It is difficult to imagine our daily tasks, life itself, the Earth, the Cosmos without the light. Our daily activities and responsibilities depend more and more on the New Information and Communication Technologies whose basic functioning depends on the light; writing articles, tasks, and scanning the bar codes of the items we buy in shops and supermarkets require light.

Likewise, light is the key component of sophisticated scientific research facilities, particularly those related to hadron accelerators, synchrotrons, and free-electron lasers, instruments that allow us to reveal the atomic and molecular details of the world we inhabit; astrophysics is practically the only source of information coming from the Universe and that thanks to the increasingly efficient telescopes, spectrographs and other types of detectors of frequencies and wavelengths of light, we better understand the origin, expansion, and destiny of the Universe of which we were created and are a part.

In Nature, we have beautiful expressions of light like the rainbow, the sunsets, the light of dawn, the blue, emerald green tones of rivers, lagoons, oceans, reefs, the colors of the fauna, the flora. Our first experience with light is through the natural world. But in the intimate nature of life, there is light; the process that converts sunlight into chemical energy is a marvel that green plants perform, called photosynthesis, and that greatly contributes to a livable, breathable atmosphere. They, in turn, are the food, the source of energy for other species, including humans.

The contemplation of the different expressions of visible light for our gaze has also been a source of inspiration for the apprehension of the diverse emotions that it provokes through art, such as painting, photography, stage lighting for plays, musical concerts.

Likewise, the growing understanding of it has allowed us to develop optical tools for the reduction of our sight, but also to see "further", or to see further into our skin, the tissues of our organs, to detect and understand the behavior of microscopic organisms aggressive to our health, thanks to optics.

The visible and invisible light

For all of us who enjoy the sense of sight, there is no doubt that light represents our main means to perceive, learn, appreciate, and enjoy the world around us. It also plays an important role in the way we communicate. However, there is light that our eyes do not perceive and this does not make it any less important.

The visible and invisible light. Photo: Andrew "FastLizard4" Adams via Flickr
The visible and invisible light. Photo: Andrew "FastLizard4" Adams via Flickr

Light is a form of energy called electromagnetic. Different colors of light represent different amounts of energy. Red light has less energy than yellow light, the green light has more energy than yellow, and the blue light has more energy than green. These colors correspond to the light that our eyes perceive. There is less energetic light than red light, known as infrared light, microwaves, and radio waves that our eyes do not perceive, as well as more energetic light than blue, ultraviolet, x-rays, and gamma rays, which our eyes do not perceive either.

All these types of light are present in our environment as we have learned to make sources of almost all types of light and some instruments to manipulate it. Of course, we have not matched our main source of light which is the Sun, but we have created sources that emulate its intensity, though not its effectiveness. Life on Earth could not be sustained for long if there were no sunlight. Animals, including us, and plants need it.

Although at night it seems that everything is dark, it turns out that light reaches us, albeit at a very low intensity, but from places so far away that we cannot even imagine them. That darkness is perhaps what has inspired us to develop instruments to study that light and to realize that we are only a small part of an immense Universe and there are very likely other places very similar to our Earth. There are many, many stars, some probably with planets similar to those in our Solar System. The light from these celestial objects is studied by astronomy and, thanks to it, we can know what these objects are made of, how old they are, and how far away they are from us, among many other things.

With light, we beat the darkness in the first place. For mankind, the invention of fire not only represented the possibility of cooking food but also of illuminating dark places. However, this form of lighting was not easy to maintain and was unsafe, so we had to develop other ways to produce safer lights. Electric power helped to achieve this goal and not only the lighting benefit, but it was used for many other things. In addition, the knowledge developed in the handling of electrical energy is that electronics emerged.

There is no doubt that we depend a lot on developments in electronics, and now light helps to produce electronic devices so small that you can have several of them in a few square millimeters. And, in addition, with the devices known as photovoltaics, the generation of electrical energy is being obtained using the incidence of light on these elements. In particular, with the desire to obtain cleaner ways of generating electrical energy, the Sun is being used as a source of light that provides us with thermal energy, which is also being used to heat water for various uses, both domestic and industrial.

The light also allowed communication at a distance, beyond the limits of our hearing and sight. Nowadays, most communications are made by means of light, which is not always propagated wirelessly. Communications have been revolutionized in their capacity and speed thanks to the light that is transported in optical fibers. However, light has other properties that, although we do not see, are useful when interacting with some material. This study and many other topics are dealt with by optics, which also includes the generation, propagation, guidance, and detection of light.

