Light: visible or invisible?

[studiot]

33 minutes ago, Eise said:

Ok, this is what John said:

What you see1 are two different things:

- a street lamp with a blueish content, by means of reflecting light

- a street lamp with a bright yellow content, by means of emitting light

But of course you see2 blue light in the first case and yellow light in the second case.

I think you still haven't appreciated the full import of John's question.

 

The title and OP of this thread and the thesis of the suspended member is that you cannot see light itself.

In particular furyan asserted that you can only see the source.

 

John's point is: If a blue source is emitting yellow light how can you see the yellow light itself if you can't see light?

[geordief]

So we detect and interpret properties of light (frequency)  ?

"Light"  itself is the old "what is reality" question?

[Eise]

23 minutes ago, studiot said:

John's point is: If a blue source is emitting yellow light how can you see the yellow light itself if you can't see light?

 I do not see a problem. Seeing1 is by means of reflecting light by an object, or by emitting light by an object. Sodium vapour is blue when it reflects light, and it is yellow when it emits light. In both cases you see1 a glass tube. Of course, you must see2 ('detect') the light to see1 the street lamp.

But this is of course all very artificial...

[studiot]

4 minutes ago, Eise said:

 I do not see a problem. Seeing1 is by means of reflecting light by an object, or by emitting light by an object. Sodium vapour is blue when it reflects light, and it is yellow when it emits light. In both cases you see1 a glass tube. Of course, you must see2 ('detect') the light to see1 the street lamp.

But this is of course all very artificial...

Just because you can't see it does not mean there is no problem.

 

If the OP premise that light itself cannot be seen is true,

then how can you see the yellow light?

How do you know there is yellow light and that it is yellow, if you can't see it?

 

Of course if you think the OP premise is untrue then that's fine, but what makes your version any better than any one else's?

[Strange]

Another example just occurred to me: we can see stars because their light is sufficiently bright, even though they are smaller than the resolving power of the human eye. So it is hard to argue that we can see the star (if it was a dot on the page, we wouldn't be able to see it) we can only see the light.

[StringJunky]

5 minutes ago, Strange said:

Another example just occurred to me: we can see stars because their light is sufficiently bright, even though they are smaller than the resolving power of the human eye. So it is hard to argue that we can see the star (if it was a dot on the page, we wouldn't be able to see it) we can only see the light.

Yes, the smallest 'object' we can see is one excited rod, which equates with a pixel.

[Strange]

49 minutes ago, StringJunky said:

one excited rod

Ooo-er, missus.

[swansont]

9 hours ago, Eise said:

 Furyan5 furiously pleads that the second meaning of 'seeing' does not count as seeing. If it would be an important philosophical topic (it is definitely not a physics topic!), philosophers would split up the two concepts of seeing with the help of two different words: one can 'see1' macro objects, and one can 'see2' light. And then try to solve the real problem behind it. In this case however, I do not see any real problem. We all agree more or less on the facts of seeing.

But it is a nice intellectual exercise: try to understand the opponent, find different concepts that can lurk behind the same word, and so try to light up the intellectual problem, see the root cause of the seemingly different views on the facts. In this case it really is nothing more than an intellectual exercise. ('Light' metaphors used intentionally...)

IOW, it's semantics, as several of us have said.

Put another way is we don't see an image of light, as we see images of objects. But not everything we see is an object, so that can't be the entirety of the definition.

[geordief]

8 minutes ago, swansont said:

IOW, it's semantics, as several of us have said.

Put another way is we don't see an image of light, as we see images of objects. But not everything we see is an object, so that can't be the entirety of the definition.

Can light cast a shadow?If a powerful laser passes between another light source and the  observer is a shadow cast?

[dimreepr]

11 minutes ago, geordief said:

Can light cast a shadow?

Yes...

[Strange]

11 minutes ago, dimreepr said:

Yes...

I think the answer is no. Unless it is within some weird non-linear medium. 

[Tub]

1 hour ago, Strange said:

Ooo-er, missus.

