A cone of photons from one (of many seen objects) hits the retina....

[ndvcxk123]

why is the result not like a movie screen where you are projecting 300 films at the same time, over each other ?
(I would get it, if each object only sent one discrete beam, the next object another one, and so on, but it is a cone, of equally strong photons, being projected everywhere, into your pupil, against your temple, everywhere. (And another surface next to it is doing the same, casting photons at ALL the same cells, and at the rest of the room...) ?

 

[berkeman]

(Thread prefix level changed from "A" = Graduate School to "B" = Basic)

What is your background in optics? Are you familiar with the Lens Equation? What reading have you done about how the eye and retina work? Can you upload a sketch or figure of what you are asking about?

Perhaps this reference can be helpful:

https://kaiserscience.wordpress.com...nment/physiology/vision-how-do-our-eyes-work/

 

[Ibix]

Because of your eye's lens. It focuses all the light coming from one point onto one point on your retina. Mine doesn't do a very good job of it, and without my glasses everything is blurred out with images overlapping and mixed up.

Pro tip: don't think about photons unless you absolutely have to. They only make things harder to understand because you can't even describe them without relativistic quantum field theory, and if you think you understand them without that level of knowledge you're wrong. Ray optics is sufficient for this problem.

 

[ndvcxk123]

(Thread prefix level changed from "A" = Graduate School to "B" = Basic)

What is your background in optics? Are you familiar with the Lens Equation? What reading have you done about how the eye and retina work? Can you upload a sketch or figure of what you are asking about?

Perhaps this reference can be helpful:

View attachment 304934
https://kaiserscience.wordpress.com...nment/physiology/vision-how-do-our-eyes-work/

In the diagram above, you see the various parts of the stop sign neatly projected on the retina. But since multiple parties can see the red, for example from standing anywhere in the room (it gets projected everywhere) - is the diagram not wrong because ALL, not SOME of the retinal cells should have red from the stop sign arriving ? And all the cells also get the wooden post. Basically, the optics novice (me) would draw brown and red and white over each retinal cell, not as selectively depicted in the diagram. (Appreciate the replies, +like the kaiserscience stuff)

 

[ndvcxk123]

Because of your eye's lens. It focuses all the light coming from one point onto one point on your retina. Mine doesn't do a very good job of it, and without my glasses everything is blurred out with images overlapping and mixed up.

Pro tip: don't think about photons unless you absolutely have to. They only make things harder to understand because you can't even describe them without relativistic quantum field theory, and if you think you understand them without that level of knowledge you're wrong. Ray optics is sufficient for this problem.

Am getting what you are saying. But look at some huge trees, w. 300 leafs each, you can really see each leaf, but the person next to you also..so from each leaf a broad cone is generated, so thinking of the retina as a screen w. pixel/emission point equivalence, for example, leaf #27 is broadcasting to all the retina pixels, not just to one. (I know you're right + it works... :) Thx.

 

[Ibix]

for example, leaf #27 is broadcasting to all the retina pixels, not just to one.

Not quite. Look at the diagram below:

Look at all the dark blue rays - they're all coming out of your leaf #27. They do go out in every direction - but the only ones that affect what you see are the ones that go through your pupil. Other rays terminate on your iris or your sclera or skin, or never interact with you at all. The few rays that do go through your pupil also pass through your lens, and it focuses all the rays that came from leaf #27 and made it through the pupil to one point.

At the same time, leaf #234 is emitting the light blue rays. Again they go in every direction (not shown - it made the diagram too confusing) but in terms of seeing we only need to worry about the ones that enter the pupil. Again, the lens focuses them all to one point, but it's a different point from the dark blue rays (it's easiest to see that this must be so by considering the ray through the center of the lens - this is not diffracted so just goes straight through, so different points on the object must map to different points on the retina). So you see an image.

If you didn't have a lens then each point on the object would map to a splotch on the retina, as you can see in the diagram below:

That would be a blurry image. Note that it does work if the pupil is very small, because the splotches are also very small and you see very little blurring (that's what a pinhole camera is). But the images are very dim because very little light makes it through a very small hole. The lens is needed to make object points map to points on the retina if the pupil size is anything more than tiny.

