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This is a question thats been bothering me a while. I don't even know if it makes sense or not (like if it is a physics question or becoming a philosophical one). But here it goes. The crux of my question basically is that we all know that we can't see light (like in its photon or electromagnetic wave form) directly when it is traveling past us. However, we also know that the way we see objects is by light reflecting off them. This then means that we are "seeing" the light reflecting from the object which then sends the signal to our brain saying that we are seeing a particular object. We know that both light traveling past us and light reflected from objects are made of photons (so they are the same kind)? So then my question is that what is happening to the photon of a light after it is reflected from the objects, that causes us to see it or the object, but on the other hand we can't see light as it is directly traveling past us.

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marked as duplicate by Yashas, John Rennie, Kyle Kanos, Jon Custer, peterh Apr 11 '17 at 1:43

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Your assumption is simply wrong. It's like someone were asking why earth is flat? $\endgroup$ – 0x90 Apr 9 '17 at 23:31
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    $\begingroup$ Possible duplicate of Why can't we see light travelling from point A to B? $\endgroup$ – WBT Apr 10 '17 at 3:06
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    $\begingroup$ Why aren't you wounded by a bullet which does not hit you? $\endgroup$ – jamesqf Apr 10 '17 at 17:59
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The key is that light must enter the eye for you to see something.

You cannot see a beam of light from a low powered laser which is not directed into your eye if the air through which the light is travelling is devoid of dust.
Adding dust to the air and you can see the trajectory of the laser beam because of the light being reflected/scattered from the dust and enters your eye.

Similarly no atmosphere on the Moon leads to a black sky even in daylight whilst on the Earth the sky is blue.

To see something light must enter the eye and the rods (and cones) must be stimulated sufficiently for the signals to be produced for processing by the brain.

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    $\begingroup$ +1 for spotting the explanation/wording that directly addressed what the asker was having difficulty understanding. Perhaps worth explicitly mentioning: this also means that there is no difference between reflected and non-reflected: you also see non-reflected light too every day, for example from the sun, light-bulbs, computer/phone screens, etc, but only because some of that light actually ends up entering your eye as well. $\endgroup$ – mtraceur Apr 10 '17 at 3:00
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    $\begingroup$ Wondering how many are now pointing a laser directly at their eye to check if they can see it. Do I need to add a hint, that this is a BAD idea? $\endgroup$ – Jens Schauder Apr 10 '17 at 12:38
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    $\begingroup$ @JensSchauder Actually, we make it out to be a worse idea than it really is. Class IIIa lasers are designed to be unlikely to cause damage to the retina. Now I have my rule about high voltage (don't trust your heart to something someone said on SE), and I suppose it applies to lasers as well (don't trust your eye to something someone said on SE), but it's supposed to take many seconds for a IIIa to cause retinal damage, and generally speaking our blink reflex will ensure this does not happen unless you take efforts to prevent it from doing so. $\endgroup$ – Cort Ammon Apr 10 '17 at 18:03
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    $\begingroup$ The US FDA has issued two notices of anecdotal damage due to a particular green laser, but general studies have not found a connection between accidental IIIa exposure and retinal damage. The only studies to show damage were those which involved prolonged viewing of the beam (10+ seconds) $\endgroup$ – Cort Ammon Apr 10 '17 at 18:05
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The reflected light is moving toward/into your eye, while the light just passing by you isn't. You can see light that's not "reflected", like the light emitted by a light bulb, there's nothing special about reflected light. All that's needed to see light is the light actually hitting your retina.

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The premise of your question is wrong - You can see light direct from a source!

This is what happens when you see the sun or a light bulb or a fire.

Most objects do not make their own light, they are illuminated by the sun or another source and we see them as a result of the sunlight reflected of them. If you go into a cave, you cannot see. Everything looks black because there is no sunlight.

Only the photons entering your eye enable you to see. Photons travel in straight lines, so you cannot see around corners. Your eye is tiny compared to the room you are in, so most of the photons bouncing off the objects around you go in other directions. Some of them may enter your friend's eye if they are with you.

So - what happens to the photons reflected off the objects around you that don't enter your eye? Well, if you are indoors, most of them will be absorbed by other objects and their energy will be dissipated as heat. If you are outside, quite a lot of them will be angled up towards the sky and out into space. Eventually, they may hit a planet or a nebula and be absorbed. It doesn't really matter.

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You can see the light which enters in your eye and is absorbed by your retina. So you cannot see the light passing by because it is not going towards your eye. If it meets an object, however, light will be reflected or scattered and part of it will go towards your eye. You will then see the light coming from the object.

I would add that if you put your eye before the object, into the light illuminating it, you will see the incoming light just as well.

