# How we can distinguish objects separately, even if light rays from them are getting mixed up in space surrounding them?

I am a high school student and I am very confused in one thing in optics (ray optics) which I think is the most basic thing but didn't find any answer on internet, before I ask let me present one thing because only if this is true my question will be understood.

My understanding :

I think when we switch on a light bulb in a dark room,

The light rays from it gets bounced off from walls and they crosses each other at several points in space of the room and these points are behaving like point sources and light rays are not coming directly from source but from these points in space that's why a room gets illuminated, I mean not only the walls but the space of the room also gets brighten up.

And that's only because of these points in space where light rays coming from many objects crosses each other.

If my understanding is correct then let's take a look at my query:

As we can see, in above image there are two point sources (for practical purposes I am considering it as point sources because all the light rays from them are diverging) you can see that :

There are several points in space where light rays from them are crossing each other in space, it means they are getting mixed (by mixed I mean if the light rays from "different" objects focus on one single point on retina when rays from them will enter eye rather light rays from each object separately), but still we can distinguish both sources separately and at the same time we can also see the illuminated space around them.How is it even possible? How we can see the brighten space as well as distinguish both sources?

I have taken only two sources but in actual life, there are many objects in room which reflects off light and all of their rays are getting mixed in space before entering eyes still our eyes (or brain) is able to distinguish which rays are coming from which object.How?

For me, everything should look like a uniform mess of illumination like this:

I appreciate the efforts made by people{for me the most important part is discussion ,not the solution} but for those who still doesn't get a clear view of what I am asking(as it seems some people are still not very clear in understanding what I am asking) I am uploading one more picture , I am just asking when we see any object why the ray diagram can't look like the second one in this image [![this image shows rough ray diagrams when we see any object]

• "and these points are behaving like point sources and light rays are not coming directly from source but from these points in space that's why a room gets illuminated," why do you think places where the light rays intersect are themselves sources? surely the light just passed through each other like any other linear wave. Jun 12, 2022 at 12:03
• You say "the light rays from it gets bounced off from walls [true] and cross each other at several points in space and these points are behaving like point sources [false] and light rays are not coming directly from source but from these points in space that's why a room gets illuminated [false], I mean not only the walls but the space of the room also gets brighten up [false] ." and similar "we can see brighten space i.e the points in space where light rays are crossing each other just like we see any real image." but what everybody here tries to tell you: This is just wrong. Jun 14, 2022 at 15:51
• Drop this conclusion of emerging point sources due to 'ray crossing' or whatever, and your question resolves. Down to a certain resolution your retina can resolve the original source of the light ray, and it does not matter how many other rays this one crossed on its way. Also remember that you cannot see light rays 'from the side' (sorry for this sloppy formulation)... At night we only see the sunlight reflecting at the Moon and the other planets, we do not see the rays that travel from the Sun to these objects. Jun 14, 2022 at 15:51
• @ArunBhardwaj Please remain calm and friendly, in some your comments you tell people 'they are not getting it' in a manner to comes across as rude. Jun 15, 2022 at 14:12
• I still do not understand why you think intersection points of light rays could be visible in any way. If you turn on a light bulb in a room, you can see the light bulb and reflections from the walls. Nothing else. There is no 'brightness around in the space' that you could see. If the room would have perfectly absorbing walls (perfectly black walls), the room would still be dark and you would only see the bulb. Jun 15, 2022 at 14:15

The basic point about light, either as described in optics by classical electromagnetic waves, or by quantum physics as a multiplicity of photons, is that light does not interact with light. In optics optical rays showing the direction of light waves go through each other, they are just a geometric model.

Light interacts with electric and magnetic fields of matter, the air in the room for example. If the room is full of smoke you just see the smoke and not the images the light would carry because it has interacted with the molecules of smoke and lost its information of direction.

See the introduction here.

If you study physics further you will see that light waves do not "wave" through a medium, the way water waves move over water. They EM waves are just sinusoidal variations of electric and magnetic fields in the directions of the light ray.

The fields making up the wave will not interact with fields of an other ray.