Light also plays an important role in preserving the health of the human body. Using visual inspection, a doctor can diagnose any alteration in an individual's health. Doctors now use both visible and invisible light to determine the degree of health of a patient, as well as to treat some disorders. For example, light is used to modify the shape of the cornea of the eyes to eliminate some eye conditions such as myopia, hyperopia, astigmatism, and presbyopia. Less painful light treatments are performed in dentistry. Light is also used to destroy stones in the bladder. Radiating with light areas with cancerous tissue is an option currently being studied. The use of light to improve the aesthetics (wrinkles, spots, and beauty) of people is very broad. In some cases, it is necessary to make a more internal inspection and light help us in this, for example, with x-rays and tomography.

We have also been able to develop instruments and techniques that allow us to see very small things or things that are transparent. In most cases, the inspection is done by some electronic element and then it is converted into an image. But this image can be altered using some digital process or computer program that allows some details to be highlighted that would otherwise not be visible. To provide vision to robots that utilize optical and electronic elements, and with the help of computational methods, emulate the human visual system, is something that is studied in the computer sciences.

The applications of all of the above are very varied at the domestic, scientific, and industrial level, and due to the impact that light has had, has, and will have in various areas of human endeavor, the aim is to highlight the importance of light and light-based technologies, as well as sustainable development and how this help to solve global problems in energy, education, agriculture, and health.

The pale light of dust

Almost everyone intuitively understands that dust is a light dimmer. When a car is left unwashed for many days, the dust leaves an opaque layer on the windows especially, where we can even write by running our fingers through them. Through a dusty window, it is very difficult to see inside the car, and conversely, the driver or passengers will have trouble seeing out onto the road if someone does not take the initiative to wash the vehicle.

The pale light of dust.
The pale light of dust.

The reason is very simple: dust is opaque to light at the wavelengths to which our eyes are sensitive. There are two reasons: the first is that our biological evolution has optimized our vision in an interval ranging from 0.4 to 0.7 microns approximately (1 micron=1 millionth of a meter), enough for us to perform most of our daytime activities and even for some nighttime activities under the moonlight. Outside that range, our eyes lack sensitivity.

The second is that most dust particles, such as those that swarm on car windows, have average sizes of about one-millionth of a meter, and being slightly larger than the wavelength of visible photons, they can either trap them - this in physics is called absorption - or bounce them off in another direction - this in physics is called scattering. The combination of the effects of absorption and scattering of light by a medium such as dust is known as light extinction.

Another very important point to note is that light with wavelengths between 0.1 and 0.4 microns is classified as ultraviolet and is even more likely to be extinguished by dust. Likewise, light with wavelengths between 0.7 microns and up to about half a millimeter is classified as infrared light, and because it has wavelengths greater than the average size of a grain of dust, it can jump or dodge like a rock in a river. Thus, it is possible to see through the dust if we use an infrared detector.

In the known Universe, galaxies are composed of three main things: stars, gas, and dust. Stars emit light, and gas also emits light but to a lesser extent. Cosmic dust, which is very similar to what we know from chimneys or braziers, has almost the same properties that we described at the beginning: it absorbs almost all visible or ultraviolet light, and to a greater or lesser extent, depending on the wavelength, it is transparent to infrared light.

Here we can raise the degree of difficulty a little more and say that there is light with shorter wavelengths than ultraviolet, which is a more energetic light but suffers from greater absorption by dust. And there is also light with longer wavelengths than infrared, such as sub-millimeter, millimeters, or radio waves, which are not absorbed by dust. What is more: dust, when heated, in a similar way to how the ashes of fire are heated, is capable of emitting light with wavelengths from a few microns to less than a millimeter: in the infrared, we can then see two types of infrared light: that which is emitted by objects located behind it, and that which is emitted by dust when it is heated.

Until the late 1960s, our knowledge of the Universe was limited to detecting waves at visible and radio lengths. We relied on telescopes and cameras, or radio telescopes (a technology inherited from the radar used in World War II). We knew that there was much more light in the Universe, but we could not see it. Ultraviolet and higher energy light is absorbed almost entirely by our atmosphere (fortunately, because it is harmful to organic matter), as is much of the infrared light. However, we knew that some of the infrared light does reach ground level, but we cannot detect it with conventional photographic plates. After a few years, the first infrared light detectors were invented, similar to the thermocouples used for heat detectors, and could be observed in the infrared using conventional telescopes.