Image references: tree, eye.

 

[ndvcxk123]

Many thx, I got to let that sink in. Fascinating is that if you look at that 300 leaf tree, you can see every sep. leaf, all in green, and it is not blurred. Of course, while focusing on one, you see the position of the others roughly, but still with some detail, and there the brain probably gets into the mix. Amazing is also, that the retinal cells have to be firing continuously to keep the image going, and it is effortless... Thx again for the illustrative diagrams !

 

[berkeman]

Fascinating is that if you look at that 300 leaf tree, you can see every sep. leaf, all in green, and it is not blurred. Of course, while focusing on one, you see the position of the others roughly, but still with some detail, and there the brain probably gets into the mix. Amazing is also, that the retinal cells have to be firing continuously to keep the image going, and it is effortless...

Vision is a very interesting subject area. It turns out that much of our detailed vision capability is concentrated in the fovea near the middle of the retina:

https://en.wikipedia.org/wiki/Fovea_centralis

Approximately half the nerve fibers in the optic nerve carry information from the fovea, while the remaining half carry information from the rest of the retina. The parafovea extends to a radius of 1.25 mm from the central fovea, and the perifovea is found at a 2.75 mm radius from the fovea centralis.[3]

There are other very interesting aspects, like how we can detect motion easier than stationary objects:

https://www.wondriumdaily.com/motion-perception/

 

[sophiecentaur]

why is the result not like a movie screen where you are projecting 300 films at the same time, over each other ?

This thought experiment of yours is too big and inconvenient to consider. And what is it supposed to represent? Human vision? A movie screen usually displays just one image and things are the other way round; small frame of film and large screen. With your eye, the scene is large and the image turns up on a 'screen (retina) that's less than 20mm across. But the same principle applies.

Because of the way light propagates, the rays from each bit of an object will cross the rays from another bit without affecting each other and arrive at different parts of any projection screen. (Except if there is dust, clouds or smoke on the optical path.)

@berkeman has pointed out that it's a very complicated system. It's important to deal with it one step at a time. The 'pinhole camera' is the simplest situation to deal with. Instead of a lens, it uses a tiny pinhole. The way it works is that an individual 'ray' passes through the hole and emerges in just one direction to form a (dim) image. All a lens does is to make sure that more of the light from any point on the object gets to a point on the screen.
You may have spotted that there is no 'focussing' needed for a pinhole camera. The resolution of the camera just depends on the size of the hole and the image gets bigger, the further the screen is from the hole..

 

[sophiecentaur]

. Fascinating is that if you look at that 300 leaf tree, you can see every sep. leaf, all in green, and it is not blurred.

It's probably best not to try to use the human eye to establish the basic principles of how images are formed and decoded in the eye. Far better to consider the basic camera and even better to think of the pinhole camera.
Everything in the field of view of the camera will be emitting (or reflecting) light in every direction. Only a small fraction of the light from the source will enter the front of your particular camera. For a pinhole camera, it's only light from a particular part of the object that will find a straight line path to just one particular part of the image. If the camera sensor has the same pixel density all over then the image will be the same 'sharpness' all over. The sensor will get no light at one point from other parts of the object.

A lens adds complexity in that all the light in a cone from a point on the object will be focussed on one place on the sensor - giving you more light energy to activate the sensor.

Incidentally the 'tree' image shows the wrong light path inside the eye because the lens focusses your diverging set of rays onto just one spot.

 

[Ibix]

Incidentally the 'tree' image shows the wrong light path inside the eye because the lens focusses your diverging set of rays onto just one spot.

The first tree image does show the rays converging to a point (although it leaves out the complexity of the cornea/lens system). The second is an attempt to show what would happen with no lens, just a hole. That does match the OP's intuition that all you'd see would be an overlapping mess.

 

[sophiecentaur]

The first tree image does show the rays converging to a point (although it leaves out the complexity of the cornea/lens system). The second is an attempt to show what would happen with no lens, just a hole. That does match the OP's intuition that all you'd see would be an overlapping mess.

More like a rubbish pinhole camera?
I still say that the eye is one step too hard.

