When I observe a visible light source, like the sun or a light bulb, why do I observe the light source as a confined object? Like why is the sun or a bulb or whatever seen as a confined ball of glowing light? Shouldn’t I Just see beams of light everywhere? Like a car passes you at night and you can see it’s head lamps as confined bright glowing things. You can see what those lamps illuminate but you don’t see anything in between. Why? I understand that the photons have to hit an object and be re-emitted and then hit your eye for you to see an object. But when I look at a visible light source I see that source as a confined glowing object. Why is it confined? Like why are glowing things able to be seen as a confined glowing thing? Like let’s change the source to fire - why do I see the flame as a confined luminous thing? If the plasma of the fire is emitting visible light why do I see it as a confined thing?! Also I am worried I can’t actually explain what I am wanting to know. So help clarifying is welcomed.
The sun emits rays in every direction. But, you can only see the rays that hit your eye. Those rays must come from where the sun is, and your eyes keep track of "Oh, I got a light signal from over here", and mentally put a big yellow blob there. You can't see the rays going in other directions because how would your eyes ever know about them, if they don't hit your eyes? Sometimes you'll see rays as in the case of a laser or bright light, but what you're really seeing is light bounce off dust or gas in the room and then into your eye.
But... this is, of course, an oversimplification. Most of what comes into seeing objects as confined objects involves a "pinhole" lens, which you can Google for some nice pictures on the subject. Without a pinhole, you'd just be getting light from every direction, since one aspect of why you're confused may be completely legitimate. An idea is, if I have coordinate 25,20 on my visible 2d plane, and I see a sun there, shouldn't photons from the tree also hit my retina at coordinate 25,20, just at an angle? The answer is yes, until you introduce a pinhole. With a pinhole, any photon that hits your retina at coordinate 25,20, must come from an object that's on the line connecting the pinhole to that coordinate. Which if that is a line pointing to the sun, it is not a line pointing to a tree. This is the big idea behind the "confinement", where you can know where objects are around you very precisely, even though photons are coming from everything and going everywhere. Decreasing the size of the pinhole will make your image sharper, but give you less light to work with, so there's a balance to be had. The limit of increasing pinhole size is a very blurry image that's very bright, until all you get is all light from all direction.
Your eye/brain system can loosely be thought of as a ray-tracer. Basically, a lightbulb spews out tons of photons in every direction. Many of those will hit your eye. When a photon hits your eye, it signals your brain to trace a straight line back in the direction it came from. When lots of these lines converge, your brain knows to put a bright spot at the point of convergence.
Light does not originate from the space around the sun, so the rays that our brain "draws" back from our eyes do not converge there. However, you may be familiar with the fact that when a speck of dust floats through a sun-filled room, you see the speck of dust as a tiny bright light. This is because photons have ricocheted off of the dust speck and into your eyes. Your eyes then trace the source of the light back to the location of the dust speck, and so it looks as though the dust emitted light.
In the case of the fire, photons are created through the conversion of chemical potential energy into heat. Those photons either
- travel directly into our eyes, creating the image of a bright flame, or
- bounce off of a nearby object, and then into our eyes, creating the image of nearby objects (i.e. peoples' faces glowing around a campfire), or
- travel off in another direction and do not end up in our eyes. Thus they are not seen.
Note that photons that travel into the empty air around the fire have very little to bounce off of, so they are extremely unlikely to be redirected into our eyes.
We can image things based on a couple of principles that hold pretty well in many situations.
- Light travels in a straight line from emission to absorption.
- Our eyes can detect the direction that light is coming from along with intensity and color.
- If the light doesn't reach your eye, it doesn't affect your image.
With those assumptions, our visual system is able to create a map of where sources are based on the light that is detected.
There's a lot of light surrounding a luminous object, but not that is also pointed at your eyeballs. It cant go out, then make a turn and reach your eye. It must travel there in a straight line. So all that other light doesn't contribute to the information used to build up the picture of the object.
If you started a fire in the dark, with objects all around you, you would see them if the flame emits with some reasonably good intensity. The less intense the fire you start is, the less you see around, because lesser photons reflect off those objects and hit your eyes.
After all, light did come from somewhere specific, which is the source, and thus you see it because there are more number of photons entering your eye from that, than those that are reflected off from other objects (the intensity due to the reflected photons decreases with distance).
the photons leaving some hot object in a vacuum (like the sun) do not scatter off each other as they stream away from that object. This means that the rays of light reaching your eye come straight from that object, and to your eye this means that the hot object appears "contained".
Furthermore, if the hot object is surrounded by air, those photons stream straight through the air because they are not significantly scattered by air molecules in their way. This means that once again, the rays of light reaching your eye come straight from that object, and to your eye this means that the hot object is "contained".