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

Question

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]

Answer 1

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.

Answer 2

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.

Answer 3

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.

Further references:
http://ffden-2.phys.uaf.edu/211_fall2013.web.dir/jessica_garvin/retina_color.htm
https://en.wikipedia.org/wiki/Retina#Spatial_encoding

Answer 4

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.

Answer 5

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.

Here is a video that may help you https://youtu.be/OydqR_7_DjI

EDIT: I think after reading thru all this, I may finally understand better what you are asking: Given, say, your 2nd diagram, how does your brain "know" where the ray of light is coming from, when it could have originated from any point along the ray? The answer lies in neuroprocessing rather than physics. From a purely physics standpoint, it is true there is no way to "know" simply from the incoming light where the object is located. The mind relies heavily on context (comparisons with other objects in the visual field, perspective) and memory to make organize objects in space into a meaningful image. If you stood in a black room or space devoid of anything but one or two generic points of light, it would be impossible to determine how far away from you or from each other they are. You could only tell the angular separation, but it could be two distant stars, or one star and one Christmas light 20 feet away, or two Christmas lights. That is why if you look up at the stars on a very dark night in a remote area, they seem to be at ceiling height, just above our heads (I've found) even though they are light years away. But if you take that second ray image and add a lens, so the refraction is correctly drawn geometrically, instead of your hand drawn sketch, it will help you see how point sources of light can both converge without being conflated. Viz.

Answer 6

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.

Answer 7

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.

The blur you drew originally would be exactly what would happened if you walked into a room that was so bright that you could not distinguish any objects -- blinding light. We see within a range of lumens that the brain can decipher. Similarly, if you turn the light intensity down very low, you see decreasing amount of detail and then nothing. The people who pointed out that your pupil functions as an aperture are correct. We see a small amount of the total light rays bouncing off objects in our environment. You can show yourself this experimentally by noting that as you move around in your environment, your view of objects changes slightly; a slightly different set of light rays are going through your pupils and you see this different view. A short answer is that an optimal amount of ambient light is required for the retina and rest of brain to convert a photon entering to an electrochemical signal via a rod (black and white retinal receptor) or cone (color retinal receptor). If you make a diagram of the physics of those rays, you will note that the projection is inverted both up and down, and left and right. That's where a massive amount of neural processing comes in to re-create our perceptual world from our sensory world. The brain reconstructs the world "out there" to a perceptual world. All sorts of transformations must take place to take visual 'snap shots' captured by moments (~200 msec) our eyes are stationary between saccades during which we are blind and reconstructs a dynamic visual world. the first level of processing is simple lines of orientation. You can read more about this on Cate's Science Corner: https://l.facebook.com/l.php?u=https%3A%2F%2Fcatebennett.wixsite.com%2Fwebsite%3Ffbclid%3DIwAR1Vz9VFtbxz7OYa4LQNd18euOAsbYv-hbJqnnPPWVw6gdU0s63Pw31MLSk&h=AT3nUNLqTyvbIaHS27SUir8kYaaOcZzhCnjB1sqn1auG8GWGlVFWKUugB30y3_bUW3ZC5ByBmJO3q6csCxcf8lW-rxHN4LWQWDqGJXbjUG9jGfb5PFGPB8im7y1d9z4ByKw

Answer 8

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.