Chapter 1 What is light?

  Light enables us to see the world around us. It provides the means by which our sense of sight gleans the most direct information about the physical arrangement of the world and how it is changing. Indeed, the capacity of light to carry and convey information is perhaps its most important, and remarkable, characteristic.

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  Seeing is believing

  Sight enables us to locate ourselves in our surroundings, defining things outside ourselves that allow us to construct a true picture of the world. And sight inspires the imagination beyond the physical sensation of vision itself. George Richmond’s painting,

  The Creation of Light, shown in Figure 1, illustrates the central place light has in our psyche. Indeed, words deriving from the idea of light-insight, illumination, clarity, for example-pertain to human, as well as physical, qualities. In fact, Latin has two words to describe light, lux and lumen, denoting both the material and the metaphysical aspects of light. It is the former with which this book is primarily concerned. The intertwining of the physical and the poetic has made light a metaphor for thinking about the world, in philosophy, theology, psychology, art, and literature. Because it is something of which almost everyone has direct experience, the physical basis of light and how it facilitates this powerful sense has made it an object of study for philosophers and more recently scientists, for centuries.

  1. The Creation of Light by George Richmond.

  Light gives life. Literally, light plays a vital role in the biological and chemical processes that underpin our and our planet's existence. Figuratively, light frames our perception of our surroundings. Our common everyday experiences illustrate the central importance of light in this regard. Of course we use it for illuminating our environment, either naturally, by the Sun or Moon, or artificially. Most common light sources use electricity, but on occasion we still use chemical reactions to generate light; burning candles, for example. The diferent character of the illumination has an efect on how we perceive our surroundings: it sets a 『mood』 for the physical space.

  Light has a very fundamental role in making possible life itself. The primary source of energy for the Earth is ultimately the Sun. And the means by which energy is transmitted from the Sun is light, both the visible components we can see, as well as some invisible ones we cannot see directly. Think, for example, of lying on a beach or sitting out in the garden in the sunshine: the warmth we feel is a consequence of some of the 『invisible light』 radiating from the Sun. This is just one example of the physiological efects of light.

  But at the very core of our planet’s ability to sustain life is an awesome biochemical process that converts 『waste』 moleculescarbon dioxide-into 『useful』 ones-oxygen, using sunlight as a source of energy. The reverse process, conversion of oxygen into carbon dioxide, occurs during respiration, as well as from the things we burn to power our everyday world.

  The Sun’s light has over the course of millions of years enabled the formation of the current biosphere and the geology that provides other sources of energy. Neither coal nor oil could have been made without energy from the Sun. And our use of these resources is changing the way in which light from the Sun afects our planet. Some of the invisible light from the Sun-ultraviolet light-is still absorbed by the Earth and its atmosphere. But the other invisible part-infrared light-is reflected back by atmospheric gases. By the same mechanism, infrared radiation is trapped on the planet, contributing to increased planetary surface temperature.

  Light enables communication

  Pictures have been part of human culture since the beginning of the species. The impact of images on how we conceive of the world, and how we make sense of our place in it, is immeasurable. Optical technology has contributed to this in ways that are utterly transformative. For instance the ability to capture images easily and rapidly, by means of film-based and digital photography, allows us to record places, people, and things as reports that can be widely distributed (nowadays by an optically enabled Internet) and which have lasting impact: images of leaders and workers, awe-inspiring scenes of the natural world, and horrifying scenes of war. These can bind or fragment people in unexpected ways: calling populations to action, inciting acts of compassion, and giving deeper insight into shared experience. Recall the astonishing sense of wonder the sight of man’s first steps on the Moon (see Figure 2) invoked. The ability to capture moving images adds a completely new dimension by enabling new narrative and documentary capabilities. Can you imagine life without television or movies, without video?

