The complex fields of light and sound may be rooted in complicated scientific theory, but their applications are felt in everyday activities. The simple acts of watching television and grocery shopping have changed tremendously due to advancements in optics and acoustics. Attending a concert can be more than just an entertaining experience when one considers the architectural, visual, and acoustical feats that all employ light and sound technology. The degree to which these branches of physics have impacted modern life is difficult to estimate, and the scientific exploration of their concepts continues to alter the world around us.
Understanding sound waves is critical to anyone beginning to study sound. Plane waves spread through space as a plane rather than as a sphere of increasing radius. Plane waves, for their simplicity, are good models for clarifying the behaviour of sound waves in general and understanding terms such as wavelength, period, and frequency. While a plane wave travels through space in a linear fashion, more typically sound waves travel spherically, increasing in radius as they disseminate. These circular waves travel forward, creating new wave fronts.
Not every waveform travels through the air as sound does. Two identical waves traveling on the same line in opposite directions form a standing wave. Both its shape and frequency match those of its two comprising waves. The bounded setting that helps produce a standing wave can be found in string and wind instruments. In a string instrument, for example, a string is stretched by fastening each end to a fixed point, and when the string is plucked, it vibrates and sends identical waves in opposite directions. Wind instruments act as bounded mediums for standing waves in a slightly different way. Instead of two separate waves composing the standing wave, it is the original wave driven into one end of the instrument that reflects off the other end and travels in the opposite direction of its original route that produces the standing wave.
By applying the same principles to the human voice, it is easy to understand how different vowel sounds are formed. The vocal column is similar to a wind instrument in both structure and function. It is a closed-tube resonator, where one end, the vocal folds, is closed and the other end, the mouth, is open. The throat, mouth, and lips change shape, impacting the frequencies of the formants, determined by the tension in the vocal folds. The resulting variations in amplitude create different vowel sounds.
Understanding how sound works is only part of the study of sound. Like the proverbial tree in the woods, there must be a receptor. As such, the ear’s range of response is huge. The ear canal acts as a closed tube, and the threshold of hearing varies depending on sound frequencies. For a low frequency, such as that of a heartbeat, the threshold for hearing rises, which explains why the human ear cannot continually detect such sounds without outside devices. For the same reason, audio components contain a feature that increases the intensity of low frequencies when the volume or loudness intensifies. The musical result equalizes the proportion of treble and bass to the ear, even as the loudness increases.
The relationship between sound and hearing is analogous to the one between light and vision. Just as understanding how sound is created and disseminated is futile without the understanding of how the ear receives that information, a discussion of light behaviour needs to include some consideration of eye behaviour. It is that very relationship between the perception of light and the nature of light that perplexed the ancient world.
The ancient Greek philosophers made the first documented theories about the nature of light. Pythagoras, Empedocles, Epicurus, Euclid, and Ptolemy all proposed theories and conducted studies on how light and vision behaved. The nature of their studies differed in how each philosopher perceived the role of vision in the study of light. As a result, the human perception of light encumbered the study of the early theories of light.
The Islamic world furthered scientific progress after the decline of Greek philosophy. Leading theories on light shifted away from the Pythagorean model, and by the 11th century, mathematician and astronomer Ibn al-Haytham correctly reversed the Greek belief that light originated in the eye and found its way toward an object. While Ptolemy was one of the first to study the behaviour of light in terms of refraction and transmission, Ibn al-Haytham advanced those concepts by providing mathematical exploration of light reflection from spherical and parabolic mirrors. He also furthered the early Greek optical studies by providing advanced sketches of the human eye. The impact of these advances can be seen in Roger Bacon’s work on the dissemination of light through simple lenses in the 13th century.
By the 17th century the focus of the scientific world returned to Europe. Dutch inventors introduced the world to compound microscopes and to the telescope. The advances in light study continued with Galileo’s astronomical discoveries of Jupiter’s moons and Saturn’s rings, Johannes Kepler’s mathematical work in the focusing properties of lenses, and Willebrord Snell and Pierre de Fermat’s work on the path of light rays between two mediums. By the end of the century, Danish astronomer Ole Rømer estimated the value for the speed of light, discovering that its speed was finite. As these empirical studies took place, other leaders in scientific thought advanced the understanding of the physical behaviour of light. René Descartes understood light as a pressure wave, while English physicist Robert Hooke described light as a “rapid vibration of any medium through which it” passes and spreads.
By the advent of the 18th century Christiaan Huygens founded the first wave theory of light. Isaac Newton became the leading expert on the particle theory of light and discovered that white light consisted of different colours. The debate between the wave models of light and the particle models of light pervades the history of optical study. The field of geometrical optics focuses on the paths of light rays as they propagate through mediums and reflect, disperse, or come into focus. Electromagnetism studies light as a wave of electric and magnetic fields. The introduction of scientific work on the subatomic level in the 20th century further fueled the debate between wave and particle theories and led to the emergence of the quantum theory of light. This paradoxical study of light provides evidence that white light consists of particles with wavelike properties.