• I don't think this answer got the main point of the question. It's not about interaction of light rays but about how the eye can distinguish the direction (and the object) from which a light ray come. Jun 19, 2022 at 10:22
• @Mattia, yes you understood the question whereas all the responses already have a picture in mind from the books that the rays only converge like the way they have shown,,,and they don't want to even give a second thought on the question Jun 21, 2022 at 7:22
• @Mattia the way the question is stated it talks as if there is intersection of light rays,"How we can see the brighten space ", there is no brightness in space if there are no (or few) molecules for the light rays to interact with. It should be a biophysics tag, perception of light. thehealthboard.com/what-is-light-perception.htm Jun 21, 2022 at 11:07

for me everything should look like a uniform mess of illumination like this: . . . shows an incorrect diagram.

A ray is the path taken by light.

Assume that without a change in refractive index light travels in straight lines.
That where light rays cross there is no "interference" between the rays.
In other words one ray is totally ignorant of what the other ray is doing and when rays cross they do not deviate in direction.

I have added to your second diagram to show a selected number of rays from the two sources passing through a convex lens and hitting a screen, which could be your eye with the retina as the screen) to form real images $$A',B'$$ on the screen of sources $$A,B$$.

In spite of rays crossing over what is seen on the screen is two distinct images of the sources.
Where you have placed white circles to show where rays cross makes no difference as far as the passage of the rays is concerned, they carry on travelling in the same direction as before.
Rays reflected/scattered off walls will produce similar images of the walls.

The photographs show the result of @EdV using two laser pointer beams, red and green, intersecting without interacting, inside a large plexiglass rod. The green laser is attenuated with a ND 2.0 filter and the plexiglass just facilitates seeing where the beams are.

• please have a look at this question physics.stackexchange.com/questions/658655/… In your ray diagram you have purposely converged the rays from distinct points separately wheras I am saying the points in the space where light rays from two objects are crossing can also converge the same way on the retina right? so if that's the case we would see that light rays COMING FROM TWO DIFFERENT OBJECTS ARE GETTNG CONVERGED OVER A SINGLE POINT ON RETINA AND WE WOULD NOT BE ABLE TO DISTINGUSH Jun 21, 2022 at 7:26
• CONTINUING: WE WOULD NOT BE ABLE TO DISTINGUISH WHICH RAYS ARE COMING FROM WHICH OBJECT{ITS DISTANCE, ITS DIRECTION AND ALL INFROMATION ABOUT OBJECT} Jun 21, 2022 at 7:27
• with lasers all rays are parallel and light rays from two sources are not intersecting at all,,,that's why we don't see those points in space,,,,,,use the software I have used,,,and track ALL the rays passing from converging lens you will see those points are also getting converged Jun 21, 2022 at 7:29
• There is a place on my diagram where one of my "green" rays crosses with one of my "yellow" rays but they are travelling in different directions so end up at different places on the screen. If those rays were to have ended up at they same place on the screen they must have changed direction as they crossed ie by being scattered off a dust particle. Jun 21, 2022 at 8:39
• I have written an answer to your other question which might help you understand what is going on? Jun 21, 2022 at 9:15

In the image above you can see that light coming from different directions is focused in different places on the retina by the lens, so your brain can distinguish different objects in space.

• this is completely vague diagram, its just showing those rays from top and bottom part which are not crossing each other. Jun 15, 2022 at 13:42
• @ArunBhardwaj If the rays cross (exactly in the lens or before entering the eye) it's the same: the lens (a part of the eye) transmits them in different parts of the retina. Because as pointed out in the answer above two rays of light crossing don't interact. Jun 15, 2022 at 14:37
• think logically, your brain doesn't know if there is any physical object or not unless is sees it,,,,,so why it will converge light rays from discrete points onto discrete points over the retina? why can't it take say for e.g two light rays FROM DIFFERENT POINTS and converge it over a single point over the retina? Jun 15, 2022 at 15:53
• because it's not the brain the converges the light rays, it's the eye lens that works, well, like a lens. And lenses converge light rays with different directions to different points. Jun 15, 2022 at 16:15
• brain decides what I wanna see and what I don't ,,,say for example you are focusing on the wall in front of you,,,light rays from all other parts are still entering eye but are not getting focused over retina{like your nose} but it will still look blurry,....have a look at my this question to know more details: of what I am asking even though its not exactly same its quite similar .physics.stackexchange.com/questions/658655/… Jun 15, 2022 at 16:22

In short: light rays emanating from an object carry two pieces of (mathematical) information. An angle and a position.

