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Suppose we have some object, from which light rays reflect off and reach our eye. In this example, we can directly see this object, and the image is formed on our retina.

However, consider a coin kept under water. Due to refraction at the surface, we perceive the apparent depth and not the real depth of the coin. The ray of light bends at the surface, and when this new ray is traced backwards in a straight line, it converges at some height above the real depth.

My question is, why do we trace this new ray in a straight line backwards, instead of the original path that it took? This 'image' in a sense, works as a virtual object, and forms an image on our retina, that the original underwater object would have formed. Hence, we perceive this virtual object and see this, instead of the actual coin that is underwater,

Why is it so, that ray-tracing backwards, only work in straight lines, as if the refractive medium wasn't even there?

The same thing happens in a mirror. It appears to us, that the object is behind the mirror. This is the virtual image of the original object, and acts as a virtual object for our eyes, that form a real image on the retina.

Again my doubt is, whether this is just an illusion that our brain perceives? That our brains only trace the light backward in a straight line, even if the actual ray bent due to refraction, reflection or something? Our brain would trace the rays to some virtual object, which in the absence of any refraction or reflection, happens to be the real object. In the presence of these phenomena, this virtual object is an image of the real object, which is somewhere else. Is this just an illusion.

Is there some special reason, why our minds can perceive objects by tracing the light in a straight line backwards? For some reason, our brains don't trace the actual path of light, it only traces the light reaching our eyes in a straight line.

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    $\begingroup$ The apparent distance is a matter of perception of the human brain ...whether there is an intervening (mirror/ lens/massive object ) angles of incidence/ refractive index/relative mass pertaining to the concerned phenomenon (electromagnetism / light/ gravity) and so on.. $\endgroup$
    – Narasimham
    Commented Jan 1, 2022 at 9:45
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    $\begingroup$ How on earth can the retina know where the ray light originally came from ? The only information it has is the angle at which it entered the eye ! The past of the light ray cannot be guessed ! $\endgroup$
    – Alfred
    Commented Jan 1, 2022 at 17:41

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After reflection the rays seem to be coming in the direction that they would if there were an object on the left.

enter image description here

image from here

The brain chooses the most likely cause for the rays to be coming in that direction - and that's that there is an object on the left.

Exactly why it does that is a psychological question, but when humans were evolving mirrors weren't around, although the refraction in water effect, or reflection from still pools could have been seen.

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  • $\begingroup$ So, in a sense, if we have any situation, that can be explained using the above diagram, the brain will always automatically assume that the object is to the left. Hence, it is a purely psychological phenomena ? $\endgroup$
    – RayPalmer
    Commented Jan 1, 2022 at 15:13
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    $\begingroup$ @RayPalmer, yes, that's the first impression, although we do realise it's a reflection, probably due to imperfections of the image, and our knowledge of the rest of the surroundings, it could also be an interesting question for Psychology Stack Exchange, if there is one... $\endgroup$
    – John Hunter
    Commented Jan 1, 2022 at 16:23
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    $\begingroup$ @RayPalmer There is a Psychology & Neuroscience. $\endgroup$
    – wizzwizz4
    Commented Jan 1, 2022 at 18:55
  • $\begingroup$ @JohnHunter yeah I'll soon ask a question there, however, while I can almost convince myself, that since the brain cannot directly trace the path of light, it 'assumes' the light came in a straight line. That's why we see the pencil bend under water, or simple reflections in the mirror and all. However, we can also take pictures of reflections using a camera. The camera doesn't have a sense of perception, it's not conscious. $\endgroup$
    – RayPalmer
    Commented Jan 1, 2022 at 22:06
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    $\begingroup$ @RayPalmer, yes it's similar to the eye in the diagram. The objects that will be in focus are the ones where the dotted lines cross. Both the eye and the camera have rays entering that could have come from an object at a point where the dotted lines meet. There is no way for either to 'know' the real routes of these rays. All the best with it. $\endgroup$
    – John Hunter
    Commented Jan 1, 2022 at 22:51
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The eyes can only perceive the rays which enter the eye. When viewing objects underwater through the flat face plate of a diving mask, the rays entering the two eyes are refracted at the glass, and the object being viewed appears to closer than it actually is.

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  • $\begingroup$ yes, this is because the object creates an image I' due to the glass. This image then acts as a virtual object that forms its image on the retina. The brain believes that this image I' is the real object, while it is nothing but an image of the original object. I'm just wondering why this happens $\endgroup$
    – RayPalmer
    Commented Jan 1, 2022 at 17:09
  • $\begingroup$ for example, if we are standing in front of a lake, and there is a mountain in the background, not only do we see the mountain directly, but also the reflection of the mountain on the lake. Our brains interpret these to be two separate entities, even though the mountain inside the lake is nothing but the reflection of the actual mountain. $\endgroup$
    – RayPalmer
    Commented Jan 1, 2022 at 17:16
  • $\begingroup$ This is because our brain traces the rays reaching our eyes directly backward, as shown in the diagram in the other answer. This is why, the place where the light rays would have met if extended backward in a straight line, is 'perceived' as the object, even though the real ray of light might have bent here and there in the actual path. $\endgroup$
    – RayPalmer
    Commented Jan 1, 2022 at 17:19
  • $\begingroup$ Is this purely psychological, or is there some real physics involved here ? Our brains perceive reflections of an object as separate from the object themselves. That is the reason why we see reflections in the first place. $\endgroup$
    – RayPalmer
    Commented Jan 1, 2022 at 17:20
  • $\begingroup$ @RayPalmer It's simply not possible to know the path that the light took. All we know is its direction when it hits the eye, and the best the brain can do is assume it came in a straight line because that's how light normally travels. $\endgroup$
    – Barmar
    Commented Jan 1, 2022 at 17:51
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After going through the wonderful answers and comments, and some more reading, I think I've managed to clarify some of my doubts.

