Hello dear friends, thanks for choosing us. In this post on the solsarin site, we will talk about “in order to see an object it must”.
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The Role of Light to Sight
The bottom line is: without light, there would be no sight. The visual ability of humans and other animals is the result of the complex interaction of light, eyes and brain. We are able to see because light from an object can move through space and reach our eyes. Once light reaches our eyes, signals are sent to our brain, and our brain deciphers the information in order to detect the appearance, location and movement of the objects we are sighting at. The whole process, as complex as it is, would not be possible if it were not for the presence of light. Without light, there would be no sight.
If you were to turn off the room lights for a moment and then cover all the windows with black construction paper to prevent any entry of light into the room, then you would notice that nothing in the room would be visible. There would be a brain present that would be capable of deciphering the information sent to it. But there would be no light! The room and everything in it would look black. The appearance of black is merely a sign of the absence of light. When a room full of objects (or a table, a shirt or a sky) looks black, then the objects are not generating nor reflecting light to your eyes. And without light, there would be no sight.
Luminous versus Illuminated Objects
The objects that we see can be placed into one of two categories: luminous objects and illuminated objects. Luminous objects are objects that generate their own light. Illuminated objects are objects that are capable of reflecting light to our eyes. The sun is an example of a luminous object, while the moon is an illuminated object. During the day, the sun generates sufficient light to illuminate objects on Earth. The blue skies, the white clouds, the green grass, the colored leaves of fall, the neighbor’s house, and the car approaching the intersection are all seen as a result of light from the sun (the luminous object) reflecting off the illuminated objects and traveling to our eyes. Without the light from the luminous objects, these illuminated objects would not be seen.
During the evening when the Earth has rotated to a position where the light from the sun can no longer reach our part of the Earth (due to its inability to bend around the spherical shape of the Earth), objects on Earth appear black (or at least so dark that we could say they are nearly black). In the absence of a porch light or a street light, the neighbor’s house can no longer be seen; the grass is no longer green, but rather black; the leaves on the trees are dark; and were it not for the headlights of the car, it would not be seen approaching the intersection. Without luminous objects generating light that propagates through space to illuminate non-luminous objects, those non-luminous objects cannot bee seen. Without light, there would be no sight.
A common Physics demonstration involves the directing of a laser beam across the room. With the room lights off, the laser is turned on and its beam is directed towards a plane mirror. The presence of the light beam cannot be detected as it travels towards the mirror. Furthermore, the light beam cannot be detected after reflecting off the mirror and traveling through the air towards a wall in the room. The only locations where the presence of the light beam can be detected are at the location where the light beam strikes the mirror and at the location where the light beam strikes a wall. At these two locations, a portion of the light in the beam is reflecting off the objects (the mirror and the wall) and traveling towards the students’ eyes. And since the detection of objects is dependent upon light traveling from that object to the eye, these are the only two locations where one can detect the light beam. But in between the laser and the mirror, the light beam cannot be detected. There is nothing present in the region between the laser and the mirror that is capable of reflecting the light of the beam to students’ eyes.
But then the phenomenal occurred (as it often does in a Physics class). A mister is used to spray water into the air in the region where the light beam is moving. Small suspended droplets of water are capable of reflecting light from the beam to your eye. It is only due to the presence of the suspended water droplets that the light path from the laser to the mirror could be detected. When light from the laser (a luminous object) strikes the suspended water droplets (the illuminated object), the light is reflected to students’ eyes. The path of the light beam can now be seen. With light, there can be sight. But without light, there would be no sight.
None of us generate light in the visible region of the electromagnetic spectrum. We are not brilliant objects (please take no offense) like the sun; rather, we are illuminated objects like the moon. We make our presence visibly known by reflecting light to the eyes of those who look our way. It is only by reflection that we, as well as most of the other objects in our physical world, can be seen. And if reflected light is so essential to sight, then the very nature of light reflection is a worthy topic of study among students of physics. And in this lesson and the several that follow, we will undertake a study of the way light reflects off objects and travels to our eyes in order to allow us to view them.
How fast would an object have to go to be invisible to the human eye?
Travelling at the speed of light is a staple of sci-fi fiction. Whether it’s Captain Picard asking for warp factor 9 or the Flash running so fast he can bend time and space, the result is often a sudden empty space where the hero once stood.
But if we really could move at an incredible pace, just how fast would something have to be going before it really was quicker than the human eye? We put on our scientific hats for a while to find out.
On the face of it, this seems like quite a simple question. However, there are a lot of external factors that might change the answer one way or another. For instance, a beach ball would have to speed past your face much faster than a ping pong ball, or even a tennis ball, in order not to be noticed at all. It also depends just how far away that object is, or even how light or dark it was that day.
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A simpler question
To make this question simpler, let’s say you were sitting in the front row of a football stadium right in the middle. It’s a clear, sunny day and you are looking straight forward as one of the player’s kicks a ball right down the middle of the football pitch. How fast does that ball have to be going before you won’t register it even going past?…also you can’t blink, that’s cheating.
Let’s start with how your eye sees the world. As we’ve shown in our eye post, when you look at something light is reflecting off that object and into your eye. That light is then focused and sent to your brain which interprets it as an image.
How fast can we really see?
In order to become invisible then, the ball would have to go so fast that the brain doesn’t have time to process any light reflecting off it. In an experiment, air force pilots were able to recognize an image of a plane that was flashed on screen for as little as 1/220th of a second. With this in mind, we can imagine that the ball would have to be in front of you for at most 1/250th before the brain just never picked it up.
So doing some quick math… An average football pitch is about 68 meters wide and 105 meters long, so to be in the centre the ball would have to be about 34 meters away. From that distance the ball won’t look much bigger than a small white circle, but not one you wouldn’t miss if it zoomed past.
The average human field of view is about 200 degrees horizontally, which is pretty huge, but our binocular vision is about 120 degrees. This is the amount in front of you that both your right and your left eye can see. For this we’ll assume you’re only really concentrating on that smaller area, staring right ahead. That means you would only be looking at roughly 70 meters of pitch at any one time.
The final result
So in order for the ball to be invisible, it would need to cross 70 meters in 1/250th of a second. That’s 17500 meters every second or 38146 mph! At this point the ball would be moving way, way faster than the speed of sound, and would almost certainly have crumpled up into a little fireball before a goal was even scored.
Of course this is all very theoretical, and completely relies on all the conditions being absolutely perfect plus a striker with the right foot of a god. It all gets a lot more complicated when you start adding bigger objects.
Ultimately our eyes and brain work together so fast, that it would often take something actually moving faster than the speed of light not to be noticed. Even in our example above, it’s possible you might still have an Idea that something went past, just because the light reflecting from the area around the ball was blocked for a split second.
Unfortunately, if the ball was kicked so hard it reached the speed of light, the resulting explosion would in fact level the city surrounding the stadium. Under standard FIFA rules this would almost certainly mean a red card and suspension for the player.