  Nowadays, the generation and transmission of images is so prevalent we hardly think of it. We use self-luminous displays every day: televisions, computers, tablets, even smartphones. All of these bring information to you and receive it from you using light as a medium. Almost all long-distance telecommunications travel on light beams, guided along thin strands of glass called optical fibres. This is the basis of fibre optical broadband services that link our homes to the Internet. Even inside computers and televisions light plays a role. For instance, the music, video, or images that are locked in a CD or DVD are accessed using light. A tiny moving head based on a miniature laser 『reads』 the disc and converts the information coded on it into electrical signals that can then be sent to the display screen. All our surfing, downloading, and emailing activity now requires such immense information capacity that light is the only feasible medium for conveying it.

  2. Neil Armstrong’s photograph of Edwin Aldrin walking on the Moon.

  Transportation in the modern world uses light as the means by which we signal and regulate our movements. From streetlights in towns, to landing lights on aircraft, light is an essential part of navigation. And it even plays a role in maintaining our vehicles. For instance, lasers are used to align the wheels of a car, and light-enabled distribution of ignition power to drive an internal combustion engine is not uncommon.

  In many, many ways, light carries the energy and information that makes modern life possible.

  Optics

  The field of enquiry that constitutes the study of light is called optics. Optics is among the oldest of the sciences, and its historical development forms one of the most important paths in the emergence of modern science. Ideas arising in optics have stimulated new ways of thinking for very disparate fields, such as the mechanics of motion of atoms and molecules. And technologies enabled by deeper understanding of light have been central in unlocking other secrets of the natural world. Galileo’s telescope designs, for instance, were critical in his observation of the moons of Jupiter, which was a vital step in moving towards a view of the solar system in which planets moved around the Sun. This, in turn, was important in developing the laws of gravity that govern planetary motion.

  The origins of optics lie in the work of the Greek philosophers of the 4th century bce, and the field has continued to flourish for the past two millennia. It is perhaps surprising that we can still discover new things about light after such a long period of attention by many clever people. Yet optics remains at the forefront of current science: more than ten Nobel Prizes in the past twenty years have been awarded for research in which light has played a central role, from controlling and measuring the motion of atoms and molecules, at unimaginably low temperatures and on timescales of breathtaking brevity, to improving the precision of clocks a thousandfold, to enabling us to look inside living cells and watch what happens as they change.

  What is light?

  A good place to start is with some of the things commonly associated with light: brightness, intensity, colour, and warmth.

  These are all tangible properties that suggest light is a physical entity. But what, exactly, is it?

  We may consider what it means for light to be bright by taking a particular source of light-a household light bulb. These come in various sizes, but all have powers of several tens of Watts (the unit in which power is measured, labelled W, and signifying the energy consumed per second of operation). A 50 W light bulb gives suicient light by which to see things inside a house. Car headlights are typically of slightly greater power, approximately between 60 W and 100 W. The floodlights at a football ground are of much larger power, up to several thousand Watts. I will discuss later exactly how these diferent sources generate light, but these powers will give a sense of the brightness of the corresponding lighting. Of course, one of the brightest sources is the Sun. It has a massive power output, more than 1025 W (1 with 25 zeroes after it), which makes it impossible to look at directly even though it is a very great distance away.

  This brings us to the next concept associated with how bright a light is. Light of the kind discussed above looks dimmer the more distant it is. So power alone is not the only criterion determining brightness. It is related in some way to the fraction of the power that we can receive from the light source. For example, a laser pointer typically has much, much lower power output than a light bulb, often only a few thousandths of a Watt (10-2 W or 10 mW). Yet it appears very bright when it is pointed at a screen.

  What is important here is the intensity of the light generated by the source-the power per unit area of the receiver. (This is more properly the irradiance, but intensity is perhaps a more familiar term.) The intensity of a light source is related to the ability to concentrate the light. A laser pointer appears to be very bright because its light beam is concentrated on a small spot on the screen, whereas the Sun’s light is difused over a very wide area.

  Therefore, although the Sun has a very great power output, the light it produces is not as intense as that of a laser pointer.

