[evan_au]

Quite clearly the wavelength's of 400nm-700nm are not observed in ''empty space'' unless we see a rainbow.

There is one star which is bright enough to fully activate our cone cells - the Sun.
This light travels to us through 150 million km of empty space.

But you should never look directly at the Sun - it is so bright that it will burn out our Cone and Rod cells.

It is safe to use a "pin-hole camera" to project an image of the Sun onto white cardboard.

You will notice that the Sun is not rainbow-colored. It is a single color, which could be (rather technically) described as "black-body radiation with a color temperature around 5700K".

Other stars have quite different black-body temperatures, resulting in the wide range of colors shown in the star-trails.

If you project the Sun's light through a prism (or raindrops), you will see it split up into its component colors. But this is a property of a dispersive medium.

Empty space is not a dispersive medium, so we see the visible light (400-700nm) from the Sun and other stars travelling through empty space without forming a rainbow. 

[guest39538]

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Bang my head against a brick wall, this is worse than talking to a brick wall, the answers you provide anyone can search on google, you are avoiding my queries, I am not making a new theory Colin so stop trying to force discipline on me that I am querying.

My question is being sidetracked with again stereotypical ''waffle''.


To all the readers of this thread, please join in and tell these people that they do not see spectral content in ''empty'' space.

Evan tut tut, for some reason you are trying to justify your sidetrack by mentioning stars and the spectral content of stars, you then perceive that spectral content travels from a star to your eye and you see this spectral content as spectral content passing through space, if this were true then you would not see the star because the space would not be crystal clear it would be coloured, I give up, you win and  keep your lies .

i see my television, i see my monitor, i see myself , I can see a rainbow, but I do not see coloured air.

Empty space is not a dispersive medium, so we see the visible light (400-700nm) from the Sun and other stars travelling through empty space without forming a rainbow.

True empty space offers no resistance, but

''so we see the visible light (400-700nm) from the Sun and other stars travelling through empty space without forming a rainbow. ''

Contradictory, see and without forming a rainbow, how do you see it if it does not form a rainbow?

I am sure you mean detect it?

[guest39538]

Something cannot be both opaque and transparent.

Quite clearly you are not a scientist Colin and an amateur scientists whom became a moderator, you do not know what you are talking about, opaque, even to your definition is transparent, you have not considered x-rays or radio waves that pass through opaque things.
You are giving out false information in the main section like Evan did earlier with the explanation of light which in respect he corrected.

Please correct your error for the public who are learning.

[evan_au]

you have not considered x-rays or radio waves that pass through opaque things.

When we talk about an "opaque" or "transparent" medium, we always use this term with respect to some radiation.

If the form of radiation is not stated, we would assume visible light - and most of this discussion has been focused on visible light (400-700 nm).

When you consider neutrinos, almost anything is transparent to them (short of a neutron star or black hole).

Something cannot be both opaque and transparent.

Opaque vs transparent is a rather binary distinction. Real matter is more nuanced:

• Empty Space is perfectly transparent to all wavelengths of light that we are likely to find on Earth.
  
• Clear Air is very transparent to visible wavelengths of light (including red and green laser pointers).
  
• Pure water is fairly transparent - if there is no algae or pollutants, you can see quite a few meters deep.
  
• Glass is somewhat transparent to visible light - but if you look from the side, the very dark green color shows that it strongly attenuates visible light over just 1 meter, and it attenuates some wavelengths more than others (giving the green hue).
  
• Both water and glass can exhibit total internal reflection at their boundaries, and partial reflection in other cases.
  
• Both water and glass are dispersive media, and can produce rainbows. 
  
• Finely divided water (eg fog) and glass (crushed glass powder) refract the light in so many directions that it no longer forms an image. Such a material is called translucent. 
  
• A clay-brick wall is opaque to visible wavelengths, but somewhat attenuates VHF radio waves and X-Rays (ie it is partially transparent and partially opaque at these wavelengths)
  

But this discussion has mostly been about empty space and air, and these are both transparent to visible light, not opaque.
So the lesson here is that transparent air is not opaque.

how do you see it if it does not form a rainbow?

Have you had your eyes lasered?
This does form a grating, and if the gratings were fine enough, would form a rainbow around every luminous object.

