Does the speed of light change, through a telescope?

[Iamme]

If you 'bring the object closer to you', in a flash....(say you are observing the moon...then look through it with 200 power scope)...what happens to the light you are observing?

[geni]

Nothing happens to the speed of the light (ok before I get hammered by a physist yes I know it will slow down while going through the lenes of a telescope). All a telescope does is collect photons over a larger area than your eye does then project this image into you eye. To put it another way ( not true but a useful lie) the telescope takes the image and streches it so it apears bigger.

[Iamme]

So....you are instantly 'stretching' light? And this doesn't affect the speed?...by 'stretching' it?

[geni]

Originally posted by Iamme
So....you are instantly 'stretching' light? And this doesn't affect the speed?...by 'stretching' it?
You note I said this was a useful lie. In no way is the light being streached.

[Elaborate]

Geni was correct in his explanation. A telesope gathers light from a larger area and focuses it into your eye, effectively making the object appear larger and more detailed. This is why larger-lensed telescopes can see farther than smaller ones.

[epepke]

Originally posted by geni
(ok before I get hammered by a physist yes I know it will slow down while going through the lenes of a telescope).

:hit: Actually, it doesn't; it takes more time to get through the glass, but the speed doesn't change.

[BillyJoe]

LINK (http://www.what-is-the-speed-of-light.com/refractive-index.html)

speed of light in a vacuum is 299 792 458 m/s
......speed of light in glass is 199 861 638 m/s

[DickK]

I was confused by this so I found this link: http://www.madsci.org/posts/archives/may98/893732585.Ph.r.html

...which explained how light propagates at C between atoms in the glass, but the time taken for the photons to be absorbed and then emitted by the atoms accounts for the increase in travel time within the medium.

PS: BillyJoe, thanks for the link, the site has an interesting page on the increasingly successful efforts to measure the speed of light from Galileo to the 1980s. Wow, those guys were inventive.

[Iamme]

I wonder if this speed differential through glass thingy can explain my post about the hologram/refraction effect I was having when I would look through layers of glass at the print underneath it. (Remember that thread of mine?)

I'nm still cornfused about the telescope light thing. I guess I can't get into my head about even how telescopes work. It is something most of us take for granted. I get a mental block with certain things, and this is one of those things. Just like, if somebody tells me they have an aunt whose sister is married to their boyfriends uncle (stuff like that)...I get lost.

[Ziggurat]

Originally posted by Iamme

I'm still cornfused about the telescope light thing. I guess I can't get into my head about even how telescopes work. It is something most of us take for granted. I get a mental block with certain things, and this is one of those things. Just like, if somebody tells me they have an aunt whose sister is married to their boyfriends uncle (stuff like that)...I get lost.

If you're having trouble figuring out how a telescope works, I would first suggest ignoring lens-based telescopes and consider mirror telescopes (all the big ones now are mirror telescopes anyways). Then you don't have to worry about index of refraction or any of that stuff, it becomes a purely geometric problem. Once you feel like you have a handle on the geometry for reflective optics, then you can go back and worry about refractive optics.

[epepke]

Originally posted by DickK
I was confused by this so I found this link: http://www.madsci.org/posts/archives/may98/893732585.Ph.r.html

...which explained how light propagates at C between atoms in the glass, but the time taken for the photons to be absorbed and then emitted by the atoms accounts for the increase in travel time within the medium.

That's one of the ways of looking at it. There is an interaction between photons and the electron shells of the atoms, and it takes some time. This is the easiest way to explain it if someone is not versed in QM.

However, it's a quantum interaction which does not change the state of the atom. (If it did, it would be like fluorescent paint.) A seemingly completely different way of looking at it (which is actually identical at a deep level) is in terms of Quantum Electrodynamics. In that way, you have to calculate the maximal probability path of the photon going through the glass based on the interactions of its amplitudes and the amplitudes of the electrons on the atoms. And when you do, you find out that the maximum probability path through glass isn't straight; it's wiggly. If you calculate using the extra distance from the wiggles with the speed of light, it's exactly the same time that light takes to get through some glass.

[epepke]

Originally posted by Iamme
I'm still cornfused about the telescope light thing. I guess I can't get into my head about even how telescopes work. It is something most of us take for granted. I get a mental block with certain things, and this is one of those things. Just like, if somebody tells me they have an aunt whose sister is married to their boyfriends uncle (stuff like that)...I get lost.

I have an idea about what might be confusing you and could be the source of your confusion and the thing about "stretching out". I don't know if my idea right, but I'll try an explanation based on it.

Our eyes make images based on the angle of light hitting them. Something nearby that looks big has light coming from all sorts of angles. That's why it looks big. The light from something far away, like the moon, is almost parallel. The angles are much narrower, so it looks small.

A telescope (and a microscope) bends the light so that small differences in angle are exaggerated, resulting in big differences. This fools the eye into the perception that it's much closer (which is why it looks closer) and also uses much more of the retina, which is why the eye can see more details.

