Chapter 4-3 Applications of Light Waves

In addition to sound waves, light waves are the most important means for humans to perceive the world.

In physics, sound waves are often referred to as "Mechanical Waves", while light waves are called "Electromagnetic Waves". The former requires a medium (such as air, water, or solids) to propagate, whereas the latter can travel through a vacuum. Electromagnetic waves are composed of photons, which travel through space until they interact with matter. Some waves are absorbed, and others are reflected. Electromagnetic waves permeate this world in various forms, enabling you to see, feel, or remain unseen and unfelt.

Electromagnetic waves are typically divided into seven types: radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays (see Figure 4.5).

Figure 4.5: Electromagnetic Spectrum

• Radio Waves

Radio waves have the lowest frequency and the longest wavelength of all electromagnetic waves. Similar to sound waves, they possess strong penetrating power and experience minimal energy loss during transmission. The wavelength of radio waves can be very long, reaching hundreds or even thousands of meters per second. Not only do mobile phones and TV transmission towers generate radio waves, but many natural stones such as crystals, agates, and jade also emit radio waves. These natural gemstones are often worn for their calming or protective properties, and rarer types are frequently used by royalty or religious institutions as ritual objects or symbols of power.

Radio waves can be used to transmit information. Using the principle of resonance, radio waves can be transmitted or received via antennas, which then convey the signals to receivers that convert them into usable information. In our daily lives, televisions, mobile phones, wireless networks, walkie-talkies, and remote-controlled toys all use radio waves to transmit information. Information can be loaded onto radio waves through modulation, with common types including Amplitude Modulation (AM) and Frequency Modulation (FM), as used in radios.

The famous Voyager 1 spacecraft, launched in 1977, has been traveling for nearly 50 years and continues to communicate with NASA using radio waves. Voyager 1 is currently about 20-25 billion kilometers from Earth, and the radio signals it sends back take approximately 18-22 hours to reach Earth, traveling at the speed of light. Additionally, stars, planets, and other celestial objects emit radio waves. The repeated reception of mysterious signals from space has reinforced the belief in the existence of Aliens.

• Microwave

Microwaves have the second-lowest frequency in the electromagnetic spectrum. While radio waves can have wavelengths of several kilometers, microwaves have wavelengths between 1 millimeter and 1 meter. Due to their higher frequency, microwaves can penetrate obstacles that interfere with radio waves, such as clouds, smoke, and rain.

The household microwave oven is the most common application of microwaves. Conventional heating methods (such as flames or steam) use heat conduction and radiation to transfer heat to the surface of the heated object, and then through conduction, gradually increase the center's temperature. This is known as external heating. It requires a certain amount of time for heat conduction to raise the center to the desired temperature, especially for materials with poor thermal conductivity. Microwave heating, on the other hand, is an internal heating method where electromagnetic energy directly acts on the molecules of the medium, converting it into heat. Its penetration ability allows the material's internal and external mediums to heat simultaneously, eliminating the need for heat conduction. Due to the lack of cooling conditions inside, the internal temperature can easily exceed the external temperature, accelerating moisture migration and evaporation. This is why heating eggs and similar foods in a microwave can easily cause them to "explode" (it is generally recommended to cover dishes when placing them in the microwave to prevent excessive drying). Household microwaves are primarily used for heating food, defrosting and cooking, while industrial microwaves can be used for sterilization, dehydration, curing, and preservation.

Microwaves are extensively used in the field of communications. Microwave communication directly uses microwaves as the medium for communication. When there are no obstacles in the direct line of sight between two points, microwaves can be used for transmission. Microwave communication has the advantages of ultra-large bandwidth capacity, high precision, low latency, fast construction speed, stable communication process, and convenient maintenance, making it an important communication method for national communication networks. The mobile and broadcasting networks we use in our daily lives all employ microwave communication.

Because the propagation characteristics of microwaves in the air are similar to those of light waves, meaning they travel in straight lines and are reflected or blocked by obstacles, the primary method for digital microwave communication is line-of-sight communication. Due to the curvature of the earth and space transmission attenuation, long-distance communication requires relay transmission. This involves using base stations to repeatedly relay signals, allowing them to be transmitted over thousands of kilometers while maintaining high transmission quality.