But most people don't have a diffraction grating burnt into their eyeballs, and they do see most things without seeing a rainbow effect around them.

I am sure you mean detect it?

I have a digital camera, and a digital sensor for my telescope. We speak about both of them detecting light.

But when I am talking about my eyes, I talk about seeing the light directly (even if I am looking through the eyepiece of my telescope).

[guest39538]

But this discussion has mostly been about empty space and air, and these are both transparent to visible light, not opaque.
So the lesson here is that transparent air is not opaque.

Air is transparent and so  is space, however what is the air if not ''opaque'', is it when L=0 and it is dark? 

The air in the dark certainty looks opaque to me in the dark?

But when I am talking about my eyes, I talk about seeing the light directly (even if I am looking through the eyepiece of my telescope).

ok, so  look through your telescope and observe the moon, you can see the moon and see the spectral content of the moon, between your eyes and the moon what do you see?

I see ''empty'' space and no spectral content in that space.

[evan_au]

I see ''empty'' space and no spectral content in that space.

This is where the contrast between transparent and opaque becomes significant.

In general:
- Something that is transparent does not absorb nor emit light (at that wavelength). You can see light from objects beyond it.
- Something that is opaque both absorbs and emits light (at that wavelength). You can't see light from objects beyond it.

So both empty space and clear air are transparent (to visible light)
- They do not absorb nor emit light (at visible wavelengths).
- You can see the wall beyond the clear air
- You can see the glow of the laser pointer beyond the clear air
- The clear air does not emit any wavelengths (spectral content) in addition to those emitted by the laser pointer
- You can see that the wall is dark outside the dot of the laser pointer
- The  wall can "see" (be illuminated by) the laser pointer in your hand, beyond the clear air.
- You can see the Moon beyond the empty space
- The empty space does not emit any wavelengths (spectral content) in addition to those emitted by the Moon
- You can see the Moon and the Wall with no extra rainbow colors.

how do you see [a star] if it does not form a rainbow?

If you look at a rainbow, you will see bands of color. These are "pure" colors (Red, Orange, yellow, etc); you will see 6 of them (if you live in the USA) or 7 (if you live in the UK).

If you have a look at the star trails, you will see streaks of many different colors. But these streaks of different color don't come from a single star; each trail is a single color and comes from one star. And none of these streaks are the "pure" colors of the rainbow; these are all mixtures of pure colors, with the exact mixture determined by the temperature of the star.

So stars come in many colors, but they don't come in rainbow colors.

Something cannot be both opaque and transparent.

There is an exception to this usual rule in the materials used in lasers - these are able to sustain a temporary "population inversion" of the electrons, where the electrons stay at a high energy state for an unusually long time (for an electron - it still seems short to humans).
- When the electrons are in the ground state, it is opaque (to the "pump" wavelengths). Any light in this wavelength range will be absorbed, boosting the electrons into the excited energy state.
- When the electrons have all been boosted into the excited state, there are no electrons left in the ground state, and so the material becomes more transparent to the pump wavelengths.
- Wait a long time, and all the electrons will all return to the ground state, and the material becomes opaque again.

But lasing materials are rather rare exceptions.

[guest39538]

I see ''empty'' space and no spectral content in that space.

This is where the contrast between transparent and opaque becomes significant.

In general:
- Something that is transparent does not absorb nor emit light (at that wavelength). You can light from objects beyond it.
- Something that is opaque both absorbs and emits light (at that wavelength). You can't see light from objects beyond it.

So both empty space and clear air are transparent (to visible light)
- They do not absorb nor emit light (at visible wavelengths).
- You can see the wall beyond the clear air
- You can see the glow of the laser pointer beyond the clear air
- The clear air does not emit any wavelengths (spectral content) in addition to those emitted by the laser pointer
- You can see that the wall is dark outside the dot of the laser pointer
- The  wall can "see" (be illuminated by) the laser pointer in your hand, beyond the clear air.
- You can see the Moon beyond the empty space
- The empty space does not emit any wavelengths (spectral content) in addition to those emitted by the Moon
- You can see the Moon and the Wall with no extra rainbow colors.

how do you see [a star] if it does not form a rainbow?

If you look at a rainbow, you will see bands of color. These are "pure" colors (Red, Orange, yellow, etc); you will see 6 of them (if you live in the USA) or 7 (if you live in the UK).