[ceptimus]

Lamme seems to be following in the footsteps of Newton. :)

Sir Isaac Newton, amongst other things, invented the reflecting telescope, and discovered the interference phenomenon known as, "Newton's rings", which occurs when a glass lens is placed on another lens, or rested on a flat surface.

Another of his experiments was to insert a bodkin (a large blunt needle) down the side of his eye, between his eyeball and eye socket. He then wiggled the bodkin about to, 'see what would happen'.

[BillyJoe]

Originally posted by ceptimus
Lamme seems to be following in the footsteps of Newton. :)

[One of his] experiments was to insert a bodkin (a large blunt needle) down the side of his eye, between his eyeball and eye socket. He then wiggled the bodkin about to, 'see what would happen'. If Iamme blinds himself attempting this one, I'm holding you personally responsible. :D

BillyJoe.

[ceptimus]

Originally posted by BillyJoe
If Iamme blinds himself attempting this one, I'm holding you personally responsible. :D

BillyJoe. No BillyJoe. I never suggested that lamme should try this one. I think, if there is a blinding now, you will be at least as culpable as I! :D

By the way, although lamme's username does start with the letter between H and J in the alphabet, the font that vbulletin uses makes spelling lamme with a lower case L look more like the version over the 'avatar'. Sort of like the words 'Me' and 'Lame' joined together.

[BillyJoe]

Ceptimus,

Originally posted by ceptimus
No BillyJoe. I never suggested that lamme should try this one. I think, if there is a blinding now, you will be at least as culpable as I! :D Oh s#!t!

Originally posted by ceptimus
By the way, although lamme's username does start with the letter between H and J in the alphabet, the font that vbulletin uses makes spelling lamme with a lower case L look more like the version over the 'avatar'. Sort of like the words 'Me' and 'Lame' joined together. I thought it meant "I am me" (yeah, look at his sig: "I am because I am" :D )

BillyJoe :cool:

[kookbreaker]

Originally posted by ceptimus

Another of his experiments was to insert a bodkin (a large blunt needle) down the side of his eye, between his eyeball and eye socket. He then wiggled the bodkin about to, 'see what would happen'.

Was this while the Plague of London was raging outside his home?

[ceptimus]

Didn't he nip away to the countryside during the plague? Smart fellow, our Newton.

[Charlie in Dayton]

The speed of light is a constant. With a telescope, what you're increasing is the amount of light reaching the optical sensors in your eye.

An adult pupil in the dark will be maybe from 5mm - 7mm across. For purposes of argument, let's say it's 5mm (call it just under 2/10"). Now, you're looking thru a 130mm aperture telescope (like mine). 130mm is 5.1". All the light that 5.1" mirror will reflect is gathered into a small bundle that will shine thru the eyepiece of the scope into your eyeball thru that 5mm pupil. You're shoving 20.43 square inches of gathered light thru an opening that unassisted would gather 3/4 square inches of light (this is an odd way to express it, but the analogy holds). So, while the light doesn't go any faster, there is more of it.

Clear as mud?

[Jon_in_london]

Let me ask this:

The light from the object we are looking at: suppose I look at it through a telescope or very thick pice of glass with one eye and just use the other eye naked, like.

Wouldnt the naked eye see the object before the one looking through the glass?

??

edit- just thought of the answer, thanks.

[BillHoyt]

Originally posted by Jon_in_london
Let me ask this:

The light from the object we are looking at: suppose I look at it through a telescope or very thick pice of glass with one eye and just use the other eye naked, like.

Wouldnt the naked eye see the object before the one looking through the glass?

??

edit- just thought of the answer, thanks.

BillyJoe already addressed this:

"speed of light in a vacuum is 299 792 458 m/s
......speed of light in glass is 199 861 638 m/s"

[pupdog]

Or read text through a rhomb of clear calcite. If oriented properly, you get good "double refraction" of the image.

Or how about moving light through atomic "tunnels" in photonic crystals (is that the right term)?

If I want to see a bigger moon, I just look when it's lower on the horizon--don't need a telescope (I won't bring up other options for seeing a bigger moon...).

[Wolverine]

Originally posted by Iamme
I'm still cornfused about the telescope light thing. I guess I can't get into my head about even how telescopes work.

Maybe this (http://www.howstuffworks.com/telescope.htm) might help.

[Mercutio]

Originally posted by pupdog
If I want to see a bigger moon, I just look when it's lower on the horizon--don't need a telescope (I won't bring up other options for seeing a bigger moon...). As you no doubt know, the cool thing about this is that the visual angle of the moon and its corresponding image on your retina is the same, whether the moon is on the horizon or at its zenith. It is only our perception of it that changes...and this cannot be explained by reflection/refraction, etc.

[sorgoth]

Originally posted by epepke


That's one of the ways of looking at it. There is an interaction between photons and the electron shells of the atoms, and it takes some time. This is the easiest way to explain it if someone is not versed in QM.

However, it's a quantum interaction which does not change the state of the atom. (If it did, it would be like fluorescent paint.) A seemingly completely different way of looking at it (which is actually identical at a deep level) is in terms of Quantum Electrodynamics. In that way, you have to calculate the maximal probability path of the photon going through the glass based on the interactions of its amplitudes and the amplitudes of the electrons on the atoms. And when you do, you find out that the maximum probability path through glass isn't straight; it's wiggly. If you calculate using the extra distance from the wiggles with the speed of light, it's exactly the same time that light takes to get through some glass.