 

[ndvcxk123]

I agree, one should leave the fovea/retina out. What amazes is, that a lens gets hit everywhere, with all the light, but keeps the source points neatly apart from each other...

 

[sophiecentaur]

Haha! That’s what lens design is all about. The optical path is the same for all parallel arriving rays. They are all in phase at just one point on the retina / film / sensor. Elsewhere they cancel. A spherical lens is not perfect of course and its aberration can be reduced by fancy shaping.

 

[ndvcxk123]

I like "elsewhere they cancel" -sounds quantumish. But if another lens lies next to thefirst one, those are available. I would get it if there were 20 separate laser beams, hitting each only a portion of the glass, but here, every mm is hit with conflicting inputs. Somehow they coexist and don't interfere. The curvature of the lens focusses them all corresponing to their source point.

 

[sophiecentaur]

I like "elsewhere they cancel" -sounds quantumish. But if another lens lies next to thefirst one, those are available. I would get it if there were 20 separate laser beams, hitting each only a portion of the glass, but here, every mm is hit with conflicting inputs. Somehow they coexist and don't interfere. The curvature of the lens focusses them all corresponing to their source point.

No QM required. Just classical wave theory with diffraction / interference. The waves from one source point only interfere constructively at one point in the image plane.
It’s a few steps further on than Young’s slits but the same mechanism.

 

[sophiecentaur]

Somehow they coexist and don't interfere.

That implies something magical or unexpected. It's very straightforward (at this level with no surprises.)

if there were 20 separate laser beams, hitting each only a portion of the glass

It's not where the beams hit the glass; it's the direction they come from and, except with some very clever optics, they will arrive from different directions. Those 20 beams will all turn up at different places on the image plane - just the same as light from different bits of the tree (above).
Cover up just half of the lens on your camera - you still see all of the scene - not just the right or left half of it. Real images are not made up of laser beam sources so some light from every point on the object will get to part of the lens and form an image.

 

[ndvcxk123]

So, to go a bit further (something, perhaps corpuscles) are hitting at a rate of millions per second all portions of the surface of a lens and unfailingly maintain their (Snell) adjusted direction, unwavering, yet they are passing in a solid, and clearly bumping into something, because they lose some speed. On emerging, DESPITE the millions of bumps they REACQUIRE c and precisely hit the correct pixel detector or fovea portion.
(Note that neither local atoms nor other photons interfere....) It;'s hard to chew on.....Cheers!

 

[berkeman]

On emerging, DESPITE the millions of bumps they REACQUIRE c

I have no idea what you are trying to say, but this part here in particular is incorrect. Take a look at the anatomy of an eyeball a bit closer, and you will see a liquid between the eye lens and the retina. Google vitreous humor.

 

[ndvcxk123]

No, that's not in dispute, light reaquires lightspeed emerging from glass. (We are not nec. talking about the eye here, only a lens, does not matter if retina is below or a photo receiver. ) What is puzzling is that the light neatly stays targeted, despite hitting matter, and separate beams stay separate, though all enter ALL areas of the lens. Sorry if I sound obscure. "Bumps" refers to the interaction of the light with glass atoms.

 

[ndvcxk123]

Now I see what you are saying, there is another medium->water. But if there is air after a lens, the image would still be recomposed..?

 

[berkeman]

Now I see what you are saying, there is another medium->water. But if there is air after a lens, the image would still be recomposed..?

Yes, of course. That's what lens systems do. How much have you studied lens systems?

 

[jbriggs444]

On emerging, DESPITE the millions of bumps

This would be the "photons are like tiny marbles bouncing in a pinball machine" theory of light.

That theory seems to get a lot of traction with first year students. However, it is just plain wrong.

 

[phinds]

@ndvcxk123 you have been asked a couple of times what level of study you are at with optics but you have not answered. It is bad form on this forum to ignore such direct questions.

In particular

What is your background in optics? Are you familiar with the Lens Equation? What reading have you done about how the eye and retina work?

 

[ndvcxk123]

Yeah, sorry, only faint memories fr. school.

 

[sophiecentaur]

Yeah, sorry, only faint memories fr. school.

Don't apologise. Just try remembering what has been written in this thread, instead and forget the garbled version from your school days. It's the way forward.