Over the next three decades, the technology for making infrared detectors advanced dramatically, as charge-coupled devices (CCDs) replaced photographic plates. These detectors were installed on ground-based telescopes and sent to satellites for use outside the atmosphere. Thus, our knowledge of the universe increased considerably: we were able to observe a multitude of regions in our galaxy and other galaxies, where dust is abundant, and which appeared with dark spots on the photographic plates and the optical digital detectors. Being able to observe them transparently revealed to us the mysteries of star formation, the emission of the dust disks from which the planets around the stars are formed, and it was possible to quantify the rate of star formation in our galaxy and other galaxies. In Mexico, several infrared detectors have been developed, such as the CAMILA spectrograph/camera, the CID double infrared camera, and the RATIR transitory infrared reionization camera, which have been used in the 1.5 and 2.1m telescopes of the National Astronomical Observatory, in San Pedro Mártir, Baja California.

As we mentioned before, interstellar dust can absorb high-energy light, from ultraviolet light to highly energetic particles such as cosmic rays, but it also can re-emit it. Energetic particles heat the material that makes up dust - mainly carbon, silicates, and complex, hydrocarbon-like molecules - and this heat is re-emitted with less energy in the form of infrared radiation. Thus, to the capacity of cosmic dust to absorb and scatter light, is added the ability to emit light. And these three properties can be quantified very precisely thanks to the detectors we have developed, also thanks to complex theoretical models and complicated laboratory experiments, which seek to explain the characteristics of the materials that make up dust and their ability to interact with light.

The thermal emission of dust takes place at wavelengths ranging from a few to several hundred microns. From the ground, very few of the thermal emission bands can be detected, because the vast majority are absorbed by the water vapor molecules floating in our atmosphere. With great effort, placing telescopes and detectors at altitudes between 2,000 and 5,000 meters, and on certain days of the year when the atmosphere in the observatory area has very little water vapor, our detectors can see dust emission down to lengths of a few tens of microns. We have also developed detectors that are mounted on antennas very similar to radio telescopes, which are capable of observing thermal emission at lengths of hundreds of microns. The latter are called sub-millimeter detectors. One of them, called AzTEC, is mounted on the Large Millimeter Telescope in Puebla.

But the best-infrared view of the universe we've had was from instruments sent into space. Two recent examples are the Spitzer Space Telescope, which carries a special camera called MIPS to observe at 24, 70, and 170 microns, and the Herschel Space Telescope, which carries two detectors, the PACS and SPIRE, capable of observing infrared thermal emission from 70 to 850 microns. While the detectors on the ground have given us many clues to the infrared universe, the Spitzer and Herschel telescopes have revealed that universe to us in exquisite detail: we have already observed the disks of young galaxies that produce from a few to even hundreds or thousands of stars per year.

The proto-planetary disks have been detected around thousands of young stars. Massive clusters of newborn stars have been revealed to us. We can trace the structure of the lumps and filaments in the giant molecular clouds where the stars are born. We can observe the expulsion of star atmospheres when they are converted from dwarfs to giants. We have seen the emission of the huge vortices of gas and dust that fall towards the centers of active galaxies where there are giant black holes. We can see the wakes of gas and dust stretched by the collisions of two or more galaxies and how new stars can form in these wakes.

The dust has thus become an enemy to the astronomer's ally. Even in its dim light, it tells the story of a constantly renewing universe.

The light from heaven to earth... and on earth

The north of the Baja California Peninsula is one of four privileged regions on the planet that has one of the most transparent skies. It is an ideal place to observe and study the Cosmos, and where the National Astronomical Observatory is located within the Sierra de San Pedro Mártir National Park (PNSSPM). To the surprise of many, not only does it have the category of Natural Protected Area and is protected by the federal government, but also, according to the Commission for Environmental Cooperation and Marine Conservation Biology Institute, from the Bering Sea, the area is surrounded by a giant geological-oceanic-climate-ecological system of transnational character, known as "Baja California-Bering Sea Region" (B2B Region for its acronym in English "Bering to Baja").

The light from heaven to earth.
The light from heaven to earth.

