[guest39538]

Does ''light'' give us night vision and allow us to see in the ''dark''?

[evan_au]

Does ''light'' give us night vision and allow us to see in the ''dark''?

Yes.

A bit of ''light'' give us night vision and allow us to see in the ''dark''.

A lot of ''light'' give us day vision and allow us to see in the ''light''.

Too much ''light'' gives us sunburn and (in extreme cases) will blind us so we never see again.

[guest39538]

Does ''light'' give us night vision and allow us to see in the ''dark''?

Yes.

A bit of ''light'' give us night vision and allow us to see in the ''dark''.

A lot of ''light'' give us day vision and allow us to see in the ''light''.

Too much ''light'' gives us sunburn and (in extreme cases) will blind us so we never see again.

You say a bit of ''light'' gives us night vision, then say a lot of ''light'' gives us day vision and allow us to see in the ''light''. which seems contradictory to the first part you said.
Should it not be that a lot of ''light'' gives us perfect ''night vision'' , meaning that although we think it is ''daytime'' it is still actually ''dark''?

How can we be sure that day vision is  just not perfect night vision and a lot of ''light'' making ''dark'' seem to be ''light''? 

And what do you mean by ''light''?, we do not see ''light'' in ''empty'' space we only see light ''interacting'' with ''matter'' , the ''light'' we don't see in space makes space ''gin-clear''.
How do we know that when we increase the intensity of ''light'' on a thing such as a dimmer switch, that we are not just intensifying our ''night'' vision?

''light'' allows us to see through the ''dark''? the light that passes through the space that is ''dark'' and ''opaque'' turns the ''dark'' space into ''gin-clear'' and not ''opaque''.


''dark space'' can be ''opaque'' or ''translucent'' relative to sight?


In simple terms how the hell do you know it is ''light'' outside of your head and you are not seeing in the ''dark''?

Does anyone disagree with that ''light'' allows us to see in the dark?

[evan_au]

I should have added another boundary condition:
No ''light'' give us no vision (we are blind).

You say a bit of ''light'' gives us night vision

"Night Vision" is produced by "Rod Cells" in the retina. They are very sensitive, so they work well on a moonlit night.
See: https://en.wikipedia.org/wiki/Rod_cell

then say a lot of ''light'' gives us day vision

Color vision or daylight vision is produced by "Cone Cells" in the retina. There are 3 types of cone cells, roughly corresponding to red, green and blue colors. However, they are not very sensitive, and so color vision only works well in daylight.
This is why we don't see colors well at night.

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

Should it not be that a lot of ''light'' gives us perfect ''night vision''?
How can we be sure that day vision is  just not perfect night vision?

There is a disease called "Cone Dystrophy" where people don't develop any cone cells. So all they have is night vision, through their rod cells.

Bright daylight saturates the rod cells, so even dark objects are overpoweringly bright. These people experience severe discomfort trying to see in daylight - extremely dark glasses are required for them to navigate in daylight.

So a lot of light destroys night vision.
https://en.wikipedia.org/wiki/Cone_dystrophy

And what do you mean by ''light''?

In physics, "light" mean electromagnetic waves with wavelengths between 400 and 700 nm, or particles of light (photons).

In common language:

• "It is now light" means that the Sun is about to rise, and we can see easily
  
• "Turn on the light" means that we activate a device which produces visible electromagnetic radiation
  

we do not see ''light'' in ''empty'' space

We do see light passing through empty space, because we see stars, the Sun and the Moon through empty space.
 

How do we know that when we increase the intensity of ''light'' on a thing such as a dimmer switch, that we are not just intensifying our ''night'' vision?

Because independent measurement equipment shows that the electrical power going into the lamp filament increases, the temperature of the filament increases, and that the power of visible light emitted by the filament increases.

These measurement devices do not rely on human night vision; they would normally use human daylight vision, but in principle, they could read out the results audibly.
 

the ''light'' we don't see in space makes space ''gin-clear''

I think the gin is not providing you with any clarity into the physics.
I suggest you quit with the gin.

[Colin2B]

Does anyone disagree with that ''light'' allows us to see in the dark?

If it is dark and we have a torch then the light from the torch allows us to see, but the area it shines on is no longer dark. If the sun is out, it is no longer dark.

There is obviously something about this topic which is giving you a lot of angst. Read through what Evan has written and see if you can pose one question at a time. Often we don't have time to answer multiple questions and so will pick the most important.