Thank you! This explains it very clearly for me. The actual SPEED of light doesn't change, but the path it takes is longer!

[ceptimus]

You can calculate where the path light takes through a lens is by just working out the quickest paths available for light, bearing in mind the reduced speed through the glass.

In one of his lectures, Richard Feynman gave the analogy of a person running across a beach, and then swimming to save a drowning person. If the person drowning is at an oblique angle to the waterline, it is quicker to not take a straight-line path to the drowner. Because you can run faster than you can swim, the quickest path is to get closer to the drowner, while still on the beach. If you do the math and work out the fastest path, allowing for the reduction of speed in the water, you will arrive at the laws of refraction.

[BillyJoe]

Originally posted by Mercutio
As you no doubt know, the cool thing about this is that the visual angle of the moon and its corresponding image on your retina is the same, whether the moon is on the horizon or at its zenith. It is only our perception of it that changes...and this cannot be explained by reflection/refraction, etc. What does explain it?
I have seen several explanations about how this illusion occurs but I have never seen an explanation the makes sense.

[Mercutio]

Originally posted by BillyJoe
What does explain it?
I have seen several explanations about how this illusion occurs but I have never seen an explanation the makes sense. Good question...I can't answer it fully.

It looks bigger on the horizon because we (quite rightly) are able to see it in a position relative to objects that we do have experience with--trees, buildings, mountains, etc. It looks absolutely huge, because we are able to see that it is so incredibly far away compared to these nearer objects.

Overhead, it does not look as large because there is nothing in the foreground to throw it into its proper perspective. With the same visual angle, but without the proper perception of distance, the image of the moon is perceived as smaller (just as a detailed model car that is close and a real car that is far can have the same visual angle and yet be seen as different sizes due to their perceived distance from your eye).

Why is it seen (or assumed to be seen) as closer when overhead? This I cannot answer. Perhaps because we just don't have experience (cannot have experience) with distances of this size. We default to something smaller, something closer to the distances we can actually handle. But I don't know for certain.

If this is true, the true illusion is not that it looks bigger on the horizon, but that it looks properly huge on the horizon, and too small overhead (the actual illusion).

[Cecil]

Here's a good explanation. http://www.grand-illusions.com/moon.htm

The explanation is believed to be as follows. We 'know' that a cloud that is overhead will be larger than when it moves towards the horizon. And an airplane that is a mere speck on the horizon becomes large when it is overhead. And we are all familiar with standing under a tree which seems enormous, yet at a couple of hundred paces seems insignificant. It would seem that so much of our world is interpreted this way that we are ill-equipped to cope with an object like the moon, that subtends the same angle at the eye, whatever position it occupies in the sky. And so our brain 'interprets' the image that it 'sees', and tells us that the moon is larger than it really is.

Also, note that the illusion disappears if you turn around and look at the moon upside down through your legs. (Are you mooning the moon? :D)

[BillyJoe]

Mercutio,

It looks bigger on the horizon because we (quite rightly) are able to see it in a position relative to objects that we do have experience with--trees, buildings, mountains, etc.

That bit makes sense.

It looks absolutely huge, because we are able to see that it is so incredibly far away compared to these nearer objects.

But this bit doesn't.
If an object looks far away, it would look smaller.
(A car in the far distance looks smaller than one closer in)

Overhead, it does not look as large because there is nothing in the foreground to throw it into its proper perspective. With the same visual angle, but without the proper perception of distance, the image of the moon is perceived as smaller

This bit makes sense

Why is it seen (or assumed to be seen) as closer when overhead?

Well this is the part that always confuses me.
For me, the horizon moon looks closer that the zenith moon. It is sort of circular to say so, but the moon looks larger because it looks closer and it looks closer because it looks larger. But at least it is consistent. To say the horizon moon looks larger and further away doesn't make sense to me.

If this is true, the true illusion is not that it looks bigger on the horizon, but that it looks properly huge on the horizon, and too small overhead (the actual illusion).

No, I'm pretty sure it's the horizon moon that provides the illusion.
The size we perceive the zenith moon to be is consistent with the size of the image on the retina but, with the horizon moon, the perceived size is larger than we would expect form the size of the image on the retina.

regards,
BillyJoe

[BillyJoe]

Cecil,

If a cloud on the horizon looks smaller than the same cloud overhead and, if a plane looks smaller in the distance where it is closer to the horizon and larger when it is closer and overhead, why is it the reverse for the moon?
And why does the horizon moon look closer than the zenith moon?

Originally posted by Cecil
Also, note that the illusion disappears if you turn around and look at the moon upside down through your legs. Yes it does and also when viewed through a cylinder to remove surrounding objects. So the presence of surrounding objects must be a big factor in the explanation for the horizon moon being larger. You can actually see it change size as you drive along in a car depending on the presence or absence of surrounding objects.

regards,
BillyJoe