The Discrete Nature of Light
From his theoretical study of the emission of radiation by blackbodies Planck concluded that they do not emit or absorb energy in a continuous fashion but only discontinuously in discrete units, which later were called photons. This means that the energy transported by an electromagnetic wave is not continuously distributed over the wave front; it is located at discrete points (the photons) along the wave and moves with the wave. In 1905 Einstein used Planck's idea of a discrete nature for the emission of light to explain a phenomenon discovered in 1887, known as the photoelectric effect. This effect cannot be understood if light has only a wave nature. Since that time an extensive body of experimental and theoretical evidence has been collected to validate the photon concept of light.
What are some of the properties of photons? They move with the speed of light, have no inertia, are electrically neutral, and are massless. Picture a radiating body as emitting photons of differing discrete amounts of energy in all directions. The photons retain their energy while traveling through space. Their arrival rate, or flux, at any point in space decreases with the square of their distance from the radiating source. Hence the inverse-square law of light can be understood in terms of numbers of photons (brightness of radiation) as well as in terms of electromagnetic waves.
The energy of each photon is inversely proportional to its wavelength. The shorter the wavelength, the more energetic is the photon; the longer the wavelength, the less energetic is the photon. That is why, for example, very-short-wavelength X-ray and gamma-ray photons can destroy molecular structures in living tissue while photons of visible light cannot. If you find that talking about the wavelength of a photon while saying that photons are the localizations of energy in an electromagnetic wave seems paradoxical, then you are perfectly normal. Just as in our discussion of relativity in Chapter 3, where we encountered a reality not evident in our human existence, so it is with our conceptual picture of light as both wave and particle. Wavelength is a characterization of the wavelike properties of light, while the energy content of a photon refers to its discrete nature. The fact that swe can link wavelength and energy content in a mathematical equation is strong validation of our conceptual picture.
Photons may be absorbed by an atom, scattered by particles of matter, or converted into matter by interaction with other photons. They are created inside atoms and in violent collisions between subatomic particles. When they lose their identity, they transfer their energy to some other physical system; and when they are created, they obtain their energy from some other physical system. Thei r creation and destruction is a classic example of the conservation of energy. The concept of light as being simultaneously both discrete photons and continuous waves is not contradictory but is a reality not borne out in our everyday life. Yet experiments are designed to inquire about either light's wave nature or its corpuscular nature; no experiment will simultaneously yield the discrete and the wave properties of light.
The theory of the discrete nature of light began a conceptual revolution in twentieth-century physics and astrophysics. It was used by Niels Bohr to formulate a new model for the atom that can be used to understand how light is created and destroyed inside the atom.
Creation and Destruction of Photons
From his theoretical study of the emission of radiation by blackbodies Planck concluded that they do not emit or absorb energy in a continuous fashion but only discontinuously in discrete units, which later were called photons. This means that the energy transported by an electromagnetic wave is not continuously distributed over the wave front; it is located at discrete points (the photons) along the wave and moves with the wave. In 1905 Einstein used Planck's idea of a discrete nature for the emission of light to explain a phenomenon discovered in 1887, known as the photoelectric effect. This effect cannot be understood if light has only a wave nature. Since that time an extensive body of experimental and theoretical evidence has been collected to validate the photon concept of light.
What are some of the properties of photons? They move with the speed of light, have no inertia, are electrically neutral, and are massless. Picture a radiating body as emitting photons of differing discrete amounts of energy in all directions. The photons retain their energy while traveling through space. Their arrival rate, or flux, at any point in space decreases with the square of their distance from the radiating source. Hence the inverse-square law of light can be understood in terms of numbers of photons (brightness of radiation) as well as in terms of electromagnetic waves.
The energy of each photon is inversely proportional to its wavelength. The shorter the wavelength, the more energetic is the photon; the longer the wavelength, the less energetic is the photon. That is why, for example, very-short-wavelength X-ray and gamma-ray photons can destroy molecular structures in living tissue while photons of visible light cannot. If you find that talking about the wavelength of a photon while saying that photons are the localizations of energy in an electromagnetic wave seems paradoxical, then you are perfectly normal. Just as in our discussion of relativity in Chapter 3, where we encountered a reality not evident in our human existence, so it is with our conceptual picture of light as both wave and particle. Wavelength is a characterization of the wavelike properties of light, while the energy content of a photon refers to its discrete nature. The fact that swe can link wavelength and energy content in a mathematical equation is strong validation of our conceptual picture.
Photons may be absorbed by an atom, scattered by particles of matter, or converted into matter by interaction with other photons. They are created inside atoms and in violent collisions between subatomic particles. When they lose their identity, they transfer their energy to some other physical system; and when they are created, they obtain their energy from some other physical system. Thei r creation and destruction is a classic example of the conservation of energy. The concept of light as being simultaneously both discrete photons and continuous waves is not contradictory but is a reality not borne out in our everyday life. Yet experiments are designed to inquire about either light's wave nature or its corpuscular nature; no experiment will simultaneously yield the discrete and the wave properties of light.
The theory of the discrete nature of light began a conceptual revolution in twentieth-century physics and astrophysics. It was used by Niels Bohr to formulate a new model for the atom that can be used to understand how light is created and destroyed inside the atom.
Creation and Destruction of Photons