When an input light ray hits a lens the angle and position of the output ray depend on both the angle and position of the input ray. For example, a ray colinear with the optical axis passes through undeflected while a ray parallel to the optical axis, but off axis, will deflect by an angle depending on how far off axis it is and the focal length of the lens.

This helps explain why multiple rays emanating from a single object and hitting a lens at different positions and angles can focus all rays down to a single point.

Likewise, it explains why separate rays from two different objects that hit a lens at the same position (but different angles) or that hit your eye with the same angle (but different position) can get focused down to different points behind the lens.

Finally, take care that you recall that the human eye (as well as most cameras) consist of TWO optical elements. A lens followed by a detector screen (the retina in the case of your eye or a ccd or cmos sensor in the case of a digital camera.) If instead you only had a screen then you would only see position information about the rays and indeed any point source would illuminate the entire screen rendering any two point sources entirely indistinguishable.

The answer to your question: we can distinguish objects using light rays because the lens in our eye works to convert the angles and positions of rays from distinct objects into distinct spots on the retinas in our eyes.

Note that this property of lenses (focusing light from distinct objects onto distinct points) only works for certain shaped refractive materials. In particular it works for a lens that has an approximately parabolic shape. Fortunately a spherical shape approximates a parabola well enough for this property to be realized. But if you instead had, for example, a triangular or quartic or something lens this property would be more and more spoiled (this is related to optical aberration). Note also, of course, that the distance between the screen and lens compared to the focal length matters as well.

• Your method of deciphering the information in theory is valid, but the eye definitely doesn't do that. It's a pinhole camera, meaning only light from a certain angle enters at a certain spot. Jun 21, 2022 at 10:24
• @jensenpaull I’ve never heard that before. Do you have a reference? My understanding of a pinhole camera is that (1) you have an aperture that is at most tens or maybe one hundred times the wavelength of light (less is better for sharper images I think?), this would be 10’s of microns, not a few millimeters and (2) there is definitely no lens. Definitely not a tunable focus lens with a focal length close to the distance between entrance pupil and retina… Jun 21, 2022 at 12:33
• Just read more about pinhole cameras. Looks like optimal pinhole size for visible light and about 25 mm distance between aperture and screen would be about 150 micron. At least an order of magnitude smaller than the human eye entrance pupil. Pretty sure the eye is not a pinhole camera. en.m.wikipedia.org/wiki/Pinhole_camera Jun 21, 2022 at 12:42

This is a good question, and I think you may benefit from an answer without a lot of technical or mathematical detail. At a basic level, the mess of color spread all over that you describe is exactly what a near sighted person sees without their glasses, or what a camera sees without its aperture and lens.

The reason your eye can resolve images is that your pupil is very small and blocks the vast majority of the light coming from any object except a narrow slice. Then, the little bit of light that makes it through is further focused to an even smaller point on the retina.

Your point about light rays crossing is not a problem, because the light passes right through other light without interacting.

• but how pupil helps in this? I am not able to understand clearly...I have already seen the video you have linked,,,but that also don't clear the question,,,,Please have a look at this question{specially the diagrams }to clearly understand what I wanna ask: physics.stackexchange.com/questions/658655/… Jun 21, 2022 at 7:32
• @ArunBhardwaj, Re, "how pupil helps...?" There's a compromise at work. A lens with a large aperture gathers a lot of light, but it has limited ability to form an image. (google "depth of field" and "optical aberration"). A tiny aperture (a.k.a., a "pinhole") can form an image that is better in some ways, but only if the aperture is so small that it hardly allows any light in at all. (google "pinhole camera.") Combining the two ideas—a lens and a small, but not "tiny" aperture—can give a better result than either one alone can give. Jun 21, 2022 at 13:40
• @SolomonSlow look at my last image,,,I am saying light from these points CAN still enter eyes like this,,,,so we should see these random points,,,but that would be a mess Jun 24, 2022 at 11:54
• @ArunBhardwaj, I don't understand what you mean by "these random points." It makes me wonder whether you still believe that when two light rays cross, their intersection becomes a secondary source of light. If you still hold fast to that idea, then you won't be able to understand any of the replies that you have received to your question. That's not how light works. Jun 24, 2022 at 12:05
• give me atleast one reason ,,why the points "o" and "P" can't converge at the retina and only A and B should converge?? only then anything would make sense Jun 24, 2022 at 12:08

I think I understand your question, but I don't think your diagrams[or you] explain your issue well.