  The underlying property that describes our ability to concentrate light is called the source coherence. This is related to the source’s propensity to send light in a particular direction. For instance, both the Sun and a light bulb radiate light into all directions-the Sun can be seen from every place on Earth, and a light bulb from wherever you stand in a room. But a laser pointer only puts out light in a single direction-that in which you point it. So you cannot see the laser beam unless you look at the surface on which it is incident. Because of the property that its beam has a well-defined direction, the laser pointer is said to be a coherent light source, whereas the light bulb is an incoherent light source.

  Another defining characteristic of light, perhaps its most evident property, is colour. The rainbow embodies the fundamental idea of a spectrum of colours-a palette emerging from the conjunction of rain and sunlight-from blue at one end to red at the other. A central strand in the development of theories of light has been the development of models of colour vision. Colour is intimately tied to perception, as well as to physics. An illustration of this is found in the experiments on the nature of colours undertaken by Sir Isaac Newton (Figure 3), a dominant figure of early 18th-century science, and whose book Opticks defined the view of light for two centuries, and by Johann Wolfgang von Goethe (Figure 3), a dominant figure of late 18th-century literature, who incorporated scientific ideas in his writings, but nonetheless believed Newton to be profoundly wrong about the nature of light.

  The first part of Newton’s famous experiment (similar to ones undertaken by Descartes and others previously to him) was to allow a small beam of light from the Sun, defined by a hole in a dark screen, to pass through a prism, and thence to fall upon a screen. The familiar rainbow colours emerge. Goethe was fascinated by this efect, and borrowed some prisms from a local aristocrat with which to experiment for himself. He concluded quickly that Newton’s experiment was wrong-in the sense that his claim of the universality of colour dissection from white light was not true. Goethe had himself discovered a very diferent set of colours.

  3. Isaac Newton (left), Johann Wolfgang von Goethe (middle), and Rosalind Franklin (right).

  The experiment Goethe undertook was to look through the prism at the mullion of a window. That is, he looked at a dark line against a bright background, the very opposite of what Newton had done. What he saw was a very diferent spectrum from Newton. Not the red, green, and blue of the Newtonian spectrum but rather a new palette of cyan, magenta, and yellow; the so-called complementary colours of the Newtonian spectrum. Combining Newton’s colours produces a white image-combining Goethe’s produces a black one.

  Goethe espoused the idea that colours are the things perceived; Newton defined them as intrinsic properties of light. They were both right. Nowadays we are content to separate the physical attribute of colour from its physiological efects, the sensation of colour. We each react diferently to colours-indeed, coloured light can even be used as a therapy. From an artistic point of view, the interpretation that our consciousness realizes from the sensations associated with light of a particular colour is a critical matter-perception is vitally important. Yet from a physical point of view there is an underlying property that we can assign to the label 『colour』 unambiguously: its frequency-at least until we get into the realm of quantum light.

  The reach of light extends beyond the visible spectrum, at the blue end into the invisible realm of the ultraviolet and the extreme ultraviolet to X-rays and γ-rays. At the other lie the infrared, microwaves, radio waves, and eventually T-rays (Figure 4). To see these, we need diferent instruments than our eyes alone. Nonetheless, we know that light of these colours exists. For instance, the warmth of the Sun is due to infrared light that is absorbed by our skin. At lower frequencies microwaves are used for cellular phone communications as well as for cooking, by heating the water in the food being prepared. The invisible colours at shorter wavelengths are also familiar. Sunburn is caused by ultraviolet light, while X-rays are used routinely in medical imaging.

  X-rays are also used in many non-medical applications. For example, the patterns made when X-rays scatter from a regular arrangement of atoms in a molecule or solid enable that arrangement to be determined, even though the atoms are spaced very, very close together, at distances more than 10,000 times smaller than a human hair. X-ray difraction images can reveal the very structure of the molecule or solid, with profound implications. Perhaps the most famous example is the structure of DNA molecules identified by James Watson and Francis Crick more than half a century ago, based on X-ray images taken by Rosalind Franklin (Figure 3) and Maurice Wilkins. Knowing how molecules replicate revolutionized biomedicine.