The study of the production, control, transmission, reception, and effects of sound is called acoustics. By studying the mechanical vibrations in the field of acoustics, physicists have contributed to developments in architecture, geology, and medicine. Architectural acoustics explores the concept of reverberation time. By studying the absorbers and reflectors in a room, an ideal construction can be reached for achieving optimal acoustic performances. If a room’s purpose is to host a speaking engagement, for example, the clarity of that sound is improved by short reverberation time and the room is constructed to that end. On the other hand, longer reverberation time is ideal in a room hosting a music performance.
Of course, not all sounds are audible to human beings. Vibrations of frequencies greater than 20 kilohertz, or ultrasonics, are greater than the upper limit of the audible range for humans. Some animals and insects can hear sounds in the human ultrasonic frequency range, a discovery that has led to practical applications such as roach and rodent repellent. These devices utilize loud sounds in a high frequency range as a form of pest control.
Ultrasonics also plays a role in sonar applications as well. The term sonar comes from the combination of “sound navigation and ranging.” By measuring the time it takes for transmitted pulses of sound or ultrasound to bounce off an object and return to the source, scientists and oceanographers have located lost ships, tracked submarines, uncovered explosive mines, and discovered optimal fishing spots for trade fishermen. Many burglar alarms also use ultrasonic technology.
The medical world has embraced ultrasonic technology as the nondestructive alternative to X-rays. Here again pulses of ultrasound are transmitted into the body, and the amount of time it takes for the ultrasound to reflect off the objects or organs it encounters reveals possible tumours or problems with blood flow. Heart valve defects and arterial diseases can be detected using ultrasonic techniques. In addition to diagnosis, doctors have also used ultrasound technology to treat ailments such as transmitting shock waves to destroy kidney stones and emitting heat from ultrasonic waves onto the area surrounding some cancerous tumours.
Just as some wave frequencies are greater than the range of human hearing, other wave frequencies fall below that range. These waves, known as infrasonics, occur in natural phenomena such as earthquakes, waterfalls, and volcanoes. Atmospheric infrasonics include wind and thunder. Some animals such as elephants are sensitive to infrasonics.
The flow of energy at the speed of light in the form of electric and magnetic fields that comprise electromagnetic waves is known as electromagnetic radiation. Electromagnetic radiation frequencies range from the low values of radio waves, television waves, and microwaves to the higher values of ultraviolet light, X-rays, and gamma rays. The various forms of radiation travel in different patterns at different speeds and lend themselves to various modern applications. Radio waves, for example, travel as frequency bands and reflect back to Earth by the ionosphere. Doctors use radio waves in conjunction with magnetic fields to produce MRI pictures of the human body. Microwaves, on the other hand, travel between parabolic dish antennas. In addition to their well-known function of heating food, microwave radar can guide airplanes and ships. At the highest end of the electromagnetic radiation spectrum, gamma rays are between 10,000 and 10,000,000 times more energetic than visible light. Their highly penetrating power makes gamma rays simultaneously hazardous and beneficial. Careful modern applications include the sterilization of medical supplies and the destruction of organisms that cause food spoilage.
A laser stimulates atoms or molecules to emit light at specific wavelengths. The result is usually a narrow beam of radiation. Lasers were invented in 1960 and have gradually developed varied and useful applications. In construction, a laser’s visible red beam can project straight lines for alignment and surveying. Doctors use lasers for eye surgeries as the favourable and less invasive alternative to cutting into the eye. Other innovations credited to laser technology include compact disc players, supermarket checkout scanners, and military target designators. The types of lasers vary depending on the medium that generates the beam. Crystals, glasses, gases, and liquids can all serve as laser media.
Optics is the comprehensive study of light, including its genesis, propagation, behaviour, and effects. Physical optics focuses on light’s nature and properties, while geometrical optics deals with the laws and principles that direct and explain the properties of light media. An optical image occurs when a lens or mirror system reflects, refracts, or diffracts light waves to form a reproduction of an object. Images are described as either real or virtual, depending on where they are formed. A real image is formed outside of the instrument, such as on a video screen. A virtual image is formed inside the instrument and is seen by looking into an eyepiece, such as with a microscope.
Optical systems comprise various components such as lenses, mirrors, light sources, and fibre-optic bundles, to name a few. Each element of the system serves a specific function. Plane mirrors, for example, may be needed to reverse an image. Nonclassical imaging systems include bifocal and trifocal spectacle lenses. Modern innovations in optics have led to the application of holography in the nondestructive testing of materials, such as analyzing auto tires for structural flaws.
Advancements in the study of sound and light have impacted modern life. The fields of entertainment and communication have seen improvement in the way information is delivered. Progress in acoustics and optics has also improved military, maritime, and architectural functions. This volume will trace those developments and explore the laws and concepts that govern the fields of sound and light, providing biographical sketches of those who have impacted these two branches of physics.