Here is a picture of an "eye", where white light from one source, and blue light from one source, do infact hit eachother at the same point in the retina, which I have circled in yellow

You would be absolutely correct, that this picture of an eye, would yield completely blurred images and you would not be able to see anything.

This is why the eye has developed a natural pinhole camera.

Here is an image of an eye with a retina. Notice how the case where the light from the 2 sources mix, is now blocked by the retina! [Which I have circled in yellow]

The pinhole shape, means that light entering one spot in the retina, can only enter the retina on that spot, if it comes in from the SAME angle.

Thus, you have a spatial location dependance, on light that enters the retina on a certain spot. Our brains can then make image.

It is the retinas job to make sure that light entering the retina at a certain spot, only comes from a certain angle, thus eliminating the possibility of light "mixing" from 2 different locations.

Is this what you think happens?

Edit 2:

Here is a better diagram of your situation in your previous question. I can always cherry pick certain rays that "converge" to a single point in space. And then I'm guessing you say "why doesn't the eye say the objects at that convergent location". Simple answer is clear from the diagram, only 1 ray will make it into the eye which registers as light coming from a certain angle.

I can artificially Cherry pick more rays that converge to another location and then only 1 ray survives.

I have highlighted the ray that makes it into the eye in orange.

Doing so for many "convergent" rays, will just say light is coming from all angles associated with the e.g wall, and the brain registers a wall.

The eye isn't perfect. So saying " go really close and then something messes up" well obviously... can you see an object from 0.0001m away from you? No. As a concept, say the pinhole is infinitely small.

• if I solely follow the diagram of pinhole camera, the problem appears to be solved,,,but still there is no reason I can think of that the rays from those intersection points in space after entering that that pinhole wouldn't converge at a single point see this link please: physics.stackexchange.com/questions/658655/… Jun 21, 2022 at 10:29
• Somewhere along your road in optics, you have severely misunderstood something. Why would light, Traveling at a specific angle, entering our pinhole camera in that angle , all change direction, and converge to a single point? A pinhole camera, is not convex lens... Jun 21, 2022 at 10:34
• Look at my edit Jun 21, 2022 at 10:36
• see you are drawing rough diagrams,,,,,but if you will use simulations and other softwares you will see the pinhole is actually bigger than you think,,,,there is no such thing as "only a single ray from a single point will cross the pinhole" ,,,make your sources closer to each other and then draw the ray diagram and then you will see the intersections points can also converge en.wikipedia.org/wiki/Pinhole_camera#/media/… Jun 21, 2022 at 10:40
• Look at my edit. Jun 21, 2022 at 10:50

great question. I'm going to try to do this without diagrams, as I don't have the technology to generate them. I'm a neuroscientist and hobbyist physicist.

• Welcome to SE. First, on this site it is mandatory to disclose that a site or article you're linking to is your own. Second, a Facebook or Wix link is all but stable over time, so you should make sure that anything useful for this answer is here and that the link isn't necessary to understand it. Thank you. Sep 11, 2022 at 16:40
• I don't understand but let me try. First, the link is to my own website, as I noted. Second, my response is a stand-alone reply. The article in my website expands on this issue and specifically how we perceive a stable but dynamic world. It goes into the neural processing of vision. I'm not sure how to "prove" that it is my own except to guarantee that it is. Sep 25, 2022 at 19:53
• @CateB, hi cate ,I revisited the question after a long time and read your answer but I didn't come to any conclusion on reading this, my only question lies on the last image I have posted in which point "O" and "p" are the points where light rays from two different point coincides and I have shown them converging over the retina but in reality the ray diagram looks the one above it and not the one with converging "o" and "p" so my question is how does our eye "knows" to converge like this only and not like I have shown in the 2nd one? Nov 10, 2022 at 13:04

When light is incident at object. The light can either be absorbed, speculary reflected, or scattered.