Additionally, wireless network communication technology (WiFi) uses low-intensity microwaves to transmit information. Due to their inability to bypass obstacles, signals quickly attenuate between different rooms. Households often install multiple WiFi routers or add signal amplifiers to maintain signal stability.

• Infrared

Infrared (IR) is electromagnetic radiation with wavelengths ranging from 0.76 to 1000 micrometers (1 micrometer = 10^-6 meters), and it is invisible light with a lower frequency than red light. In Physics, any substance with a temperature above absolute zero (-273.15℃) can emit infrared radiation (as well as other types of electromagnetic waves), hence it is also called infrared radiation or thermal radiation. Infrared has a "Thermal Effect", which can cause resonance with most molecules, converting the energy of light (electromagnetic wave energy) into molecular internal energy (thermal energy). The heat from the sun is primarily transferred to the earth through infrared radiation.

The significant feature of infrared is its thermal effect. Unlike microwaves, which have wavelengths nearly a thousand times longer than infrared wavelengths and thus heat deeply, infrared heating is primarily surface heating. Longer wavelength infrared rays (far-infrared) generate more heat and are emitted by hot objects like fire, the sun, and incandescent lamps. Shorter wavelength infrared rays (near-infrared or short-wave infrared) do not produce as much heat and are used in remote control and imaging technologies.

Infrared applications are ubiquitous in daily life. For instance, high-precision detection and disease diagnosis convert infrared radiation images of objects into visual images; forehead thermometers measure temperature; night vision devices (surveillance cameras) are used in security; household infrared remote controllers control TVs and air conditioners.

Far-infrared rays, which penetrate the skin and subcutaneous tissues and have frequencies close to the vibration frequencies of molecules within human cells, are widely used in the medical field. They can raise the temperature of deep subcutaneous layers, dilate microvessels, help clear blood vessel deposits and harmful substances in the body, and assist in treating muscle soreness, pressure sores, burns, and slow-healing wounds.

Infrared night vision equipment also plays a crucial role in modern warfare. From the Gulf War in 1991 to the recent Russia-Ukraine conflict, US-supported firearms and artillery are equipped with infrared night vision devices. These devices can receive infrared radiation emitted by various substances and present them visually. Thus, even in night-time or smoke-filled battlefields, it is possible to detect enemies first, open fire, and gain a tactical advantage.

• Visible light

Visible light allows us to see the world around us. It is a very narrow band of the electromagnetic spectrum that can affect the human eye and produce vision. The frequency ranges from 380 to 750 terahertz (1 THz = 10¹² Hz), and the wavelength ranges from 400 to 780 nanometers (1 nm = 10^-9 meters).

The sunlight we see is typically described as white (or "colorless", hence the Buddhist saying "form is emptiness"), but it is actually composite light (or aggregate light) made up of a mixture of seven colors: red, orange, yellow, green, blue, indigo, and violet. In 1666, the British scientist Newton was the first to reveal the properties of light and the secrets of color. He demonstrated through "Prism Dispersion Experiment" that sunlight is mixed light of various colors and discovered that the color of light is determined by its frequency. Among them, red has the lowest frequency and the longest wavelength, while violet has the highest frequency and the shortest wavelength, with different frequencies manifesting as different colors.

On "Newton's Color Wheel" (Figure 4.6), if the circle is divided into six equal parts, each segment filled with red, orange, yellow, green, blue, and purple hues, it represents the relationships among primary colors, secondary colors, adjacent colors, contrasting colors, and complementary colors. Among these, red, green, and blue are the three primary colors that cannot be further decomposed and are known as the three primary colors of light. Any two primary colors mixed together produce a mixed color, and the combination of all three primary colors results in white. "Newton's color wheel" laid a theoretical foundation for the establishment of subsequent color systems, which later developed into the 12-color wheel, 24-color wheel, and 100-color wheel.

Figure 4.6: Newton's Color Wheel and the three primary colors of light

The most natural source of visible light is the sun. Rainbows, commonly seen after rain, result from sunlight entering the atmosphere and being refracted and reflected by water droplets, allowing us to see the seven colors (frequencies) that make up sunlight. Human eyes perceive objects as different colors based on the wavelengths of light they absorb and reflect. When sunlight shines on an object, the object absorbs certain colors, and the remaining colors are what we see. For example, we see green leaves because the physical properties of the leaves cause them to absorb other colors and only reflect green light. This reflected light enters the retina, forming a colored image, and we see green leaves.