Let us imagine the following scenario.

I'm standing in front of a lake. There is a mountain behind the lake.

The rays of light from the mountain reach my eye, and form an image on the retina. My brain interprets this, by assuming the light travelled directly from the object to my eye ( which it did in this case ), and I perceive the mountain in my vision.

Now suppose, the rays of light bounce off the lake surface and reach my eye. Again, an image is formed on the retina. The brain, assumes that the light travelled directly, in a straight line, from the object to the lens, even though, in reality, the light has reflected, and thus changed its course. My brain perceives an object that is inside the lake. This is nothing but the virtual image, the reflection of the mountain on the lake, that acts as a virtual object for my eye.

The brain always assumes that the light rays came in a straight line directly from the object to our eyes, and this is why we see phenomena such as reflection, bending of pencil underwater and so on. In case of reflection, light from our body bounces off the mirror and reach our eye. Our brain assumes that there is an object behind the mirror, from which this light came. So we perceive some object behind the mirror. What is this object ? Our reflection !

Now consider a camera. If we take a photo of the mountain lake, we'd see both the mountain, and the reflection in the camera. Note that if the water is perfectly still, and without any side references, we won't be able to tell from the photo, which is the real object and which is the reflection. Remember, the virtual image is acting as a virtual object. Our brains just trick us into beleiving that this is the real object.

Now my question was, why can we see the reflection of the mountain on the lake, in the camera photo. Our brains assume that light travels in a straight direct path, and so we might perceive that there is some object under the lake i.e. the reflection or the virtual image. However, camera is not conscious. So, if the reflection in the lake is visible in the photo, does it mean, that the camera assumes that there is a real object under the water ? Does the camera also treat this virtual image as a real object ?

The answer is an obvious NO !

The camera records the image and nothing more. Our brains record the image on the retina, and analyzes it to give us a sense of vision, a sense of where the object is. A camera does nothing of the sort. It records the image and is done ! There is no perception or analysis involved.

The reflection of the mountain appears in the image, because when it bounced off the surface of the lake, it flipped upside down before reaching the camera. The camera simply records and prints this image. It draws no inference whatsoever, regarding the position of the objects - that's what our brain does !

And since our brain assumes that light travels in a straight path directly from object to the lens of our eyes, the mountain and it's reflection in the lake, appears to be 'two separate' and 'distinguishable' objects to us !

In case of vision, we see or rather perceive the 'objects' that form an image on our retina. In case of a camera photograph, we see the image that formed inside the camera i.e. the image that formed on the retina.

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  • $\begingroup$ A lot of this seems right, and it's great it's all making sense. The rays of light from the mountain enter our eye or the camera from two different routes. Both are actually real images, in the sense they both make a real image at the retina and both are equally valid. It's our brain, as you say that makes us realise that the upside down mountain is not the usual way for us to perceive a mountain. Although we could stand on out heads, then it could happen that the upside down image was from the 'real' mountain without reflection. So you are right, the brain must interpreting the images. $\endgroup$
    – John Hunter
    Commented Jan 1, 2022 at 23:04
  • $\begingroup$ All this reminds me of an old Comedian Harry Worth, who had the photo on this link at the start of his show, made using a shop window - it was actually a moving film, he could apparently lift both legs in the air at the same time! en.wikipedia.org/wiki/Harry_Worth $\endgroup$
    – John Hunter
    Commented Jan 1, 2022 at 23:06
  • $\begingroup$ @JohnHunter yes ! and moreover, it is the brain that seems to add depth, that a camera doesn't. Suppose there is an object, not directly in our line of sight, but the light from this object bounces off a mirror and reach us. On our retina, there is a 2-d image of the surrounding, the mirror, and the object too ( on the mirror ). What our brain does is, it says, well the reality must look exactly like the image that has formed, and all objects must by away from us, in a straight line. So our brain assumes that the object must also be in front of us, even though it actually isn't. $\endgroup$
    – RayPalmer
    Commented Jan 2, 2022 at 8:02
  • $\begingroup$ So our brain, makes the assumption, that the object is inside the mirror, and that is what we see. In a sense, it just adds depth to the 2-d image that forms on the retina. I think that is how vision works. The brain first convinces us that the image that forms on the retina must be exactly what the world looks like. For example, suppose the book isn't visible to us directly, but through a mirror. So, the book is there, in the image that forms on the retina. The brain tells us, well, since there is an image of a book on the retina, there must be a book in our line of sight. $\endgroup$
    – RayPalmer
    Commented Jan 2, 2022 at 8:05
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    $\begingroup$ This all seems right, but just to add that the matter of depth is due to us having two eyes. If we cover one, then it's similar to the camera, it could probably be shown with ray diagrams that the apparent depth in the mirror will be the same as for the original object. As @kkm mentioned in a comment, we probably can manage without psychological input from the brain to explain most of these things. Interesting question, all the best with it. $\endgroup$
    – John Hunter
    Commented Jan 2, 2022 at 10:15

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