 

54 minutes ago, Strange said:

I think the answer is no. Unless it is within some weird non-linear medium. 

Shouldn't you have said " Nay, nay and thrice nay " ?

[geordief]

57 minutes ago, Strange said:

I think the answer is no. Unless it is within some weird non-linear medium. 

Would my laser act as a gravitational source  and so at least distort signals passing through to it or in its vicinity?

 

A "shadow" of sorts. (gravitational lensing)

[swansont]

2 hours ago, geordief said:

Can light cast a shadow?If a powerful laser passes between another light source and the  observer is a shadow cast?

Light generally does not interact with other light in free space; it's only something that's been seen with very high-energy photons, and I don't know if you'd call it casting a shadow.

edit: as Strange notes, in some non-linear media all bets are off, as the photons interact mediated by the medium.

[John Cuthber]

12 hours ago, Eise said:

That is simple: you see the colour of the apple change. Not of the light itself.

I checked with Captain Kirk. The colour of the apple didn't change; it still matches Scotty's shirt..

So that's obviously not what I saw change.

However, in passing through space, from a source to a moving observer, the wavelength of the light changed. It's not just a  visual perception thing. Any instrument that measures wavelengths and intensities will confirm- the light isn't red any more.

The property of the light has changed, as shown my any available means- a spectrometer or an eye (or anything else- you can, in principle, shine a really bright light on the apple so that a lot of light is reflected off it. When that light reaches me I can use it to illuminate a scene and that scene will look and behave exactly the same way as if I used any other source of green light. . 

I can detect that change with my eyes.

That's because I have seen the light.

 

6 hours ago, Eise said:

What you see1 are two different things:

- a street lamp with a blueish content, by means of reflecting light

- a street lamp with a bright yellow content, by means of emitting light

The trouble is that they are not two things; they are one thing which is- in your explanation- simultaneously blue and yellow.

I'm not sure that helps.

 

At best all this shows is that whether or not you can see light is a matter of what you mean by "see".

However in a very abstract system where relativistic Dopper shifts happen you can only describe the light that reaches you- you don't have any way to know how that light relates to the object that emitted or reflected it.

So the only thing that you can form a cognitive model of is the light itself.

Whatever version of "see" you use; you can see light.

[MigL]

What happens when that apple that you 'know' to be red ( Captain Kirk said so ), is blue shifted into the ultraviolet, or the infrared ?
Can you still see the light, John ?

Photons are still getting to you are they not ?
But they are carrying information about the apple in another frame.

Similarly, a gas arc can glow in the infrared or ultraviolet, can you still see the light ?
Or are the photons just carrying information about temperature and composition ?

 

 

 

[Eise]

14 hours ago, studiot said:

Just because you can't see it does not mean there is no problem.

Oops, that would be see3... 

14 hours ago, studiot said:

If the OP premise that light itself cannot be seen is true,

then how can you see the yellow light?

You don't see1 yellow light. You see1 a bright yellow tube. I think that if you want to have a clear discussion, you must distinguish between seeing1 and seeing2 (forget about seeing3...). As long as you mixup the two, we cannot have a clear discussion.

15 hours ago, studiot said:

How do you know there is yellow light and that it is yellow, if you can't see it?

I see1 the yellow tube, because I see2 its yellow light.

15 hours ago, studiot said:

Of course if you think the OP premise is untrue then that's fine, but what makes your version any better than any one else's?

That it accounts for the difference how we see macro objects (that do not need to enter the eye to see them: we see them by the light they reflect or emit), and how we see light (because it enters the eye). Do you see this difference, and do you see how I use the words 'seeing1' and 'seeing2' to account for this difference?

13 hours ago, swansont said:

IOW, it's semantics, as several of us have said.

Put another way is we don't see an image of light, as we see images of objects. But not everything we see is an object, so that can't be the entirety of the definition.

Exactly. But if you want to distinguish between these 2 ways of seeing, then one make such a (artificial, I fully agree) distinction. As it solves no real physical or philosophical problem, it is of course an empty intellectual exercise.

