Some optical curiosities

1995/09/01 Bandres Unanue, Luis Iturria: Elhuyar aldizkaria

• Fisika

In recent months and under the title “Enjoying Physics”, in a simple and pleasant way, in my opinion, at least we have touched some points of optics. With this work today I want to end the optics of this great variety of articles. Of course, there are still a thousand things to play and, if necessary, on one occasion I will return to the optics, but I think the time is coming to change the subject to enjoy more of Physics. So, for the last time, we will bring here some curiosities so abundant in the field of optics and optics to leave the topic with good flavor.

Black and white and white

Look at the following image and answer the question: How many black circles can be included in the clean space between the left circle and any of the right? Three, four, five? You will surely say that four circles fit easily, but for the fifth there will be no space. But if you are told that in that interval only three enter, you will not easily believe it. However, the truth is round and if you do not believe it, take a rule and check.

On the other hand, this white section seems much larger than that of the two black circles on the right margin, but they are equal. The representation that makes us see the black parts smaller than the white of the same size is called “irradiance” and appears because our eye does not meet all the requirements that optics require. Due to the so-called spherical aberration, all the white figures carry around a clear donkey and therefore their measurements appear as if they were greater in the retina of the eye. Consequently, the sizes of a white image seem to us larger than those of a black of the same size.

For this reason, the German poet Goethe in his work “The science of colors” wrote:

“A dark object of the same size seems smaller than a lighter one. If we look together two equal circles, one white on a dark background and another black on a white background, the latter will seem 1/5 times less than the first...

When people wear dark garments they seem thinner than when they wear lights...”

While the first and third of these three statements are true, with the second the same does not occur, that is, a white circle always seems to us greater than another black of its size in that fixed proportion, since this apparent increase depends on the distance of observation.

Of black and white color we bring another curiosity. This curiosity will allow us to know an error of our eye, astigmatism.

If we look at the upper letters with a single eye, we will surely not see them all equally from the point of view of black. Let's see what is the blackest. Let's now put the image to ninety degrees and, as expected, the one who was the blackest seems now more grey and the blackest seems to us another.

In fact, the four letters are equal in terms of blackness. The only difference between them is the direction of the lines. If our eye were as accurate as the optical instruments we use, the direction of the lines would do nothing and the four would look the same. But our eye does not equally refracte the rays of light in all directions. Therefore, we do not see with the same clarity vertical, horizontal and oblique traces.

Most people have the error said, that is, they have astigmatism and in some cases, when they go through a border, those people should wear special glasses.

Living Photographs Living Photographs Living Photographs

Surely we all know the photos that bear this name, that is, those that look at us when we look at it, and that if we move them, in their sight, they follow us. This peculiarity is known from ancient times and has often been taken as something mysterious. This causes some nervousness in some people. Gogol, prestigious writer, explains in his writing “Photo” a case:

“As if the eyes did not want to see anything else, they nailed to him... The photo, without looking any further, looks fixed, as if I wanted to see its viscera...”

There are a thousand stories related to this mysterious visual peculiarity of photos, but in reality it is nothing more than an optical curiosity.

To achieve this effect, the eyelashes should be painted in the center of the eyes. When a person looks at us fixedly his glasses are in that position. (When he looks elsewhere his silk is not slightly deviated in the center of the eye, but). If we move from the photo to one side, your glasses do not change, that is, they continue in the center of the eyes. And on the other hand, as we see the face in the position of always with respect to us, as it should be, it seems to us that photography has turned itself and follows us with its eyes.

The advertisers know this peculiarity perfectly: when a friend, stretching his arm, points us with a finger of his.

Nailed lines

The nails that we initially see in the image do not present any particularity. But we will close one eye and take the magazine so that we will look at the lines at the height of the other eye passing our sight above them (we must put the eye at the point where the lines are joined). Then it will seem to us that nails are vertically nailed more than those drawn on paper. And with the head slightly inclined to one side, it seems that the nails tilt towards that side.

Our ease of falling into optical curiosities should not be considered as an error of our vision. That also has its good side that we often forget. If our eyes did not have the opportunity to fall into any optical anecdote, painting, or the rest of fine arts, would be perfect. The painters produce these special emotions through these false visual errors.

XVIII. The prestigious scientist Euler of the twentieth century wrote in his work “Letters on different physical themes”: “All the art of painting is based on these curiosities. If we were accustomed to capturing things through their real reality, this art (that is, painting) would not be, as if all of us were blind did not exist, the Painter would mix paintings to dissipate his alfer alfer art. We would come to say that if that tablet has a red stick, another blue, another black and several whitish lines.

All this in a plane, without any difference of distance being seen or any object can be formed. Everything painted on the table would seem like something written on a paper... With all that perfection, without the pleasure that gives us that art so useful and delicious every day, would we not give any straw?”