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Seeing an object means that the photons coming from the object (either by reflecting photons from the object or by emitting photons from the object itself) reach your retina which sends signals to your visual cortex, where an image of the object is created.You don't see the photons but the object.

So to see an object like a photon passing you, the photon has to emit itself photons in all directions (or reflect photons shined on it, which is ruled out by QM, because it alters the state of the photon you want to "see", and because the frequencies of photons and the photons you shine on the photons must have frequencies which lie outside the part of the photon frequency spectrum in which the photons are "visible"), a part of which is reaching your retina after which an image of the photon is created in your visual cortex.

Now we all know that photons don't emit photons, so no photons coming from the photon will reach your retina, which is why we can't form an image of a photon in our visual cortex. It just passes by without sending photons to your retina.

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I think you have a fundamental misapprehension of the way light interacts with an object. Most objects do not reflect light. They absorb light and re-emit it. When you see regular objects like a house or tree, you are not seeing reflected light. You are seeing light which is emitted by the object. The reason why objects have color is because the objects only emit light in certain frequencies. For example, the leaf of a tree absorbs all the red and blue light and re-emits light in the green part of the spectrum. At the atomic level what happens is that the light hits atoms of the object and the electrons of the atoms are momentarily excited. The electrons then return to their normal state and emit new photons in the process. The photons that are emitted are DIFFERENT photons than the ones that came in originally.

Reflected light usually is whitish yellow because that is the color of the sun. Reflected light is what we call "glare". When you see the glare of the sun off the water of a lake that is reflected light. Reflected light bounces off of materials that for whatever reason cannot absorb the light. Reflected light is always whatever color it was originally.

We do not see things from light that is reflected. We see things from the light that they EMIT.

If you want a more technical description description of the interaction of light with matter read this paper from the Yale astronomy department or better yet the lectures of Feynman on the interaction of light with matter.

In short, even though you might read that in "diffuse reflection" the light "bounces" off of the materials atoms, this is not true. It is actually absorped and re-emitted at the same wavelength. This can happen by two mechanisms: Thomson Scattering or Resonant Scattering. Either way, all the photons are absorbed and partially re-admitted (unless the object is black and they are all absorbed.

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    $\begingroup$ Not to nitpick your terminology, but arguably, "absorb light and re-emit it" is the very definition of reflecting light... $\endgroup$ – fluffy Apr 9 '17 at 23:34
  • $\begingroup$ @fluffy Did you even read my answer? When light is reflected, the light bounces off the object. There is only one photon. When light is absorbed and re-emitted, the photons hitting the object are consumed and NEW photons are emitted. It is a completely different type of event. $\endgroup$ – Ambrose Swasey Apr 10 '17 at 1:50
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    $\begingroup$ It sounds like you're saying that most of the light we see from objects is the result of fluorescence rather than diffuse reflection. I don't think that's true; after all, if I shine some blue light on a leaf, it will appear blue, not green. Do you have a source for any of this? $\endgroup$ – Tanner Swett Apr 10 '17 at 6:54
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    $\begingroup$ A reflected photon is always a "new" photon, at the very least because it has a different wavevector. Sure, it is generated in a fixed phase with respect to the incident photon, but it's still a different one. It's counterproductive to limit oneself always to the corpuscular interpretation of em-radiation. $\endgroup$ – LLlAMnYP Apr 10 '17 at 9:14
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    $\begingroup$ Reflected light is also absorbed and immediately re-emitted by electrons, in the same sense and by the exact same mechanism as the Thomson scattering described in the astrophysics paper you linked to. It's just that when you have a flat solid surface instead of a cloud of isolated particles, the re-emitted waves interfere with each other, producing a coherent reflection instead of random scattering. Whether you call this "bouncing off the atom" or "absorption and re-emission with the same wavelength and fixed phase difference" is just semantics. $\endgroup$ – Ilmari Karonen Apr 10 '17 at 15:54
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Well done. You grasp a concept which many can't. The simplest explanation is that we don't see light, we feel light. By light I mean photons, not brightness. We see brightness because it is a visual sensation created by our brain. When our retina detects a photon it sends a message to the brain and the brain interprets this message as an image. Everything you see is created by the brain. A 3 dimensional visual representation of our surroundings.

Seeing something means detecting the shape, size, location and what wavelength of light it reflects (colour), without touching it. We only detect light that strikes our retina. Therefore we feel light, we don't see light. Feeling light is part of the process we call seeing.

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  • $\begingroup$ As I said, very few people grasp this concept but if you are interested in the truth, read page 108 and 109.books.google.co.za/… $\endgroup$ – zane scheepers Apr 10 '17 at 10:51

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