Different colors absorb and reflect light differently. Red reflects 67% of light, yellow reflects 65%, green reflects 47%, and cyan reflects only 36%. Because red and yellow reflect light strongly, they are prone to glare and can be dazzling, which is why warning signs typically use red lights or red colors. Green and cyan have moderate light absorption and reflection, making them more suitable for the human nervous system, cerebral cortex, and retina. For example, grass and green trees can absorb harmful ultraviolet rays from strong light and significantly reduce the glare's stimulus on the retina. Therefore, it is recommended for teenagers and office workers to look at distant trees during breaks from studying or working. This helps relax the nerves and quickly relieves eye fatigue. However, looking at objects within 33 centimeters, even if green, has limited benefits for the eyes, so it is important to often gaze into the distance.

The advancement of science and the widespread use of electricity have led to the invention of many visible electric light sources, such as incandescent lamps, fluorescent lamps, LED lamps, and neon lights. Incandescent lamps were once the most produced and widely used electric light sources in China. However, their high filament temperatures cause most of the energy to be consumed as infrared radiation, resulting in a short lifespan of generally no more than 1,000 hours. In recent years, they have been replaced by LED lamps, which use solid-state semiconductor components (light-emitting diodes) to convert electrical energy directly into light. LED lamps are characterized by their simple structure, low cost, long lifespan, high luminous efficiency, and resistance to breakage, making them the preferred choice for household night-time illumination.

• Ultraviolet

Ultraviolet (UV) light is electromagnetic radiation with wavelengths ranging from 5 to 400 nanometers, making it invisible light with a higher frequency than blue-violet light. In 1801, German physicist Johann Wilhelm Ritter discovered that a segment of the sunlight spectrum beyond the violet end could sensitize photographic plates containing silver bromide, thereby discovering the existence of ultraviolet light. Since its discovery, ultraviolet light has been closely related to our lives.

The primary source of ultraviolet light in nature is the sun. Ultraviolet light can be further divided into four types: UVA (Ultraviolet A, with wavelengths of 320-400nm, low-frequency long wave), UVB (wavelengths of 280-320nm, mid-frequency medium wave), UVC (wavelengths of 100-280nm, high-frequency short wave), and EUV (10-100nm, extremely high frequency). Among these, UVA rays can cause skin pigmentation, leading to dark spots or tanning, and may potentially cause cancer by further damaging the chemical bonds of DNA. However, due to its ability to strongly stimulate the skin, accelerating metabolism and enhancing skin growth and thickness, it is also an important band for clinical treatment of skin diseases such as psoriasis and vitiligo. UVB has shorter wavelengths, and most of this type of ultraviolet light is absorbed by the epidermis of the skin, not penetrating deeper. Prolonged exposure, however, can burn the skin, causing erythema, inflammation, or skin aging. Although sunlight is milder in winter, UV radiation is only about 20% weaker than in summer, and even on cloudy days, UV levels can reach up to 90% of those on sunny days. Therefore, UV radiation can still harm the skin and eyes, making it advisable to apply protective creams or sunscreen during outdoor activities in winter to avoid prolonged UV exposure.

Fortunately, the ozone layer in the earth's atmosphere absorbs most of the UV radiation. In the stratosphere, some oxygen molecules can absorb UV light with wavelengths shorter than 290 nm, breaking down to form oxygen atoms. These oxygen atoms combine with oxygen molecules to create ozone (O₃), forming the ozone layer. Ozone can absorb UV light, breaking down again, and can also recombine with oxygen atoms to revert to oxygen molecules. Most of the UVC and EUV radiation in UV light is absorbed by the ozone layer and does not reach the earth's surface.

UV radiation has a strong stimulating effect on human eyes. Due to its short wavelength, high frequency, and high energy, it can penetrate the retina, and prolonged exposure can cause retinal damage. UV radiation is also the culprit behind "snow blindness". In places with strong sunlight reflection, such as snowfields and mountains, people's eyes may feel a sharp pain. Therefore, when climbing snowy mountains or exploring polar regions, protective goggles are often needed to prevent UV damage to the eyes. Ultraviolet light generated during welding processes can cause welders to suffer from arc eye (photokeratitis).