Mountains and oceanic trenches, ocean and atmospheric currents fronts, the crossing of the Tropic of Cancer, and three and a half million years of global biological evolution have created an enormous ecoregion that manifests itself as one of the richest and most spectacular expressions of biodiversity on the planet. Three countries, Mexico, the United States, and Canada, inevitably share one of the most important sources of wealth in natural resources in the northwest of the American continent; today, unfortunately, with serious environmental threats such as habitat destruction, over-exploitation, and light pollution.

The "transparency of the sky" characteristic of PNSSPM is defined as the capacity we have to visually perceive the clarity of the sky and the stars by their brightness or size. From the physical point of view, it is a condition of the Earth's atmosphere affected by a variable, which are the particles of matter suspended in the atmosphere. The variable can be broken down into factors such as water particles, pollen, smoke, dust, and others. Although it is necessary to consider that natural light and non-visible radiation from the Sun, the Moon, other stars, and artificial light of anthropogenic origin, are added factor that affects transparency due to their interaction with the suspended particles. Non-transparency", then, is the interposition of matter between the emitter of electromagnetic radiation from a given star and its detectors such as an astronomical instrument, the human eye, or another animal. Formally, these factors cause a phenomenon called "extinction" (of light of course), which is nothing more than the reflection, refraction, and scattering of light emitted by celestial bodies that collide with the molecules of the atmosphere.

On the other hand, there is also a phenomenon known as "image distortion", which depends on the variable air temperature and wind. The distortion of the image of a star is caused by the turbulence of the air; that is when light continuously passes through media of different densities due to thermal variation, which is why we see stars or "mirages" on the horizon chiming. Finally, the wind is formed by air convections (circular movements) and horizontal atmospheric movement by the Earth's rotation.

The B2B region has 28 Priority Conservation Areas or PCAs, where the APC-19 Bahía de San Quintín/Bahía El Rosario and the APC-25 Alto Golfo de California, influence oceanically and biologically the continental part, from the coastal coasts to the Sierra de San Pedro Mártir. The biodiversity on the mainland and islands of all of Baja California is about 18 species of amphibians, 150 species of reptiles (not birds; 51 endemic or typical of the area), 238 species of birds (more than 50 endemic), 64 species of non-marine mammals, 37 species of marine mammals, and perhaps more than 4,500 species of terrestrial plants. As if this were not enough, the PNSSPM contains one of the few relict forests (the only ones that exist) from the Pleistocene era for 10,000 thousand years. The forest is composed of several species of conifers and at least one of them is endemic, such as the capricious and sympathetic "San Pedro Martir Cypress" (Cupressus montana), plus another 25 (or 102 along with those of the Sierra de Juárez) species of herbaceous and shrubby plants that are also endemic.

In the region's seas there are some 16 species of marine animals of common concern because they are threatened and have some kind of ecological and/or economic importance, such as the blue whale (Balaenoptera musculus), gray whale (Eschrichtius robustus), and humpback whale (Megaptera novaeanglie); as well as the vaquita (Phocoena sinus), the xanthus murrelet fish (Synthlibiramphus hypoleucus), and the leatherback (Dermochelys coriacea), black (Chelonia midas) and loggerhead (Caretta caretta) turtles. The latter are directly affected in their reproduction by light pollution from coastal cities, as they are diverted from nesting areas during nesting. It is surprising that the APC-19 and APC-25 register one of the "highest rates of productivity and endemism in the B2B region since 8 of the 16 species of marine animals of common concern are found there; that is, in only 2 of the 28 recognized APCs, there is 50% of the biodiversity for conservation, and therefore, the same percentage of direct responsibility among the three beneficiary countries.

Since 2009, there is a reform to the Environmental Protection Law for the State of Baja California that led to the creation of the so-called "Baja California Sky Law" and was created by the astronomer community to regulate the emission of artificial light in nearby cities, which affects the celestial transparency needed for research. However, this is only a partial solution, because as we see, the conservation of these natural conditions implies the conservation of the ecosystem with its geophysical-atmospheric interactions and the network of living elements or ecosystems that exist there, but with a multidisciplinary and global vision. It is necessary to make the citizen and his or her rulers aware of this as a human benefit and not as an ecological or astronomical whim.

