Part III the wave theory and the wave-particle duality

(1) An early proposal of the wave theory of light, as opposed to the corpuscular, was made by Christian Huygens in 1690. But it was not until 1803 that Thomas Young’s experiments in light diffraction and interference provided strong support for Huygen’s theory. Young’s research showed that light is a wave in motion. Other researchers added to the evidence; for example, Augustin Jean Fresnel, who showed that light is a transverse wave.

(2) In 1864 James Clerk Maxwell set forth the mathematical theory that led to the view that light is of electromagnetic nature, propagated as a wave from the source to the receiver. Each wave is made up of two components superimposed on one another: an oscillating electric field and a correspondingly fluctuating magnetic field. About twenty years later, Heinrich Hertz discovered experimentally the existence of electromagnetic waves at radio frequencies. He also demonstrated that electro­magnetic radiation had the classic properties of waves, including interference, refraction, reflection and polarization. Isaac Newton’s corpuscular view of light was shunted aside. In its place was a comprehensive wave theory which showed that all types of radiation, from candle light to radio signals, had the same nature.

(3) In 1895, William Conrad Rontgen discovered X-rays. He showed, among other things, that like light, X-rays were propagating in straight lines but, in contrast to light, penetrated through matter.

(4) At the turn of the twentieth century, a few puzzles still persisted which could not be explained by wave theory alone, for example the photoelectric effect. The research leading to an understanding of light and the emission and absorption processes which was conducted during the twentieth century has been of paramount importance. It began in 1900 with the development of quantum physics. The research reached a high peak in the 1920s and there was another high point in the mid-century years, which ended in the completion of the important Quantum ElectroDynamic (QED) theory.

(5) Discoveries relating to the particle nature of light also belong to this century. By 1910 a heated debate over the nature of X-rays occurred between two physicists; one holding that the rays are waves like light, the other that they consist of "streams of little bullets". In 1927, Arthur H. Compton found that there was a gradual change in the characteristics of X-rays in the extreme frequencies — a scattering of some parts of X-rays away from the beam direction, resulting in a longer wave length than the incoming radiation which could not be explained by wave theory. (This is now called the Compton effect). In 1938, Compton demonstrated that cosmic radiation consists of charged particles. He did so by using a spectrometer which showed that X-rays scatter as particles, a clear indication of the duality of light.

(6) At the beginning of the century, Max Planck formulated the quantum theory, one of the most important discoveries of the twentieth century. The term "quantum" was coined to mean the minimal amount by which certain properties, such as energy or angular momentum, can change. In waves and fields the quantum can be regarded as an excitation, giving a particle-like interpretation to the wave or field.6 Thus the quantum of the electromagnetic field is the photon and of the gravitational field is the graviton. Quantum theory laid the groundwork for Einstein's theoretical resolution of the particle vs wave paradox. Einstein proposed his theory of the duality of light in 1905, but it was not generally accepted until Arthur Compton's experiments showed that X-rays were behaving like individual solid particles.

(7) The explosion of scientific investigation and the resulting understanding of the photon's behavior and the nature of radiation provided insight into the way atoms are arranged in materials. Just as each human being has a unique set of fingerprints, each chemical element has a unique arrangement of its component parts. As a result, each is capable of absorbing and emitting only certain wave lengths of light.

(8) Although we have focused on visible light and X-rays in this section, scientists have learned a great deal about other forms of electromagnetic radiation and have been able to apply that knowledge. For example, much of what has been observed and learned about the structure of the cosmos in the past forty years has been revealed by examining data taken in wavelengths other than visible light.

(9) Let us now turn to the subject of lasers. The subject is just one example of the many that illustrate the application of theory and of experimental data to the 60 development of new fields and technologies.

Exercises based on PART III Exercise 1: Vocabulary

A. Which word underlined in the text is a synonym or near synonym for each of the following

process of energizing _______________

across, at right angles________________

inclusive, all-inclusive _______________

go through ____________________

sign _______________________

placed on top of ___________________________

data supporting a position ____________________

incorporation ________________________

show (v) ___________________

periodic motion _________________________

Exercise 2: Concepts

Match the following concepts with their definitions below:

1. DIFFRACTION

2. INTERFERENCE

3. OSCILLATION

4. FLUCTUATION

5. FREQUENCY

6. POLARIZATION

7. PHOTOELECTRIC EFFECT

8. PHOTON

9. QUANTUM

Definitions:

1. quantum of visible light or other electromagnetic radiation usually considered as an elementary particle.

2. process of confining the vibrations of the electric vector* of light waves to one direction.

3. spreading or bending of waves as they pass through a small hole or around the edge of a barrier.

4. random deviations in the value of a quantity about some average value.

5. number of cycles in a wave (or some other oscillation or vibration)per unit time.

6. general term for the indivisible unit of any form of physical energy.

