doesn't change the fact that it already emitted a bunch of them already. So, the light that has already started doesn't "go out," it simply continues on its way.

 

The reason it appears to us on Earth that light "goes out" after the source is shut off is that light travels so fast that we can't tell the difference between a) the light source shutting off, and b) the "existing" light traveling to wherever it ends up.

 

It is surprising for me that light travels through space while the source (star) is finished or turned dark.

I always thought, “ Light is there, until the source of light is active”.

I have two more questions,

· Why in space “light is not absorbed or finished before reaching earth”?

· If we place a box with reflecting mirrors inside (with low capacity to absorb light ) and

Torch inside the box. Then place this box in space, will the light remain even after the torch is switched off.

And thanks for information you have given.

The density of space is something on the order of one molecule per cubic meter. To give you an idea how little that is, the density of air at sea level (which light passes through easily enough) is about 25,000,000,000,000,000,000,000,000 molecules per cubic meter. In order to keep the light from reaching us, it has to change directions or be absorbed. The former can be accomplished by forces like gravity and magnetism, but the most dramatic cause of both is collision with things like atoms. Since there are so few atoms in open space, light is free to pass through almost indefinitely.

 

As for your second question, yes, the light would remain even after the torch is switched off. If the mirrors were ideal (reflecting 100% of the light), the light would probably bounce around forever, although any attempt to "see" the light would probably have to involve absorbing it (like you do with your eyes). As has been said, though, the light would collide with the mirrors so many times (because c is so fast) that even a 99.99% reflecting mirror would soon absorb it all.

 

That's not the reason at all.

That's not entirely fair; the second two reasons given are very good. The stars are grouped into galaxies (and the light that comes from stars doesn't always make it around other objects, due both to actual collision and to gravity), making it very difficult to argue that they would light up the entire sky, even if the other problems were circumvented. Also, the fact that there are finitely many stars is something I would consider to be the main reason the entire sky is not glowing.

[Severian]

That's not entirely fair; the second two reasons given are very good. The stars are grouped into galaxies (and the light that comes from stars doesn't always make it around other objects' date=' due both to actual collision and to gravity), making it very difficult to argue that they would light up the [i']entire[/i] sky, even if the other problems were circumvented. Also, the fact that there are finitely many stars is something I would consider to be the main reason the entire sky is not glowing.

 

No. If the universe was infinitely old and infinitely large, then the sky would be lit up in every direction at night. But even with an infinite number of stars in the universe, the finite lifetime of the universe would stop this happening. The number of stars in the universe has nothing to do with it - it is the number of stars within your horizon which is important, and your horizon is fixed at 14 billion light years.

[Pangloss]

I could have phrased it a little better. I meant, of course, that "occupied" space is not infinite. Thanks for the correction.

[Woxor]

No. If the universe was infinitely old and infinitely large, then the sky would[/b'] be lit up in every direction at night. But even with an infinite number of stars in the universe, the finite lifetime of the universe would stop this happening. The number of stars in the universe has nothing to do with it - it is the number of stars within your horizon which is important, and your horizon is fixed at 14 billion light years.

You are right; I retract my statement that the finitude of stars is the main reason. I don't know why the finitude of light speed and history didn't occur to me as the best reason.

 

However, the argument (that there are finitely many stars) is still a good one in countering the original idea (that the sky should be extremely bright), for the simple reason that your first sentence (after "No.") is false. Age and time, no matter how infinite, do not produce radiation. If you want infinite light bombardment, you probably need an infinite source; in other words, a finite number of stars will not produce infinite radiation, no matter how much time and space you give them.

 

Similarly, the other argument (that stars are not evenly distributed across the sky, and that not all light radiated in our direction actually makes it here) is also a good counter.

 

And just to be mind-bogglingly picky: why say "at night"? Wouldn't the light from an infinite number of stars probably overcome the intensity of the sun?

[Edisonian]

That's not the reason at all. Space is[/b'] infinitely big in the sense that we could keep travelling in a straight line forever without coming to the 'edge' of space. The reason why we don't see stars in every direction is because the further the light has come, the longer ago it must have been emitted. Since the universe is 'only' 14 billion years old or so, the furthest away stars that we see light from can't be any further away than 14 billion light years (actually they must be a wee bit less than this because the stars took some time to form).

 

Yeah, but how do you really know that you can travel in a straight line forever? Kind of reminds me of when early scientists believed the world was flat. They laughed at the theory that it was round. How do we really know this? We have not done it. It seems more plausible that space envelopes or repeats itself; like the loop symbol we use to represent infinity.