To this end, a conservation strategy could be the use of "flagship species", defined as those that are charismatic and that are used as symbols, and which would serve to raise awareness and gain the support of governments, donors, sponsors, and the public for natural conservation programs. Two examples to use would be the golden eagle (Aquila chrysaetos, national patriotic symbol) and the leatherback turtle (Dermochelys coriácea, the third largest reptile in the plan). Also, "umbrella species" can be used, which are those whose conservation of their populations due to their ecosystemic importance implies the protection of populations of other coexisting species, and even an appreciable part of the ecosystem. Examples could be the puma (Puma concolor), the mascot of the National University, or the California Condor (Gymnogyps californianus; at critical risk of extinction).

The same light that allows us to know the Universe, see in the dark, create medical devices, tools and have entertainment; is affecting the physiological processes of the species and modifying their reproductive, ambulatory, and migratory behaviors due to their abusive conditions. Like astronomical telescopes, many organisms need a night sky free of artificial light to adequately perform the function for which they were made or created. The most affected creatures are amphibians, sea turtles, nocturnal reptiles such as geckos, gregarious and migratory birds, flying insects, bats, and a significant part of the surrounding vegetation that produces photosynthesis, among other groups. All of them forming food chains where some of us depend on others. If we want transparent skies to understand our existence in the Cosmos and to be benefited by the natural wealth; not to mention the aesthetic and landscape value that they provide, the three governments involved must coerce natural conservation. And understanding this ecoregion as a complex system that requires a global approach and multidisciplinary research.

Us and the everyday light

Humans are beings of light and not in a mystical or metaphysical sense. Light is present in all aspects of our daily lives. However, we have not been able to fully appropriate the knowledge related to it to put it at the service of society.

Us and the everyday light.
Us and the everyday light.

Light, as well as the science and technology developed around it, provide solutions to global problems in various fields such as energy, health, or agriculture, while at the same time they have revolutionized medicine and telecommunications. In developed countries, photonic (light) technologies drive up to 30 percent of their economy, but in other nations, such as Mexico, we buy products and knowledge abroad. It is time to take advantage of the opportunities in this field especially in science and industry, to do it ourselves.

The nearest star

When we talk about the stars, we almost always think of a dark sky full of these twinkling objects. But there's one star that's absolutely and inevitably visible in the daytime, the Sun. In the words of the Nobel Prize winner for chemistry, Ahmed Zewail, the sun is "an essential ingredient of the universe and of life," not least because it provides living things with food, energy, and an atmosphere.

Light is present in our lives at all times, from the moment we open our eyes (and even sleep!) in the morning and can perceive reality. The photons that come from the sun interact with our proteins and molecules in our eyes so that we can capture the different colors that reflect all the objects around us.

One of the best-known benefits of the Sun's ultraviolet radiation is the increased ability of the body to synthesize the vitamin D that we humans need for healthy calcium metabolism, neuromuscular, and immune system function. But ultraviolet radiation at the same time can be harmful, it is known that prolonged exposure to ultraviolet rays promotes the appearance of skin cancer.

The most abundant source of energy on our planet is solar radiation. Humans have learned to harness it in different ways. Some examples are solar collectors that convert the energy of the Sun into heat to heat the water that a house needs, or photovoltaic panels that convert it into electrical energy.

Artificial lighting

Throughout the history of our consciousness, the setting of the Sun has announced total darkness and the time to light the dim light of a candle or a fuel lamp. In the middle of the 19th century, some European and American cities began to use electric lighting on their streets and since then the lighting systems were evolving from kerosene lamps to modern light-emitting diodes or "LEDs" that we see today in urban spaces.

Humans need light, but we also need darkness. On the one hand, according to the International Year of Light website, more than 1.5 billion people are living with poor or no lighting at night, with a negative impact on their health and educational opportunities. They could benefit from solar-powered LED lamp technologies.

Conversely, while these populations lack quality lighting, in large cities where lamps are abundant, light pollution makes it impossible to appreciate the beauty of the night skies with the naked eye, and professional and amateur astronomers have trouble observing the stars, and also has a negative impact on the behavior of various species of animals and insects, which ends up being to our detriment.

Even though we like illuminated spaces very much, it has been proven that excessive exposure to artificial light is harmful to our body, interrupts the circadian cycle (our biological clock), alters the natural course of sleep and decreases the production of melatonin in our body, a hormone that participates in a wide variety of cellular, neuroendocrine and neurophysiological processes.