  These applications are indicative of the importance of light-in its broadest sense-in making possible our modern world, and impact our ability to enjoy it to its full. They rest on the fundamental work of a number of scientists in the 19th century-Michael Faraday,

  4. The spectrum of electromagnetic waves.

  Hans Christian Oersted, André-Marie Ampère, Charles Augustin de Coulomb, Alessandro Volta, Georg Ohm, James Clerk Maxwell, and Heinrich Hertz. That there exists a connection between visible light and other apparently disjoint things like microwaves and X-rays is remarkable, and it was the triumph of scientific enquiry by these men and others who made and identified these connections.

  The gamut of colours, or spectrum, provides a tool for art and science. Whereas a painter or artist explores ways in which colours themselves are juxtaposed or combined, a spectroscopist explores ways in which matter responds to diferent colours. For example, in the early 19th century Joseph von Fraunhofer determined some of the types of atoms that are present in the Sun by looking carefully at the particular colours of light that the Sun emits. He noticed that characteristic colours were missing from the Sun’s spectrum and noted that these colours were 『fingerprints』 for particular atoms. The study of spectra is the domain of spectroscopy, which uses light to identify diferent atoms or molecules. It is a vital research activity today, with impact in many areas from health monitoring to remote sensing of the atmosphere for pollutants.

  Apart from these familiar properties of light, there is one further property I want to point out. It, too, is something that we all know about from our everyday lives, though perhaps in a less explicit way than we experience the other properties of light. It is polarization.

  If you have watched a 3D movie, then you have seen this property being exploited. Watching such a movie requires you to wear special spectacles with cardboard or plastic frames that have pieces of plastic for the 『lenses』. If you take two pairs of these spectacles and slide the left lens of one over the right lens of the other, then look through them at a light bulb, you will see a very,very dim image of the bulb. Alternatively if you rotate one of the spectacles through 90 degrees with respect to the other and put either the two left lenses or the two right lenses over one another you will see something very similar-almost no transmission of light through the pair of glasses.

  This can be explained by assigning a property of 『orientation』 to the light. Most common light sources emit light without a preferred orientation. When you look at a light through these spectacles it will appear dimmer, indicating that they have selected a particular orientation. The left lens allows one orientation to pass, the right lens an orientation that is at right angles to the first. That’s why when you put a second lens oriented at right angles to the first, nothing will be transmitted through it: the light passing through the first lens has the 『wrong』 orientation to pass through the second. This characteristic of orientation is called polarization. It took a great deal of careful exploration to come up with and to understand the idea of polarization, but it is an important feature for technologies based on light, and indeed for understanding what light actually is.

  These physical characteristics of intensity, colour, and polarization are what enables light to be used to discern, to measure, and to control properties of material substances, and therefore to form the basis of a host of tools for studying and manipulating matter and even small objects. In the examples above, light almost always plays the role of an information carrier. Whether it is conveying an image or a spectrum or a phone conversation, it acts as a messenger. But light can also be used in an active way. For instance, the heating properties of light can be used to cut metal and other materials very precisely. Thick plates of metal, up to a centimetre or so, can be machined using high-power lasers more rapidly, with a higher quality finish and less waste, than using a saw. And light is used in medicine in various ways, from correcting vision through laser surgery to activating drugs for anti-cancer therapies.

  Light enables us to view the natural world at every time scale and distance scale imaginable; from the earliest moments of the universe to the unimaginably fast motion of electrons in atoms and molecules; from the large-scale arrangement of clusters of galaxies across the universe to the atomic arrangement of carbon atoms in graphene. It provides us with insight into the very foundations of the natural world, from the weirdness of quantum physics to the structure of DNA molecules.

  The story of optics is one in which new discoveries about light have enabled new technologies that have, in turn, given rise to new discoveries across many fields of science. At each stage, from the invention of eyeglasses to the most precise atomic clocks, to modern technologies for imaging, measuring, and communications, light has given new applications that have revolutionized how we live. This cycle of discovery and innovation makes the study of light a vibrant discipline, even as it is among the most venerable. How we came to a modern understanding of what light is, and therefore how we could use it, both for new understanding of the world and for new capabilities that change the world, is the tale that forms the rest of the book.