10 hours ago, John Cuthber said:

I checked with Captain Kirk. The colour of the apple didn't change; it still matches Scotty's shirt..

Yes. Kirk sees both still red. But you see1 Scotty's shirt and the apple both blue. Really, tell me what you see. And I am sure: you see a blue shirt and a blue apple.

10 hours ago, John Cuthber said:

The trouble is that they are not two things; they are one thing which is- in your explanation- simultaneously blue and yellow.

But you see them thanks to two different processes. But both fall under the definition of see1. When the lamp is turned on, you just see2 no reflected light anymore, you see2 yellow light. So you see1 a bright yellow lamp.

Don't you see that seeing light and seeing macro objects are not the same? 

10 hours ago, John Cuthber said:

At best all this shows is that whether or not you can see light is a matter of what you mean by "see".

Yep. That's the whole point. 

10 hours ago, John Cuthber said:

Whatever version of "see" you use; you can see light.

But that is wrong again, and inconsistent with the sentence I quoted from you above.

'Seeing' in daily life includes both meanings. And then of course it is at least very funny when somebody reduces the meaning to seeing1, and therefore concludes that light is invisible. And then furiously arguing that seeing1 is the only correct meaning of seeing.

I hoped that distinguishing between both meanings would clarify the discussion, but obviously that did not work... 

So once again: do you see3 the difference I make between seeing1 and seeing2? 

And do you see3 that in daily life we do not make this difference, and so we can see things, but also light.

[John Cuthber]

Speed the ship up a bit and the light from the apple and the shirt turns blue.

There isn't a blue apple in the universe- so that's obviously not what I am seeing.

What I see is light from the apple which has been made blue.

The light changes, not the apple.

What I see changes.

So what I see must be the light.rather than the apple.

Incidentally, I forgot my glasses.
What I "see" is an image of the imperfections in my eye's lens; regardless of the source.

Edited February 2, 2018 by John Cuthber

[Eise]

1 hour ago, John Cuthber said:

There isn't a blue apple in the universe- so that's obviously not what I am seeing.

Wrong. You see1 a blue apple. Only observers in the same inertial frame see1 the apple red. There is no objective reason to say that some inertial frame is preferred above another.

So once again: do you see3 the difference I make between seeing1 and seeing2? 

And do you see3 that there is a difference between seeing light and seeing objects by means of light?

 

Edited February 2, 2018 by Eise

[Tub]

12 hours ago, John Cuthber said:

 

 

 

Whatever version of "see" you use; you can see light.

Would it help to clear away the semantics roadblock if we used " sense " instead of " see "?  So we can " sense " visible light, but we can't " sense " invisible light.

[Strange]

30 minutes ago, Tub said:

Would it help to clear away the semantics roadblock if we used " sense " instead of " see "?  So we can " sense " visible light, but we can't " sense " invisible light.

That seems to open a whole new can of worms.  For example, we can sense infrared but can’t see it. And what, exactly, does “sense” mean?

[Eise]

31 minutes ago, Tub said:

Would it help to clear away the semantics roadblock if we used " sense " instead of " see "?  So we can " sense " visible light, but we can't " sense " invisible light.

One could do that. Your 'sensing' however would be the same as my seeing2. But if you do not introduce a different concept for 'seeing1' the confusion will continue. One should exclude the meaning of 'sensing' from 'seeing'. To clear this up I am using 'seeing1'.

Your use of the concept 'semantics roadblock' is interesting. If one wants to try confusions as illustrated in this thread, one should be prepared to introduce new concepts for words, making them more precise, and sometimes introduce new ones. But obviously the resistance against such semantic moves is very strong. A semantic roadblock, as you say.

19 minutes ago, Strange said:

That seems to open a whole new can of worms.  For example, we can sense infrared but can’t see it. And what, exactly, does “sense” mean?

Yep. Therefore I introduced non-existing words: see1 and see2.

[Tub]

1 hour ago, Tub said:

Would it help to clear away the semantics roadblock if we used " sense " instead of " see "?  So we can " sense " visible light, but we can't " sense " invisible light.