Electronic products often emit a small amount of ultraviolet light and a significant amount of violet and blue light, which are close to the UV spectrum. Prolonged use of electronic devices can cause irreversible damage to the eyes from these high-energy UV and blue-violet lights, leading to symptoms such as decreased vision, blurred vision, yellowing, and dimness, potentially causing retinal macular degeneration. Therefore, it is advisable to limit continuous use of electronic products to no more than three hours and take regular breaks to rest the eyes, look at green plants, or go outdoors to prevent eye strain. When using a phone in the dark, lower the screen brightness and enable eye protection mode (blue light filter) or night mode to effectively prevent damage from UV and blue-violet light.

Ultraviolet light has many applications in daily life. Besides its chemical action that sensitizes photographic film, it has a strong fluorescence effect. Fluorescent lamps, daylight lamps, and agricultural black lights used to attract and kill pests all use UV light to excite fluorescent materials to emit light. Most insects' compound eyes are particularly sensitive to 365nm UV light. At night, turning on a UV light creates a bright world for insects, thereby achieving the effect of attracting and trapping them. UV light can also be used for anti-counterfeiting, such as in currency verification. It has physiological effects as well; for instance, 10 minutes of outdoor activity under natural UV light daily can help adolescents synthesize the vitamin D they need to prevent rickets. In household and medical fields, UV light is commonly used for sterilization, disinfection, and deodorization.

UV light has strong particle properties and can cause various metals to exhibit the photoelectric effect. Extremely high-frequency UV light (EUV), also known as extreme ultraviolet, has a very high frequency, very short wavelength, and strong energy and resolution, making it suitable for photolithography machines used in the production of precision CPUs and various electronic chips. UV photolithography machines are cutting-edge equipment in chip manufacturing and are considered a bottleneck technology in China. The global chip manufacturing giant, the Dutch company ASML, launched a new extreme ultraviolet (EUV) photolithography machine in 2024, priced at 350 million euros and weighing 150,000 kilograms, equivalent to two Airbus A320 planes. Behind this weight is its irreplaceable advanced EUV photolithography technology.

• X-ray

We know that in the spectrum of light waves, the higher the frequency, the shorter the wavelength, and the greater the energy of the photons. X-rays are a type of electromagnetic wave with extremely high frequency, very short wavelength (ranging from 0.01 to 10nm), and significant energy.

In 1895, the German physicist Wilhelm Conrad Röntgen discovered X-rays (also known as "Röntgen Rays", with "Röntgen" later becoming a unit of radiation measurement) while researching cathode rays. Due to the differences in density and thickness between various human tissues, X-rays are absorbed to different extents when passing through different tissues. After imaging processing, different images can be obtained. Röntgen discovered that X-rays could penetrate a thousand pages of a book, 2-3cm thick wooden boards, several centimeters of hard rubber, and 15 mm thick aluminum plates, but they could be almost completely blocked by a 1.5mm thick lead plate. He also accidentally found that X-rays could penetrate muscles and reveal the outline of hand bones. He then asked his wife to place her hand on a photographic plate wrapped in black paper and irradiated it with X-rays for 15 minutes. The resulting film clearly showed the image of his wife's palm bones wearing a wedding ring (Figure 4.7). This photograph holds historical significance, as it demonstrated that humans could use X-rays to see through flesh and reveal bones. Röntgen was awarded the first Nobel Prize in Physics for this discovery.

Figure 4.7: Physicist Röntgen and his wife's palm bones

 

The combination of technology and fashion always leads the times. X-rays are invisible and imperceptible rays, yet they are filled with a sense of technological wonder, sparking infinite imagination. When a person is in an X-ray beam, they do not feel any discomfort in a short period, making high-tech products centered around X-rays extremely popular in Europe and the United States for a time. Reviewing the history of X-rays, we must admit that they are a "Double-edged Sword", capable of bringing benefits to humanity while also posing potential dangers.