Not only humans are affected by the lack of darkness, but also other species of plants and animals, including migratory species in both urban and rural areas and protected areas, cannot live in places flooded by artificial light.

The challenge then is to learn how to light more intelligently and efficiently, only doing so where and when necessary. Lighting in streets, public places, advertisements, and monuments should avoid emitting light into the sky. UNESCO, through its Man and the Biosphere program, is promoting a new culture of protection in the world's biosphere reserves, which includes ensuring the quality of night skies and promoting sustainable lighting.

In Mexico, the city of Ensenada already has a Sky Law that should regulate outdoor lighting and protect the National Astronomical Observatory, which is located nearby in the Sierra de San Pedro Mártir, one of the sites with the darkest, clearest, driest skies and without much atmospheric turbulence in the world.

Ana María Cetto Kramis, a researcher at the Institute of Physics and director of the Museum of Light of the UNAM's Directorate for the Dissemination of Science, says that raising the general public's awareness of the consequences of light pollution will lead to the more rational use of energy and that human beings will revalue natural light and darkness.

Technological devices

The technological devices that young and not-so-young people like so much, such as tablets, smartphones, and screens, as well as the Internet connectivity that brings them to life, would be impossible without light-related technologies.

For example, the Internet would not be possible without electrical drivers and currently with fiber optics, one of the most important light-based technologies for our daily lives. Fiber optic threads are thinner than a hair and are made of glass or plastic. Through them, information is converted into pulses of light that travel very fast and later are decoded and converted into information that we can understand. Information travels through them much more easily than it would through electrons on metal wires. Without a doubt, it is one of the technologies related to light that has most revolutionized our lifestyle.

Today many people have a camera or video camera in their pocket or on their phone. The simple act of taking a photograph or a video and uploading it to social networks involves many physical phenomena in which light is involved, as well as very interesting social phenomena.

Some examples are the operation of the digital camera that involves letting a certain amount of light pass through that will impact on a material sensitive to it, which in the case of digital cameras is a photoelectric sensor (in which photons are converted into electrons and these into magnetic impulses); the decomposition of light into its red, green and blue components, which are the colors shown in the points or pixels on the screen or the interaction of light with the liquid crystal molecules of LCD screens.

Nowadays we use many sophisticated photonic devices, however, and according to Dr. Ana Maria Cetto, there is a general lack of knowledge about how gadgets work. "We use them as black boxes, that is, we know what they do but we completely ignore how they do it, so it would be very good if we became a little more familiar with how they work, and with the science and the materials to make them work".

A bright future

Increasing knowledge of light and related technologies is a good investment. Lasers for health are a very promising and growing field; technologies related to the use of solar energy could change energy consumption and the massive burning of fossil fuels in countries that receive huge amounts of sunshine, and the considerable resources invested in imitating plants photosynthesis in laboratories will bear fruit.

Likewise, a better understanding of how sunlight interacts with our planet's atmosphere will serve to more effectively combat global warming. Telecommunications will also be revolutionized not only on Earth but also on the International Space Station and Mars, the neighboring planet. There is no doubt that in the future, light and its associated technologies will help solve many of humanity's most pressing challenges.

Light is important for our good health

Light is at the heart of the good health of plants, animals, and human beings on the planet. Seeing the Sunrise and set is a daily and natural thing for us, but we do not always have a clear awareness of how the light emanating from that star interacts with our body.

Light is important for our good health.
Light is important for our good health.

Photons are energy and our eyes are a sensor-transducer of light because they capture it and translate it for us into shapes and colors in the visible zone (the rainbow) of the electromagnetic spectrum. At the back of our eye, in the retina, we have cells called cones and rods that are photosensitive and convert the light energy of photons into electrical energy that travels to the brain so that we can form images of our environment.

Light and our biological clock

A clear example of how light impacts our health is the circadian rhythm. In addition to the photosensitive cells in our eyes that allow us to see, other cells are equally sensitive to light, called retinal ganglion cells, which are intrinsically photosensitive and whose function is not to form images but to synchronize our biological clock with day and night.

Light through these cells also regulates behaviors such as the sleep cycle, the humor cycle, and even the learning capacity. Irregular lighting conditions, such as night shifts, the winter months - when daylight is shortened - or long-distance travel across time zones can have direct effects on health, particularly on moods such as depression, anxiety, or learning.