Note that you arn't using opaque in the sense used by most users of English, you clearly have your own definition which doesn't help communication.

[guest39538]

[


Note that you arn't using opaque in the sense used by most users of English, you clearly have your own definition which doesn't help communication.

Thank you Colin for the advice and I will approach Evan's post with one question at a time. 

I understand I am using opaque with a variation to the definition, that is why I have put opaque as ''opaque''.

opaque
ə(ʊ)ˈpeɪk/Submit
adjective
1.
not able to be seen through; not transparent.

When I turn out the ''light'' it is dark, relative to me ''dark'' is not ''transparent'' to my sight.   ''light'' is needed to make ''dark'' ''transparent''.   (''gin-clear clarity of ''empty space'').


Glass is not ''opaque'', water is not ''opaque'', space is not ''opaque'' when light is present.


We can see and observe that darkness exists in a shadow. I am now observing out of my window, the sun is shining and I can clearly distinguish  where there is an ''edge'' of ''light'' that is ''adjoined''  to the ''edge'' of ''darkness'' , i.e the shadow off a neighbours house.


The shadow is at a distance away from me, the darkness/shadow I am observing is not inside my head,   I am seeing and observing the shadow in it's exact location relative to the suns position in the ''sky''.   


Darkness can not be just a thing where we can't see, darkness exists behind/under the light.



Try to understand this please, if you have any questions or do not understand please ask a question and one at a time, if you can understand the shadow then you will understand that darkness exists .  I will ask Evan a question and it may help you understand me.

[guest39538]

I should have added another boundary condition:
No ''light'' give us no vision (we are blind).

Thank you for the interesting post, I will take Colin advice and ask one question at a time , then maybe we can reach an equal understanding.

You say if there is no ''light'' that we are blind, I do not believe this to be true, I believe there is simply ''nothing'' to see and there is evidence that suggests this is the correct thought.


I have spoke before about a laser dot, even in absolute darkness, we can observe a laser dot at a distance ''interacting'' with a wall.

You will argue that photons are reflected into your eyes so this allows you to see and observe the laser dot.


I would disagree and say that all the space surrounding the laser dot was still ''opaque'', however between our eyes and laser dot the space was ''transparent'' and not opaque.


Between our eyes and the laser dot we do not observe a beam of ''light'', we only observe the dot.   


It would be quite difficult to ''judge'' the distance the dot was away from you.

If we added smoke we would observe the beam of light but we would not observe a reflective ray or photons or any sort of light travelling towards our eyes, the beam is observed in it's exact location ' '''shining'' in the dark.


In the smoke filled room, it would still be relative absolute darkness, we could not see the walls but could observe the beam in the smoke.



So the question is how do you know you are blind in the dark when the dot suggests there just has to be something to see and seemingly we see objects in their exact location?

[Colin2B]

I would disagree and say that all the space surrounding the laser dot was still ''opaque'', however between our eyes and laser dot the space was ''transparent'' and not opaque.

Before we go any further, we need to understand your meaning of opaque. From our dictionary opaque means light will not pass through that area. However, if someone stands to your right or left they will also see the dot so the space cannot be opaque - even by whatever definition you use - because between their eyes and the dot is transparent.

[evan_au]

When I turn out the ''light'' it is dark, relative to me ''dark'' is not ''transparent'' to my sight.   ''light'' is needed to make ''dark'' ''transparent''.

Let's try some more definitions:
"Dark" is the absence of light.
- The presence of light overcomes the darkness.

A "Transparent" medium allows any light to pass through.
- If light is present, you will see the light.
- If no light is present, you will see the darkness (ie the absence of light).

An "Opaque" medium means that it blocks transmission of light.
- Opaque overcomes the transmission of light.
- If there is an opaque substance between you and the light, opaque produces darkness.

I say that all the space surrounding the laser dot was still ''opaque'', however between our eyes and laser dot the space was ''transparent'' and not opaque.

This is not correct.
- The area slightly away from the laser dot is dark (ie light is absent).
- The volume of air between your eye and the wall is transparent, because it faithfully transmits the glow of the laser dot (where it is present), and faithfully transmits the darkness (where the laser dot is absent)
- The volume of air between your eye and the wall is not opaque, because it faithfully transmits the glow of the laser dot (where it is present),
- I think your confusion arises because darkness can also be caused by the presence of an opaque medium between your eye and the source of light.