 

34 minutes ago, Strange said:

That seems to open a whole new can of worms.  For example, we can sense infrared but can’t see it. And what, exactly, does “sense” mean?

 

15 minutes ago, Eise said:

One could do that. Your 'sensing' however would be the same as my seeing2. But if you do not introduce a different concept for 'seeing1' the confusion will continue. One should exclude the meaning of 'sensing' from 'seeing'. To clear this up I am using 'seeing1'.

Your use of the concept 'semantics roadblock' is interesting. If one wants to try confusions as illustrated in this thread, one should be prepared to introduce new concepts for words, making them more precise, and sometimes introduce new ones. But obviously the resistance against such semantic moves is very strong. A semantic roadblock, as you say.

Yep. Therefore I introduced non-existing words: see1 and see2.

Thank you, Strange; thank you Eise. That was my best shot - should i say " Shot in the dark "?  Perhaps not! ( It's difficult to avoid puns in this  particular thread ). There are so many words to use here: seeing, looking, watching , sensing, sight, vision, perception, so i'm sorry if i've opened-up a whole new can of " words ". 

If it's not off-topic too much, i've noticed that, while watching a TV programme that's not too interesting, i sometimes drift-off into some train of thought, or some reverie ,and then, when i eventually  "come back to my senses ", i notice that the programme has finished; so my eyes have been gathering the EM signals from the TV but i have.'t been " seeing " them.    Oh dear.

[studiot]

3 hours ago, Eise said:

A)Oops, that would be see3... 

B)You don't see1 yellow light. You see1 a bright yellow tube. I think that if you want to have a clear discussion, you must distinguish between seeing1 and seeing2 (forget about seeing3...). As long as you mixup the two, we cannot have a clear discussion.

I see1 the yellow tube, because I see2 its yellow light.

That it accounts for the difference how we see macro objects (that do not need to enter the eye to see them: we see them by the light they reflect or emit), and how we see light (because it enters the eye). Do you see this difference, and do you see how I use the words 'seeing1' and 'seeing2' to account for this difference?

C)Exactly. But if you want to distinguish between these 2 ways of seeing, then one make such a (artificial, I fully agree) distinction. As it solves no real physical or philosophical problem, it is of course an empty intellectual exercise.

 

 

I really don't understand how you can refute what I have been saying consistently throughout this thread which is in short:

 

It's complicated.

 

Whilst at the same time agreeing with Swansont ( C "Exactly") and introducing see1, see2 and now see3 (A).

 

Quote

Light: visible or invisible?

 

I have a question about light. We all know that we see objects because they reflect light into our eyes. But we never see the actual light. So my question is why can't we see light. Or can we in fact see light. If so, how?

Let me remind you of the OP.

 

I have maintained consistently that the word see has multiple meanings and that the OP has not introduced sufficiently clarity or limitation.

 

That's your (A) and (C),

As to your (B)

I do not see the tube because I added the context that I am not looking at it, but I can still see the light when standing under the streetlamp and see that it is yellow.

That would be two of your different 'sees'.

 

As far as I can make out (see) the process of seeing depends upon a chain of events and the wrord see is used to indicate both the process reaching  stage partway through the chain and the stage at the very end.
The process is further complicated by the fact that additional factors may be introduced at several of the stages along the way, including between the stage of formation of a retinal image and the final stage.

 

Throughout this thread I have attempted to draw out the complications arising from both the multiple inputs and the multiple meanings.

Your see1 etc was a valiant effort to try to bring some order to this jumbled up situation.

 

But the bottom line is that unless the person using the word somehow indicates what's in and what's out, confusion will reign.

 

[geordief]

3 hours ago, Strange said:

That seems to open a whole new can of worms.  For example, we can sense infrared but can’t see it. And what, exactly, does “sense” mean?

With "sense"  ,I feel  one is collating and processing all available sensory    inputs (including remembered inputs )

 

Edited February 2, 2018 by geordief