A green object, Absorbs all light apart from green, and then scatters green light in all directions.

This green light then travels to my eye, and my brain processes it as a "green object".

The light scattered from the green object that does not travel towards me, does not get registered by my brain, and therefore I will not see those rays.

I hope everything upto this point is clear.

You talk about "mixing light rays" and then draw a diagram of those rays and then I presume you think that we see those rays? We do not.

We do not see light that doesn't directly travel from the source to the eye in straight lines.

Light rays do not interact with eachother.

The reason that the "entire room" is lit up, is because the rays that do not directly enter our eye, are incident on something like a wall.

The rays from that green object that miss our eyes, instead reach the Wall, get absorbed and remitted by the molecules the wall.

The light from the wall gets scattered in all directions, the rays from the wall that don't travel directly to our eye We don't see. The rays from the wall that DO travel directly to our eyes, we do see.

The light from the wall is registered by our brains as what ever colour the wall happens to be.

Ofcourse the light directly from the torch ALSO gets directly scattered by the wall aswell (along with the light scattered off the green object)

This is also a reason why if you put a green object close to a wall, the wall appears slightly greener. The light from the green object is scattered by the wall, so that our brains register the wall having a greener tint to it. As there is more greenlight that is now scattered, in addition to the variety of light scattered by the torch.

For simplicity I have only added a single point of scattering

Here we have white light that is incident an object. When it hits the object, light scatters in ALL directions. The light the enters the eye, is the ONLY light that I see. no mention of "the light ray that doesnt enter acts as another source that propagates to the eye," this is wrong.

So as of now, if I were the eye, I would register the only light I see, is a green object(or a single point of green as I have only included a single point of scattering), and the rest of my vision is black. This is similar to the sun in space. In space everything is black, apart from the point at which the sun is, as the rays from the sun move in straight lines towards my eyes.

Now what If we add walls?

For simplicity I have only added a single reflected Ray from the green object. The light that doesn't initially reach my eye, gets scattered by the wall in all directions!, the light that doesn't get scattered in my direction, I don't see, the light that does get scattered in my direction, I do see.

(Including all of the scattering elements from the wall and not just a single one) From this POV, you should technically see a green wall as green light is all that is scattered. However in the diagram I have drawn blue. This is because the white light from the torch is also scattered by the wall, which I have chosen is predominantly blue that is scattered. However in reality, it should be blue + a tint of green.

I hope this clears it up.

In general, white light enters an object, only part of that light is scattered, giving it a specific colour, the light that is scattered directly towards you , you see, and your brain registers it as a coloured object.

Huygens principle is what is confusing you I'm pretty sure. This is already accounted for in the fact that you consider the net EM wave propagating and not a single element, these cancel out the apparent "source" of light from a direction not travelling straight toward you

Mixing elements:

There is no trouble in "unmixing" if the rays Cross paths, light does not interact with light so it doesnt effect anything.

In the diagram you see 2 light rays crossing paths. The eye will not see the rays as coming from the same location.

In the diagram I have drawn a huge eye in black and have circled where the eye registers each ray on the retina in yellow. The eye picks up the light at different locations on the retina and will perceive the light as coming from 2 distinct locations, ie , the places I have drawn on the diagram.

• I think I have not said anywhere that light rays interacts on the point of "INTERSECTION" ,,I know they just passes each other without changing direction I am just saying that the points in space where they are "CROSSING" should also converge on our retina and in this way rays CMING FROM TWO DIFFERENT SOURCES gets converged OVER A SINGLE POINT ON RETINA,By "MIXING" i just meant that but everybody took a different meaning out of it ,,,,please take a look at this question to understand in detail Jun 21, 2022 at 7:46
• physics.stackexchange.com/questions/658655/… Jun 21, 2022 at 7:46
• here in your last diagram,,why you have not converged the white light as well as the blue light entering the eye on the same point over retina? If you would be able to answer this in reality then i would think you have answered my question....the point where they are crossing can also be converged over retina right? its like saying because I already know rays are coming from which part i would converge those parts only,,,it isn't fair Jun 21, 2022 at 7:50
• I don't think you explain your issue very well. However I have added a new answer. I also think this is a biology not a physics question. Jun 21, 2022 at 10:23