You can easily prove that the entire volume of air is transparent (and not opaque) by rapidly wiggling the laser beam so it travels all over the wall (followed by your cat, if you have one). This quickly samples the entire volume of space between your eye and the wall, and shows that the entire volume of air faithfully transmits the light (where the dot is shining) and faithfully transmits the darkness (where the dot is not shining). So the entire volume of air is transparent.

So the question is how do you know you are blind in the dark when the dot suggests there just has to be something to see

If there is no light, we are blind. They demonstrate this on every cave tour; you should try it.
However, a dot of light is some light; there is something to see, so we are not blind.
Whether we are blind or not, the real world still exists around us; we just can't sense it by sight.

It would be quite difficult to ''judge'' the distance the dot was away from you.

This is due to three effects:
- One eye can estimate the distance to close object by the amount of effort to focus. A wall is too far.
- Two eyes can estimate the distance to a nearby object by the amount they turn inwards. A wall is probably close enough, except for:
- A laser beam is coherent. It refracts from small crystalline areas of the wall, so the image is quite different between the two eyes. This makes it hard for your eyes to focus on a single object.

A dot of light from a torch (non-coherent light) would make it much easier to focus and estimate the distance.
See: https://en.wikipedia.org/wiki/Stereopsis

[guest39538]

I would disagree and say that all the space surrounding the laser dot was still ''opaque'', however between our eyes and laser dot the space was ''transparent'' and not opaque.

Before we go any further, we need to understand your meaning of opaque. From our dictionary opaque means light will not pass through that area. However, if someone stands to your right or left they will also see the dot so the space cannot be opaque - even by whatever definition you use - because between their eyes and the dot is transparent.

I understand Colin your definition, opaque is the closest word I can think of that closely describes what I refer to as ''opaque''.   
My definition variation is -  sight will not pass through that area,  i.e a rainbow obstructs the vision and is ''opaque'' to sight.
Sight will not pass through darkness, the darkness is relatively ''opaque''.

A piece of thin low opacity coloured plastic is ''translucent'' to sight


Water is less ''opaque'' than milk to sight.


Ok?

However, if someone stands to your right or left they will also see the dot so the space cannot be opaque - even by whatever definition you use - because between their eyes and the dot is transparent.

Which shows that we can see in the dark and are not blind there just needs to be light interacting with something that allows us to see that something?

Yes the darkness is transparent and allows light to pass through,  even without light the darkness is still transparent , however without light/matter interaction the darkness is ''opaque'', add a little bit of light  i.e a dimmer switch, the darkness becomes ''translucent''?

[Colin2B]

My definition variation is -  sight will not pass through that area,  i.e a rainbow obstructs the vision and is ''opaque'' to sight.
Sight will not pass through darkness, the darkness is relatively ''opaque''.

This is due to you defining opaque differently to everyone else.
We do not consider that sight passes through anything, sight does not move through space, we believe it is light which moves.

Which shows that we can see in the dark and are not blind there just needs to be light interacting with something that allows us to see that something?

As we said, how you define whether a room is dark or light depends on how much light is there.
A dark room with a single torch beam will look light where the beam shines, and dark everywhere else because light from the beam is reflected back to the eyes, whereas where the beam does not fall there is no light to reflect back and so it looks dark.

Yes the darkness is transparent and allows light to pass through,  even without light the darkness is still transparent , however without light/matter interaction the darkness is ''opaque'', add a little bit of light  i.e a dimmer switch, the darkness becomes ''translucent''?

Again your definition leading you to think that the space between you and an object changes, whereas we say it remains the same - always transparent to light, but if there is no light then we can't see anything.

I understand that you don't believe in reflection or photons so if you want to discuss that you need to open up a new theory and we'll see what you say: http://www.thenakedscientists.com/forum/index.php?topic=66954.0
At least I now understand why you believe what you do.

[evan_au]

opaque is the closest word I can think of that closely describes what I refer to as ''opaque'

Since you seem to enjoy pseudo-mathematical malarky, I'll try to represent dark and light, transparent & opaque in pseudo-mathematical terms for you.

Lets assume we have a source of light (eg a laser pointer, a torch or the Sun), which can either be ON (1) or OFF (0).