Light is thought to have a direct effect on problems such as so-called seasonal affective disorder, a form of depression that occurs in late autumn and during the winter when daylight is shorter and natural light is dimmer.

Other parts of our body, such as our skin, are sensitive to other types of radiation such as infrared or ultraviolet. What gives color to our skin is a pigment called melanin. The production of melanin is stimulated by damage to the DNA of the epidermis induced by ultraviolet radiation from the Sun. Thus, skin color is a result of our body's adaptation to defend itself from the sun's radiation.

However, our ability to receive radiation from the Sun has a limit because UV rays break down keratinocytes, the most superficial cells in the skin that contain a very hard protein called keratin. They also promote the appearance of oxidizing substances that end up damaging the DNA of the cells of the epidermis. That is why it is very important to protect our skin from the sun, seeking information in the press about the rates of ultraviolet light.

Instruments for treatment and diagnosis of light-based diseases

The light of the colors of the rainbow that we can perceive and that of other wavelengths such as infrared and ultraviolet, interact with our organism. When the physical phenomena of the interaction of light and matter are well known, useful tools can be developed for the diagnosis or treatment of diseases.

Dr. Mathieu Hautefeuille, professor at the Faculty of Sciences at UNAM, explains that one of the applications of light to the biomedical sciences is microscopy. When cell tissues, bacteria, viruses, or fungi are placed under the microscope, the reason they can be detected with the eye is that they are not transparent, but interact with light. Since its invention by Jansen, Kepler, Hooke, and Leeuwenhoek (1608 - 1611) this instrument has been very useful in the study of cells and tissues, and therefore in the knowledge of dysfunctions and treatment of diseases.

The phenomenon of fluorescence, in which a substance has the property of absorbing light and then remitting that electromagnetic radiation with a different wavelength - a different color - has been widely used in medicine.

One example is the test for the detection of the Human Immunodeficiency Virus (HIV) known as ELISA (enzyme-linked immunosorbent assay, for detecting antibodies). "In this test, if the antibody to the virus is present, it binds to an enzyme that fluoresces, and a color change is observed in the sample; this indicates that the individual is infected".

Mathieu Hautefeuille said that the pulse oximeter is another example of a simple and inexpensive tool that has revolutionized biomedical and physiological science. It measures the oxygen saturation in patients' blood; it consists of a pair of light-emitting diodes as opposed to a photodiode (light receptor) and works through a translucent portion of the patient's body. In the past, samples had to be sent to a laboratory, analyzed by gas chromatography and waited for hours or days to obtain a result, which was also costly.

The pulse oximeter is placed on the patient with a kind of finger clip, to which pulses of red and infrared light are sent alternately. The blood, if it has more or less oxygen, absorbs the red and infrared light differently. By making a relationship between how much the blood absorbs, with each of the two radiations, it is possible to know the amount of oxygen there is and, at the same time, measure the heart rate. A student from the Faculty of Science at UNAM is already working on the development of an oximeter that would be wireless and have the capacity to be used in telemedicine.

How photonic technologies will revolutionize light in the coming years

There are technologies and tools based on the light that promises to radically change the diagnosis and treatment of diseases.

Computed tomography is a technique that uses x-rays to obtain images of the body. This technique has been known for a long time and is routinely used in hospitals, but there is a variant, optical coherence tomography, which uses visible light instead of x-rays. One of its main fields of application is in ophthalmology.

Dr. Juan Hernández Cordero, from the UNAM Materials Research Institute, said that optical coherence tomography is used when you want to study the retina, a structure at the back of the eye, in which any radiation other than in the visible wavelengths would be catastrophic.

A ray of light is sent to this organ, the most frontal part is transparent to this radiation, while the retina absorbs some of the light and reflects another, processing the information you can know for example the thickness of the retina without using an invasive technique.

These new retinal inspection techniques make it possible, in a way that was not possible before, to see how far congenital diseases or retinal degenerations have progressed and even to make a diagnosis on what type of treatments can be done.

Although their best-known application is in telecommunications, optical fibers also have a future in the field of health. Since the thickness of optical fibers is very small - similar to that of a hair - they can be delivered via catheters to almost any corner of the body. Since the fibers carry light and bring it back, work is already underway on special fiber-optic tips that will enable optical coherence tomography to be performed in previously unthinkable places on the body.