If you have a look at the star trails, you will see streaks of many different colors. But these streaks of different color don't come from a single star; each trail is a single color and comes from one star. And none of these streaks are the "pure" colors of the rainbow; these are all mixtures of pure colors, with the exact mixture determined by the temperature of the star.

So stars come in many colors, but they don't come in rainbow colors.

I know stars do not come multi coloured like a rainbow you misunderstand my  meaning.

You seem to understand clear air because the air has a low refractive index and does not ''compress'' the ''light'' to form visible wavelengths.

We have to have a clear line of sight to observe ''things'', I deem if we do  not have a clear line of sight in that such as where a rainbow obstructs the line of sight, the once clear air is now ''opaque'' relative to sight where the rainbow occupies. Our line of sight is obstructed which also shows us that ''something'' is obstructing the ''light''.



I am not sure if you understand my ''argument'' about opaque or not?

[Colin2B]

You seem to understand clear air because the air has a low refractive index and does not ''compress'' the ''light'' to form visible wavelengths.

I don't know what you mean by this. This is not why air is transparent to visible light.

We have to have a clear line of sight to observe ''things'', I deem if we do  not have a clear line of sight in that such as where a rainbow obstructs the line of sight, the once clear air is now ''opaque'' relative to sight where the rainbow occupies. Our line of sight is obstructed which also shows us that ''something'' is obstructing the ''light''.

I am not sure if you understand my ''argument'' about opaque or not?

I understand your argument about the rainbow,( although I have posted a picture in another thread of yours that shows you can see through a rainbow.) what is happening here is that the light reflected by the water droplets is brighter than what is behind, note that the background is usually very dark clouds, if you have a white house behind part of the rainbow light from the house will be brighter and you won't see the rainbow so clearly - mixing of light.

What I don't understand is why you consider dark areas eg shadows to be opaque.
Consider a light in the centre of a room. Light off, room and bulb dark. Turn dimmer switch up slightly there is a little light but the corners of the room are still dark (further away from light=less light =darker)
The more you turn up the light the brighter the room becomes. Light from the bulb is hitting the walls and some is bouncing off in all directions, when this bounced light hits your retina you 'see' the wall. Where the light is obstructed eg by opaque furniture there will be a shadow=less light=darker. You can even stop the light with your hand and cast a shadow. It is not the air changing from transparent to opaque, but the amount of light illuminating the wall and other objects to cause light and dark.

[evan_au]

You seem to understand clear air because the air has a low refractive index

By this reasoning, I also have an excellent understanding of Solar System space, because the Solar Wind has an even lower refractive index than air.
- And it suggests that I have an even better understanding of intergalactice space, which has an even lower refractive index than Solar System space.
- By the way, I also think I have a reasonable (if classical) understanding of light propagation through water, glass and diamond, all of which have a far greater refractive index than air.
 

air has a low refractive index and does not ''compress'' the ''light'' to form visible wavelengths.

Perhaps you are getting confused here by the relationship between the wavelength and frequency of light, and the refractive index of the material through which it is passing?

When light enters a material with a higher refractive index (ie a higher refractive index than a vacuum), the speed of light slows down, which also compresses the wavelength proportionately.

See: https://en.wikipedia.org/wiki/List_of_refractive_indices

For example, as soon as light strikes a diamond, it slows down by a factor of 2.4, and the wavelength also reduces by a factor of 2.4. But as soon as the light exits the diamond, it returns to its original speed and also its original wavelength.

When it comes to wavelength, the only material that matters is the gel inside your eye, because this is the material through which light is traveling when we see it. But this is about as transparent as air.

The frequency of the light has not changed when it was in a vacuum (with refractive index n=1), in the air (n=1.000277), in glass (n≈1.5), in diamond (n≈2.4), or in the gel of your eye (n≈1.3). It is the frequency of light that determines the energy of the photons. It is the energy of the photons which determines whether the photon will trigger a red, green or blue Cone cell in your eye (or none at all), or reach a Red, Green or Blue sensor in your camera.

So I don't care what material or sequence of transparent materials the light has passed through, if the photons are in the visible frequency range, you will still be able to see it (provided it is bright enough).

This is how you can see the same colors when you are in air, when you are wearing contact lenses, when swimming underwater, and also when a spaceman is in his spacesuit.