7. periodic motion about an equilibrium position.

8. the interaction of two or more wave motions resulting in parallel dark and line bands or concentric circles.

9. liberation of electrons from a substance exposed to electromagnetic radiation

* vector - quantity possessing both magnitude and direction

Read the text “Unlocking the Comets’ Secrets”

PART I What Do We Know About Comets?

(1) Earliest man wondered at the objects he saw in the heavens above, particularly about those that we call comets. These would appear temporarily, shine brightly, and then disappear. Myths, predictions (often terrible) and explanations were developed to explain these unknown phenomena. In fact, curious and sometimes bizarre explanations are still given and unusual and catastrophic events linked to their appearance ¹. (line 5)

(2) Comets, unlike the other small bodies in the solar system, have been known since antiquity. In 240 ВС the Chinese recorded Halley's appearance. It is one of the brightest, most visible comets and makes a "regular" appearance every 76 years. Being a periodic comet (one whose orbital period is less than 200 years), it has been the source of theorizing and speculation by both serious stargazers and common folk alike. (line10)

(3) What is a comet? How many are there? Where do they come from? Until the

Twentieth century, little definitive was known about their paths, their composition, or 15 about their activities and "life". Indeed, it was during Halley's last appearance, in 1986, that a gigantic step was taken in understanding the nature of comets and a number of then-held hypotheses were confirmed, or discarded. Today we know quite a bit, although there are still holes in our knowledge. But even these may soon be filled. (line16)

(4) Comets are celestial bodies orbiting the sun. Their orbits are eccentric, highly elliptical paths that bring them very close to the Sun and then swing them away deeply into space, often beyond the orbit of Pluto ³. They contain a mixture of non-volatile grains or dust and frozen gases. Comet structures are diverse and very dynamic, but they all consist of a fuzzy "head" (comprising a frozen nucleus of dust and other materials, resembling a dirty snowball, surrounded by a cloud of diffuse material, called a coma) and a "tail" that tends to grow and be brighter as the comet approaches the Sun. Comets are invisible except when they are near the Sun; then some of the frozen materials vaporize. It is these gases and other granular particles that one sees in the tail. (line 25)

(5) The current, generally accepted view is that comets are the "leftovers" from the formation of planets and are made up of the original material comprising the Solar System. There may be billions of comets orbiting the Sun in the so-called Oort cloud ⁴, far beyond the planet Pluto. Occasionally the Oort cloud is disturbed and a comet escapes into the inner part of the Solar System. But there are questions, such as: how active or changeable the Oort cloud is and might it contain "new" comets that have come in from interstellar space. (line 31)

(6) A 1995 count catalogues 878 comets whose orbits have been calculated, at least roughly. Of these, 184 are periodic; some of the others may also be periodic, but not enough is known about their orbits. (line 34)

(7) About a dozen comets pass the Sun each year, but a truly brilliant one comes into public view about once a decade. Since Comet Halley in 1986, a number of other comets have been observed; three major ones will be mentioned. Comet Hyakutake in 1996 was described by astronomers as one of the grandest comets of the millennium. The Shoemaker-Levy 9, discovered only in 1993, violated the Roche limit ⁵ by passing too close to Jupiter and broke into 21 fragments. For the first time, astronomers observed the collision of two extraterrestrial bodies from ground-based telescopes and from several spacecraft. For six days in July 1994, the fragments burned up in Jupiter's atmosphere or impacted on its surface, creating craters and lines. Comet Hale-Bopp, discovered in 1995, was visible to the naked eye from July 1996 through October 1997. It provided a great deal of data, but is remembered because of its role in the suicide of a small cult ⁶. (line 45)

(8) The large amount of images and data gathered from the Halley Comet probes, particularly from the ESA's 1986 Giotto, contributed greatly to our understanding of comets. Giotto, in one of the most extraordinary achievements of recent astronomy, managed to photograph Comet Halley as it spit out jets of dust and vapour at rates as high as fifty tons a second ⁷. But there were few images with close-up views of the surface of the nucleus ⁸. The then-new information established its seeming composition (ice, snow, dust), colours, shape, and other superficial features. Still to be answered are questions about a comet's mass and density, what it is made of, and whether it has any internal structure. (line 53)

(9) The last-mentioned is of particular interest. Dr. Mayo Greenberg, Professor of Astrophysics at Leiden University, Holland, has long argued that comets could сану organic contents that could land on Earth in the form of dust. In his opinion, the nucleus has a fairly open structure— the inner core containing complex organic materials; the outer shell being made of ice, water, and sulphur dioxide. New evidence seems to indicate that comet dust contains some complex carbon compounds. One scientist has gone even further, stating that cosmic rays could turn simple carbon compounds into biological molecules. Thus, if interstellar dust holds bacteria from other planets—a massive if they might survive to infect the Earth. (line 61)