The Story of Optics - Electromagnetic Waves

Gautam Gangopadhyay

Translation: Sunando Patra

Photo: Sunando

Once we accept that light is a wave, the question arises: what kind of wave is it? In the case of sound or water waves, the particles of the medium vibrate, which is why these are called mechanical waves. Scientists like Huygens suggested that light is a wave in the medium of ether. But ether can't be a regular substance. So, what exactly is a light wave?

Let’s briefly explore the history of another branch of physics: electromagnetism. People have known about the attractive force of magnets since ancient times, and they also knew that rubbing certain materials together could produce static electricity. In 1785, French scientist Charles Coulomb (1736-1806) demonstrated that the force between two charges is inversely proportional to the square of the distance between them and directly proportional to the product of the two charges. In 1791, Luigi Galvani (1737-1798) discovered electric current, although he didn't fully understand it. In 1800, Alessandro Volta (1745-1827) created the first battery, sparking a surge of research in electricity. Hans Christian Ørsted (1777-1851) and André-Marie Ampère (1775-1836) showed that an electric current creates a magnetic field. Michael Faraday (1791-1867) demonstrated that changing a magnetic field applied to a circuit induces an electric current, meaning it creates an electric field. James Clerk Maxwell (1831–1879) theoretically proved that the reverse is also true, that changing an electric field creates a magnetic field. This shows that electric and magnetic fields are not separate but are interconnected.

Faraday's Law - demonstration. (Source: YouTube)

Around the same time, various experiments showed that light has a special relationship with the electrical and magnetic properties of different materials. One of these discoveries was the photoelectric effect, where sunlight could be used to generate an electric current using a special type of battery. Michael Faraday demonstrated that the polarization of light depends on a magnetic field. John Kerr (1824-1907) showed that the speed of light in a medium could be changed by applying an electric field. Faraday suggested that light might actually be a vibration of electric and magnetic fields. He even questioned whether aether was necessary at all.

The James Clerk Maxwell Monument in Edinburgh, by Alexander Stoddart. (Source: Wiki)

One of the most significant discoveries in theoretical physics during the 19th century was the understanding of the nature of light. Inspired by Faraday, James Clerk Maxwell showed that electric and magnetic fields are closely connected. An electric and magnetic field can exist in any space, and when an electromagnetic wave passes through, the values of these fields increase and decrease. This is the vibration of the electromagnetic field.

Let’s understand a bit more about the nature of electromagnetic waves as explained by Maxwell’s theory. In the image above, an arrow shows the direction of an electromagnetic wave. The image illustrates that the magnetic field and the electric field both oscillate perpendicularly to the wave’s direction, and they are also perpendicular to each other. The distance between two wave peaks is the wavelength; the wavelength is the same for both the electric field and the magnetic field. Maxwell's theory allowed the calculation of the speed of electromagnetic waves using two properties of any medium. He showed that this speed is given by the formula \(1/\sqrt{\mu\epsilon}\), where \(\mu\) is the permeability (magnetic susceptibility) of the medium, and \(\epsilon\) is the permittivity (electric susceptibility) of the medium. When the values for a vacuum are used, the speed of the electromagnetic wave matches the speed of light in that medium. This led to the conclusion that light is, in fact, an electromagnetic wave. From then on, the study of light waves became a part of the field of electromagnetism.

The image clearly shows that electromagnetic waves are transverse waves because both types of oscillations (electric and magnetic fields) are perpendicular to the direction of wave propagation. Generally, the direction of the electric field is referred to as the polarization of the light wave. In linear polarization, the electric field remains in the same plane, and the magnetic field does too. In circular or elliptical polarization, these planes rotate with respect to the direction of propagation.

Our eyes can only detect a very small portion of the electromagnetic spectrum. For a long time, scientists were unaware of the existence of many types of radiation that we cannot see. Infrared and ultraviolet rays were discovered through astronomical observations. In 1800, the famous astronomer William Herschel (1738-1822) placed thermometers in different parts of the solar spectrum to measure temperature changes. He found that the temperature increased the most in the dark area just beyond the red light, which we now know is due to infrared rays, which have a longer wavelength than red light. The following year, German scientist Johann Wilhelm Ritter (1776-1810) observed that certain chemical reactions occurred more rapidly in the violet part of the solar spectrum and even more rapidly in the dark area beyond violet light. This was the first observation of ultraviolet rays, which have a shorter wavelength than violet light.

Radio waves have wavelengths ranging from fractions of a centimeter to many meters. In 1888, Heinrich Hertz (1857-1894) was the first to produce such waves in a laboratory setting. However, creating radio waves is not enough; you also need to detect them to prove they exist. The image below shows the equipment Hertz used in his experiments. The device on the left is called a spark gap transmitter, which generates the radio waves. When the switch (\(SW\)) is turned on or off, the electric current in the left coil changes. This change causes a high voltage in the right coil through electromagnetic induction, producing a spark across the gap labeled \(S\). This process is similar to how a car’s spark plug works. When this spark occurs, radio waves are emitted. By adjusting the capacitor (\(C\)) in the spark gap transmitter, Hertz could change the frequency of the radio waves produced. The circular circuit on the right acts as the receiver, and its size determines which frequencies of radio waves it can detect. If the transmitter and receiver are tuned to nearly the same frequency, resonance happens. In this case, an electric current flows through the receiver, and a spark appears across the gap labeled \(M\). Hertz observed this tiny spark in a dark room using a microscope. Today, we recognize that Hertz successfully created and detected high-frequency radio waves through these experiments.