Let's look at the classic scenes of the first "adopters" (Figure 4.8):

• Hair removal: Due to racial and dietary differences, hair removal is a perpetual topic in Europe and the United States. Before the discovery of X-rays, European and American women commonly used wax to pull out hair by the roots, which was quite painful. When scientists used X-ray irradiation for a total of 20 hours over 12 days to successfully remove the dense back hair of a large man, countless beauty-conscious women joined what seemed to be a "safe and harmless" hair removal movement.
• Shoe-fitting machine: Even more universally popular was the "X-rayshoe-fitting machine". This machine had a very simple structure: an X-ray tube was placed inside a wooden shell, and customers could simply put their feet into the box and clearly see the fit of the shoes through a lens on top of the box. Obviously, "shoe-fitting" was just a gimmick to attract customers. Due to its surprisingly good effect, the X-ray shoe-fitting machine almost became an essential feature in shoe stores of that time. At its peak in the 1950s, short-term sales of X-ray shoe-fitting machines in the United States once reached tens of thousands of units, with sales in the UK also reaching 3,000 units.
• X-raycamera: European and American royalty and aristocrats eagerly took X-ray portraits and wedding photos, quickly making X-ray photography a trend in high society and fashion. To keep up with the trend, people would take a skeletal X-ray photo showing their "inner-beauty" in addition to a wedding photo before getting married.
• Beauty pageant: In beauty pageants of the 1950s, contestants not only needed to have beautiful faces but also perfect postures. Judges used X-rays to assess the contestants' spines, using the curvature of the spine to evaluate the perfection of their standing posture.

Figure 4.8: X-ray Shoe-fitting Machine (top),

Bridal Photography and Beauty Contest (bottom)

Such an entertaining and groundbreaking innovation would certainly not be without the involvement of the contemporary inventor Thomas Edison. When Edison learned about this new technology, he began searching for fluorescent materials to improve image clarity, ultimately inventing the portable X-ray imaging machine. Each time it was started, his assistant would put his hand inside to "observe" if the machine was working properly.

Playing with fashion at the cost of life, X-ray machine practitioners and enthusiasts paid a heavy price. Just three to four years later, Edison experienced blurred vision in his left eye and gastrointestinal discomfort, while his assistant, after long-term exposure to X-rays, had both hands amputated, followed by the entire arm, and died shortly afterward. At St. George's Hospital in Hamburg, Germany, there stands an "X-ray Stele", engraved with the names of over 160 scientists and doctors from 15 countries, all of whom became "martyrs" due to X-rays. It is said that the Roentgen Society held a banquet in 1920, where all the attending experts were missing arms or hands, almost a complete defeat. They were unable to enjoy the delicious roast chicken served and many wept bitterly. Society at large also paid a "tuition fee" for following the trend; many of those who were enthusiastic about the shoe-fitting machines suffered from skeletal deformities, those who underwent hair removal developed terrible skin cancers, and others faced infertility, leukemia, and various cancers.

There is no easy path in science. As one of the greatest discoveries of the 20th century, X-rays, despite the irreversible harm they once caused to users, have allowed later generations to avoid pitfalls and risks, truly benefiting humanity. The hard work of the pioneers has made it possible for future generations to enjoy the shade of their trees. The application of X-rays has made significant breakthroughs in medicine and innovation, providing unprecedented insights into the human body's interior.

Today, the X-rays and CT scans we undergo during hospital check-ups are forms of X-ray examinations. The difference is that an X-ray captures a single "photo", whereas a CT scan takes multiple "photos" from different layers and angles, resulting in higher radiation levels than a standard X-ray, and is typically used for further diagnosis. Doctors can determine whether a specific part of the body is normal by analyzing the different density shadows displayed on a fluorescent screen or photographic film, combined with clinical symptoms and lab results, thus providing more effective treatment plans. During X-ray exposure, patients' non-exposed areas are covered with heavy lead aprons, and staff stand behind control panels to avoid scattered X-ray radiation. Since patients are not frequently exposed to X-rays and are usually only subjected to localized exposure, the risk of X-ray harm is minimal and generally safe.

• Gamma-ray

Gamma rays (γ-rays) are high-energy electromagnetic waves with wavelengths shorter than 0.2 nanometers. In 1900, French scientist Paul Villard discovered this phenomenon while experimenting with cathode rays from radium. In layman's terms, γ-rays are a type of nuclear radiation, high-energy electromagnetic radiation released during nuclear reactions, and one of the rays emitted during atomic decay.