The amount of light L illuminating the scene can be represented as follows:
a) Dark + Dark = Dark; L=0+0=0 (ie dark)
b) Dark + Light = Light; L=0+1=1 (ie light)
c) Light + Dark = Light; L=1+0=1
d) Light + Light = Light; L=1+1=2 (ie very light*)

Now let's assume that we have a medium M through which the light may travel (eg air, water or black cardboard), which may be either TRANSPARENT (1) or OPAQUE (0). The amount of Brightness seen by your eyes may be expressed as B=L*M.

The amount of Brightness seen by your eyes may be represented as follows:
e) Dark*Opaque = Dark; B=0*0=0
f) Dark*Transparent = Dark; B=0*1=0
g) Light*Opaque = Dark; B=1*0=0
h) Light*Transparent = Light; B=1*1=1

I think you are confusing equations (e) and (f).
They both give the same answer: They both suggest that the area outside a laser spot is dark.

However, getting the right answer is not enough in physics or mathematics - you must get the right answer for the right reason, otherwise you will end up with the wrong answer in other scenarios.

The right answer is that the air inside your house is transparent, and
- the laser spot appears light, as suggested by equation (h)
- the area away from the laser spot appears dark, as suggested by equation (f)

*This ignores the theoretical possibility of local light cancellation by anti-phase light.

i.e a rainbow obstructs the vision and is ''opaque'' to sight.

I have no idea where the rainbow came from - it seems to have come out of thin air. Is it something to do with color vision??

But a rainbow does not obstruct light and it is not opaque - it is made of water droplets (transparent) in the air (transparent).

A rainbow is a source of light: sunlight reflected back towards your eye by the raindrops, and each color is refracted at a slightly different angle. It adds to the light from the background, as defined by equation (d).

I suggest that you take a closer look at a rainbow next time - you can see the background behind it, whether that be blue sky, clouds or hills.

[guest39538]

My definition variation is -  sight will not pass through that area,  i.e a rainbow obstructs the vision and is ''opaque'' to sight.
Sight will not pass through darkness, the darkness is relatively ''opaque''.

This is due to you defining opaque differently to everyone else.
We do not consider that sight passes through anything, sight does not move through space, we believe it is light which moves.

Please slow down, I did not even finish replying to Evan's post there is a lot to get my head around and to question in just that first post and now there is even more.


I agree that light seemingly moves from although I also believe it is the ''tip'' of ''light'' that moves , I am unaware of any movement after the ''tip'' has reached it's destination although I am aware of waves which I feel is an invert process of ''force''  of once the ''tip'' has reached it's destination.

You say ''you'' do not consider that sight passes through anything, yet we can not see through brick walls but can see through glass, the glass is transparent but the brick wall is opaque, the space between your eyes and the glass is transparent also and like the glass we can see through the space.  However we can not see through the brick wall it is opaque, like we can not see through the dark because that is not see through like the glass but more like the brick wall and ''opaque'' although the dark remains transparent to light like it remains ''transparent'' to sight, light allow us to see through the dark.
Things that are transparent and allow light to pass through also allow sight to pass through although I do not mean in a sense that there is any travelling involved of/from  the eyes.
Sight ''passes'' through water like light passes through water.


Please just reply with an ok if you now understand that bit and what I mean by sight passes through, if not please further question before I answer the rest of the post , thanks.

[guest39538]

And what do you mean by ''light''?

In physics, "light" mean electromagnetic waves with wavelengths between 400 and 700 nm, or particles of light (photons).

I am well aware that ''light'' is electromagnetic radiation and also the CBMR which we have yet to discuss in this thread which I feel is  later required.
I must correct you though in that wavelengths between 400-700nm  is visible light observed as spectral content like the colours of a rainbow, that is different to the electromagnetic radiation observed passing through space, in that is observed to be ''invisible'' light, there is also ''invisible'' light such as radio waves, so to be clear in our understanding we must be certain that we refer to visible light or invisible light rather than a group terminology of just light.

ok?

we do not see ''light'' in ''empty'' space

We do see light passing through empty space, because we see stars, the Sun and the Moon through empty space

So you claim that you can see visible light of 400-700nm passing through space?   

I can see stars because they are visible light, I do not observe visible light between me and the star , I observe invisible light do you disagree with this observation?

[evan_au]

My definition variation is -  sight will not pass through that area....
I do not mean in a sense that there is any travelling involved of/from  the eyes.

The Greek philosopher Empedocles believed that there was a fire in your eyes that came out and illuminated the world around you. In such a theory, vision would not pass through an opaque object.