The treatment of tumors or other tissues that grow abnormally inside the body will also benefit in the coming years from advances in fiber optics. Juan Hernández Cordero's research group is already working on fiber optic tips that concentrate light and function as micro heaters. The purpose is to test if these tips can serve for hyperthermia treatments, inducing cell death in this type of tumor, and try to control, reduce, or even eliminate them.

Each material has a very characteristic way of interacting with light, and that allows it to be used to detect and identify substances. According to the specialist, there are two tendencies, one is known as lab on fiber, in this type of device the light would travel through fiber, and in some specific place of it. The light would interact with some molecule of interest that is sensitive to light and would allow detecting its presence and concentration.

The other trend is known as lab on a chip, in this case, the light is sent to a chip or device which has micrometric channels that are where the samples are traveling. The advantage is that, unlike conventional chemical analysis, the amount of sample needed is very small and the analysis can be done quickly. Specialists believe that they will allow chemical analysis to be done almost instantaneously.

Specialists agree that this is the century of photonics and that its medical applications are almost as important as telecommunications and technology applications, and the only limit is the creativity and imagination of scientists and innovators.

The art of writing with light: José María Velasco

Painters, like photographers, are the kind of artists who can write with light. There have been all kinds: academicians, impressionists, expressionists, naturalists, futurists, cubists, and many others identified with all kinds of the avant-garde.

José María Velasco (1840-1912), Mexico Valley, 1877, Oil on canvas, 160.5 x 229.7 cm.
José María Velasco (1840-1912), Mexico Valley, 1877, Oil on canvas, 160.5 x 229.7 cm, National Museum of Art, INBA

Among them, there is a type of painters that calls our attention. This refers to those who, stimulated by science, used their techniques to illustrate scientific publications. Some were recognized for the quality of their work, as was the case of the painter José María Velasco (the State of Mexico, 1840).

Velasco was one of those painters interested in recording the nature of Mexican territory in the second half of the 19th century. When he was still a student at the San Carlos Academy, which would later be called the National School of Fine Arts, he took some courses in botany and zoology at the School of Medicine in Mexico City.

His work includes, in addition to the landscapes for which he has been called a "scientific landscape painter", illustrations of the Mexican Flora of the Valley of Mexico, the Mexican Ornithology prepared by the naturalist Alfonso Herrera, and several articles published in La Naturaleza, a magazine edited by the Mexican Society of Natural History of which he was a member and vice president.

But what do we see when we contemplate a painting by José María Velasco? We can formulate an answer from the properties of light, the properties of bodies, and the properties of the human eye. Velasco's landscapes, flora, and fauna are a kind of graphic, writing whose main component is light.

José María Velasco (1840-1912), Cañada de metlac, 1893, Oil on canvas, 105 x 160 cm.
José María Velasco (1840-1912), Cañada de metlac, 1893, Oil on canvas, 105 x 160 cm, National Museum of Art, INBA

First: more than three centuries ago the English physicist Isaac Newton observed that the light of the Sun broke down into various colors when passing through a prism. Later we learned that colors are the visible wavelengths of the electromagnetic spectrum, which includes gamma rays, X-rays, radio waves, ultraviolet, and infrared radiation, and that each color has a wavelength. The wavelength is the distance between one wave and the next.

Second: Since Aristotle (384 BC), one of the most influential Greek philosophers in European thought, it was said that color was one of the attributes that we could perceive of a body. Today we know that the colors we perceive from things, from people's skin, are because a part of the light they receive is reflected and the rest is absorbed.

Third: The retina of the human eye is made up of a set of light-sensitive cells or photoreceptors, called cones and rods. These cells, which are sensitive to certain wavelengths, perceive the light that bodies reflect. Both the cones and the rods send that information through the optic nerve -in the form of electrical impulses- to the brain, where it is identified with a particular color.

Source: Noche de las Estrellas, Authors: Rolando Ísita Tornell, David Iturbe (researcher at the INAOE Optics Coordination), Carlos Román Zúñiga (Institute of Astronomy, UNAM), Carlos Jesús Balderas-Valdivia, Abraham Rubí Vázquez (General Directorate for the Dissemination of Science, UNAM), Alfonso Andrés Fernández Medina (DGDC - UNAM) and Carlos Ortega Ibarra. Photos: Pixabay