Nuclear radiation is a term used to describe the energy (particle stream) emitted during the decay of unstable atomic nuclei. This energy can take various forms, including alpha (α), beta (β), and gamma (γ) rays. Alpha particles have weak penetrating ability and can be blocked by paper or the outer layer of human skin; beta particles do not cause much external harm to the human body and can be blocked by clothing or a few centimeters of metal. However, if these particles enter the body, they can cause severe damage to internal organs. Gamma rays, with extremely strong penetrating ability and high energy, pose significant dangers both externally and internally to the human body. Multiple layers of concrete can block most of it, but once it enters the body, it can disrupt cellular structure and function, leading to genetic mutations, loss of hematopoietic function, reproductive issues, and cancers.

Today, people are well aware of the dangers of nuclear radiation, but over 70 years ago, it was the high society of Europe and America, eager to embrace the "high technology" of the time, who paid the price. Back then, people did not know that radium was radioactive. Watch manufacturers coated dials with radium to achieve a "glow-in-the-dark" effect; fashionable women even used radium on their nails and lips as decoration.

Figure 4.9: U-238 Atomic Energy Laboratory Toy

Figure 4.9 shows the highly popular "U-238 Atomic Energy Lab" toy set, which encouraged future "little scientists" to bravely seek knowledge and explore by measuring the radioactivity of samples using enclosed instruments and observing the decay process of radioactive substances through these instruments. The accompanying instruction manual was 60 pages long. This high-tech toy was both "useful and affordable", selling over 5,000 sets at $50 each upon release. However, little did people know that mishandling these radioactive "gravel" toys could cause fatal harm to the body. Some highly toxic substances, such as polonium-210, are even more toxic than the well-known cyanide, with less than 0.1 micrograms being lethal to an adult. Fortunately, after people "woke up" to the dangers, such ignorant "dangerous toys" were finally withdrawn from the historical stage.

The primary groups affected by nuclear radiation are biological organisms. The danger lies in nuclear radiation's ability to cause both internal and external exposure using its rays (α, β, γ rays). Nuclear radiation can damage DNA, proteins, and other small molecules within organisms, thereby destroying their biological functions.

In 1986, the explosion at the Chernobyl nuclear power plant led to countless direct and indirect casualties and severely damaged the surrounding environment. To mitigate the damage, the Soviet government sealed the facility with reinforced concrete to contain the pollutants and control radiation contamination.

The movie "Oppenheimer" tells the story of the creation of the atomic bomb, which gave humanity the power to cause massive destruction. In 1945, the United States dropped atomic bombs on Hiroshima and Nagasaki, turning both cities to ashes as the mushroom clouds rose. The survivors of the time, having been exposed to severe nuclear radiation, suffered from painful diseases for the rest of their lives, finding life worse than death. The mayor of Hiroshima once said, "Japan is the only country that has suffered from nuclear weapons, and we hope that people will learn from this and prevent the recurrence of the Hiroshima tragedy."

However, the Japanese government contradicted itself. In 2011, an earthquake and tsunami in Japan caused the "Fukushima nuclear power plant" to explode. Ten years later, despite international opposition, the Japanese government decided to discharge millions of tons of nuclear-contaminated water into the sea, formally beginning the "ocean release" project in 2023. It is foreseeable that marine life will suffer massive deaths or mutations, leading to unpredictable negative impacts on other animals and humans higher up the food chain. As the memorable line from the movie "Oppenheimer" states, "You gave humanity the power of self-destruction, and the world was unprepared."

Technological inventions have always been a "Double-edged Sword". Although γ-rays are dangerous, their reasonable use can benefit humanity. For example, there are over 600 nuclear power plants worldwide (more than 50 in China, with plans to increase to 150 in the future). Rockets and spacecraft require nuclear fuel for launch. Astronomers have discovered that γ-rays can be emitted by high-energy cosmic objects such as pulsars, neutron stars, supernovae, and black holes, which can be used for star exploration and positioning. Additionally, lightning storms on Earth can also produce γ-rays.

In medicine, γ-rays can kill cells and thus can be used to target and kill cancer cells. In the 1980s, medical irradiation became popular, using γ-rays produced by Cobalt-60 to cause irreversible damage and destruction to microorganisms, thus sterilizing and disinfecting cancer treatment methods. My father underwent "Cobalt-60 Radiotherapy" to combat tongue base cancer, which controlled the cancer cells but also caused irreversible organ damage.