This theory was not totally discredited until the time of Newton.
See: https://en.wikipedia.org/wiki/Emission_theory_(vision)

Perhaps what you mean is a "line of sight", which is a geometrical construct linking an illuminated object to your eye; it is effectively the path along which light will pass from the object to your eye, providing there is no opaque object in the way.

I must correct you though in that wavelengths between 400-700nm  is visible light

Yes, I could have phrased that better. I suggest:
In physics:

• "light" means electromagnetic waves, or particles of light (photons).
  
• "visible light" means light with wavelengths between 400 and 700 nm
  
• Other forms of light that are not visible to the human eye include radio waves, microwaves, infra-red, X-Rays, and gamma rays
  

wavelengths between 400-700nm  is visible light observed as spectral content like the colours of a rainbow

The same wavelength range can produce colors like white, magenta and cyan, which are not present in the rainbow.

I do not observe visible light between me and the star, I observe invisible light do you disagree with this observation?

When I look at a star, my eye detects visible radiation which has passed through empty space - the atmosphere is spanned in less than 1 millisecond of a journey that may have taken hundreds of years (eg in the case of Betelgeuse).

When a radio telescope looks into space, it detects invisible light like microwaves (which includes the CMBR and radiation from dust clouds) which has passed through empty space - and again, the atmosphere is spanned in the last 1 millisecond of that journey.

So I disagree with your assertion - I see that both visible and invisible light pass freely through empty space (although I need a computer to translate the invisible wavelengths into a wavelength range that my eyes can see).

[guest39538]

So I disagree with your assertion - I see that both visible and invisible light pass freely through empty space (although I need a computer to translate the invisible wavelengths into a wavelength range that my eyes can see).

Thank you for your reply but I certainly feel you side stepped the question, do you or do you not see visible light of 400-700nm passing through space?

The obvious answer is no, we do not see visible light passing through space, we see space as relatively ''empty'' space, do your eyes work differently to mine or the random people I ask in real life?


and moving onto your further post  -

I have no idea where the rainbow came from - it seems to have come out of thin air. Is it something to do with color vision??

But a rainbow does not obstruct light and it is not opaque - it is made of water droplets (transparent) in the air (transparent).

Is  it something to do with colour vision?  Colour is the only vision we have, we only see colour and light in colour form. A rainbow is not opaque it is transparent, but a rainbow obstructs the line of sight and is ''opaque'' relative to sight.

A rainbow is not made of water droplets, a rainbow is a ''projection'' from a water droplet at a 45? degree angle to the incident ray.

Do you see the rainbow is different to the space that surrounds the rainbow? 


Also mentioned earlier was a point about being blind when there was no light, I  have had chance to think about this, would ''you'' then say we was part blind if the dimmer switch was turned down to half of the power giving half the luminosity?

[evan_au]

do you or do you not see visible light of 400-700nm passing through space?

Yes, I see it.
But since I don't like boiling and freezing my lungs at the same time, I prefer to view it through a window of air about 100km thick.

would ''you'' then say we was part blind if the dimmer switch was turned down to half of the power giving half the luminosity?

No.
It is said that Rod cells are about 100 times more sensitive than Cone cells.
- So if you turned the dimmer down to 1%, the cones would struggle, and we would start to become color-blind.
- And if you turned the dimmer down to 1% of 1%, then the rods would be struggling too, and we would start to become blind.
- And if you turn all light off, we would become blind.
See: https://en.wikipedia.org/wiki/Rod_cell#Sensitivity

Colour is the only vision we have

Not so; we also have night vision, which comes from the Rod cells.

There is a third type of vision that has been investigated in detail only since the 1990s. This acts below the conscious level to control pupil size and the circadian rhythm, among others.
See: https://en.wikipedia.org/wiki/Intrinsically_photosensitive_retinal_ganglion_cells

Do you see the rainbow is different to the space that surrounds the rainbow?

The astronomer William Herschel discovered infra-red by placing a thermometer off the end of the visible rainbow, past the red. He found that the thermometer showed a temperature rise.
A similar effect occurs beyond the violet, with ultra-violet radiation.

So I believe that the visible rainbow is not fundamentally different to the space which surrounds it, which consists of a spectrum of light which is invisible to unaided humans (although parts of this invisible spectrum are visible to snakes and bees).
See: https://en.wikipedia.org/wiki/Infrared

Infra-red is my favorite color []...