Hertz's apparatus to produce radio waves in a laboratory. (Source: Wiki)

In this context, it’s important to mention Jagadish Chandra Bose (1858-1937). Like Hertz, Bose used a receiver in his experiments. Early researchers in radio waves faced a common problem: the metal spheres used in the spark gap (S) would quickly wear out and become uneven, making it difficult to stop the electric spark promptly. To solve this, Bose used platinum spheres, as platinum is highly resistant to wear, thus avoiding this issue. Instead of Hertz's circular receiver coil, Bose employed a spiral spring circuit. He was also the first to use a semiconductor as a receiver. The radio waves he generated had wavelengths ranging from five to twenty-five millimeters, which we now refer to as millimeter waves. Through his experiments, Bose demonstrated the reflection, refraction, and polarization properties of these waves.

Instruments used by Jagadish Chandra Bose, now in Bose Institute, Kolkata. (Source: Wiki)

In 1895, German scientist Wilhelm Röntgen (1845-1923) discovered X-rays. He was working with a cathode ray tube, which is a device where a high voltage is applied to gas at very low pressure. This causes the gas atoms to break apart into ions and electrons. These electrons, propelled by the high voltage, strike the target within the tube with great force, resulting in the emission of X-rays. However, electrons hadn’t been discovered at that time, so Röntgen couldn't provide this explanation. Röntgen observed that X-rays have a very high penetrating power—they can pass through paper, cardboard, and even muscle tissue, but are blocked by metal or bone. X-rays also leave an image on photographic plates. A famous photograph taken with X-rays was of the hand of Röntgen’s wife, where the bones and her ring were clearly visible.

Röntgen didn’t fully understand the nature of his discovery, so he named it X-ray, meaning “unknown ray”. In 1906, British scientist Charles Barkla (1877-1944) demonstrated that X-rays have polarization properties. Later, in 1912, German scientist Max von Laue (1879-1960) showed that X-rays can also undergo diffraction. However, because X-rays have very short wavelengths, their diffraction can’t be observed through ordinary openings. Only the tiny spaces between atomic layers in crystals can show this diffraction effect.

Röntgen’s discovery inspired many scientists to work on X-rays. In 1896, in France, Henri Becquerel (1852-1908) placed a photographic plate and some potassium uranyl sulfate compound inside a drawer. Later, when he examined the plates, he found that the compound emitted a new type of radiation, leaving an impression on the plates. Upon further investigation, Becquerel realized that this radiation came from uranium and was different from X-rays, as it carried a charge. X-rays are electrically neutral and are a type of electromagnetic wave. In 1898, French scientist Paul Ulrich Villard (1860-1934) showed that radioactive substances emit not only charged particles but also neutral, or uncharged, radiation.

Ernest Rutherford (1871-1937) discovered that charged radiation could be divided into two types. One type of radiation is easily absorbed by materials, while the other has a longer range. Rutherford named these two types of radiation alpha (\(\alpha\)) and beta (\(\beta\)) rays. Interestingly, Becquerel had noticed two different levels of penetration in the radiation two years earlier but hadn't thought to categorize them separately. In 1900, Villard demonstrated that neutral radiation had even greater penetration power than beta rays. Rutherford named this third type of radiation gamma (\(\gamma\)) rays. It took more time to understand the true nature of gamma rays, but finally, in 1914, Rutherford and Edward Andrade (1887-1971) showed that gamma rays could be reflected from surfaces. Since reflection is a property of light, it became clear that gamma rays are a type of light or electromagnetic wave.

Electromagnetic Spectrum (Source: Wiki)

Just like visible light, gamma rays, X-rays, and radio waves are all electromagnetic waves, but they have different wavelengths. This is clear from the image above. Maxwell’s theory made our understanding of light waves much clearer. Light is an electromagnetic wave, and so are radio waves—the only difference between them is their wavelength. As mentioned earlier, visible light has a wavelength between 400 to 700 nanometers (\(10^{-9}\) meters). Gamma rays have a wavelength measured in picometers (\(10^{-12}\) meters), while radio waves have wavelengths ranging from a few meters to hundreds of meters. However, one problem remained: what medium do these waves travel through? Despite many efforts, no evidence of the existence of ether was found. That story will be covered in the next chapter.

Let’s end this chapter with the story of the discovery of the electromagnetic waves that fill the entire universe. A hundred years after Maxwell, in 1964, two scientists at Bell Labs in New Jersey, USA, were testing a new horn antenna designed to detect microwaves. Jagadish Chandra Bose was the first to create a horn antenna, which he used for millimeter waves, but this had been largely forgotten. Millimeter waves fall within the range of microwaves. The two scientists kept detecting a signal on their device, but they couldn’t figure out its source. Initially, they thought it was just noise or interference in the circuit. They even suspected two pigeons that had nested in the antenna. However, after removing the pigeons and cleaning their droppings from the antenna, the signal persisted. They eventually realized that the signal wasn’t coming from any specific source but was equally present in all directions in the sky. After discussing with others, they understood that this was a remnant from the birth of our universe. When the universe was born from the Big Bang, it was incredibly hot, emitting gamma rays as the initial radiation. As the universe expanded, it cooled down, and the wavelength of that radiation stretched, transforming into microwaves. This is known as the Cosmic Microwave Background. From microwave observations, we now know that the temperature of the universe is 2.72 Kelvin (or -270.43 degrees Celsius). For this discovery, Arno Penzias (born 1933) and Robert Wilson (born 1936) were awarded the Nobel Prize in 1978.

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