[guest39538]

do you or do you not see visible light of 400-700nm passing through space?

Yes, I see it.

Ok! let us ''stick'' with this for a moment and I will ask you to describe your observation of ''empty'' space which you claim to see ''light''   that is in the range of 400nm-700nm.

Below is an illustration and fact of what the wavelength's of 400nm-700nm looks like with sight that you claim to observe in the ''empty'' space between your eyes and object.

 [ Invalid Attachment ]


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


Do you still claim to see the wavelength of 400nm-700nm passing through  space?



added-

Since you seem to enjoy pseudo-mathematical malarky, I'll try to represent dark and light, transparent & opaque in pseudo-mathematical terms for you.

ok

Lets assume we have a source of light (eg a laser pointer, a torch or the Sun), which can either be ON (1) or OFF (0).

ok

The amount of light L illuminating the scene can be represented as follows:
a) Dark + Dark = Dark; L=0+0=0 (ie dark)
b) Dark + Light = Light; L=0+1=1 (ie light)
c) Light + Dark = Light; L=1+0=1
d) Light + Light = Light; L=1+1=2 (ie very light*)

Dark + Dark?   just L=0=D  where L is light and 0 equals intensity and D is darkness ?


D+m₁+1_c=L 


I will try to work out your version , the kids are playing up sorry.

  *This ignores the theoretical possibility of local light cancellation by anti-phase light.

What does this mean?

They both give the same answer: They both suggest that the area outside a laser spot is dark.

Yes it is dark but the dark is also transparent at the same ''time''.  You see the dot  through the dark? which is ''opaque''and ''transparent'' ,



   

[Colin2B]

.....describe your observation of ''empty'' space which you claim to see ''light''   that is in the range of 400nm-700nm.

He already has.  Reread all of reply 14.
Note that a mixture of wavelengths can create the colour of the light we see from a star. It does not have to be all the separate colours of the rainbow unless we separate them with a prism.

You see the dot  through the dark? which is ''opaque''and ''transparent'' ,

Something cannot be both opaque and transparent. Your misuse of words is confusing you.
The reason for what you see has also been explained before. You can't keep asking for explanations of things that have already been explained, either try to understand the explanation or post an alternative theory.

[evan_au]

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

There is an astronomical instrument called a spectroscope which breaks the light up into a rainbow of colors. It breaks the light up into far more bands than the meagre 3 bands which can be seen by the human eye. The spectrum is usually created by a prism or a grating, using all the light collected by a large telescope.

From this spectrum you can tell a wide variety of information, like the star's temperature, chemical composition and even how many planets it has.

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

However, my eyes collect only a small amount of light. While some of the brightest stars show a bit of color (eg Betelguese, a red giant), most stars are too dim to activate your Cone cells, so you see them only in monochrome, using your Rod cells.

My eyes aren't a grating; they are a prism, but the chromatic aberration is so slight that I don't notice it (and I suspect that the wiring of the retina to the brain partly cancels the chromatic aberration).
See: https://en.wikipedia.org/wiki/Chromatic_aberration

So, for multiple reasons, I do not see the stars as a rainbow.

Do you still claim to see the wavelength of 400nm-700nm passing through  space?

Yes. I believe that the light from stars at night contain all the colors from 400 to 700nm (and more beside).
The closest I have come to seeing these colors for myself is when I created star-trails with a long exposure on old-fashioned color film.
For a gallery of spectacular star-trails, see: http://www.lincolnharrison.com/startrails/

Dark + Dark?

If you are in a dark room (the room lights are off: L₁=0), and you turn off your laser pointer (L₂=0), the total illumination L_t=L₁+L₂=Dark+Dark=0+0=0
 

Yes it is dark but the dark is also transparent at the same ''time''.  You see the dot  through the dark? which is ''opaque''and ''transparent''

You are getting confused between the different concepts here:
- The level of illumination L determines whether the wall is bright or dark
- But the Brightness you perceive (B) is also affected by the transparency of the Medium M. The Medium can be opaque or transparent.

In the case of the laser dot, the central area is bright. The surrounding area is dark.
The medium (plain air) is transparent, in both cases.
This leaves an area on your retina which is bright, surrounded by an area which is not bright.

As all the mathematics textbooks say, I leave extension of this model as an exercise for the reader: incorporating non-binary illumination and transparency, and the inverse-square law.