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S. Stübinger et al. (eds.)Lasers in Oral and Maxillofacial Surgeryhttps://doi.org/10.1007/978-3-030-29604-9_1
1. Physical Fundamentals
Abstract
The chapter gives a short introduction into the physical fundamentals of light propagation and the interaction of light with matter. The chapter is neither a strict scientific description nor does it replace a textbook. It should only help the reader to understand the book more easily, and it should be a starting point for further studies.
Physical fundamentals of lightLight propagationLight-matter interaction
1.1 Prequel
This chapter tries to summarize important physical fundamentals to enable the reader to understand the remainder of this book. However, it cannot replace a good textbook on optics or describe these fundamentals in a strict scientific manner. For this, the bibliography of the chapter contains several references: [1–3].
Light is a physical phenomenon that is fundamental to human life. For example, the energy from the sun is transferred to the earth by light, and furthermore, approximately 80% of the information input to humans is by light through the eyes. Due to this, for several millennia, humans have been thinking about the nature, behavior, and properties of light.
1.2 Basic Properties of Light
Over time, several models have been developed, which allow to explain light. The simplest model uses the so-called geometrical optics.
1.2.1 Geometrical Optics: Light as Rays
Geometrical optics assumes that light consists of rays. This means that light starts at a certain point and propagates in a straight line until it hits a surface that absorbs it or changes its direction. Geometrical or ray optics can be used to explain phenomena of light reflection or refraction, so the basic behavior of optical elements like lenses, mirrors, or prisms can be explained. However, effects like diffraction or polarization cannot be described by ray optics, neither is there a good explanation for the idea of “color” in terms of ray optics. To explain these effects, a different model is needed, which describes light as waves.
1.2.2 Wave Optics
The base of wave optics is the Maxwell equations [4]. This is a set of partial differential equations that describe the behavior of electromagnetic fields, and as light is an electromagnetic field, light propagation can be described by the Maxwell equations. Making a few assumptions (nonconducting medium, no space charges), two equations can be derived from the Maxwell equations, which are similar to wave equations, and thus, light can be described as a wave. Two equations are needed because light consists of both an electric field/wave and a magnetic field/wave in terms of wave optics. These two waves oscillate perpendicular to each other as well as to the propagation direction of light, so light is a traversal wave: it oscillates perpendicular to its propagation direction. As the electric and magnetic radiation fills the whole space, they are also called fields. Thus, in terms of wave optics, light consists of an electric field and a magnetic field, which both contribute to the behavior of light. Both fields are vector fields, i.e., at each point, the field has not only a value but also a direction.
The remainder of this chapter will concentrate on the electric field because the electric field/wave is often better suited for an explanation for the behavior of light. Besides this, the direction of the electric field also defines the polarization of a light wave. A light wave with linear polarization has an electric field where the vectors of the field always point in the same direction. Another important property is the wavelength of light: it is a physical unit associated with its color. By dividing the speed of light by its wavelength, one gets the associated frequency of the light wave.
While wave optics covers many effects, some effects cannot be explained by it. One example is the photoelectric effect: by irradiating matter with light, it is possible to break away electrons. If this effect is present, it shows a threshold regarding the wavelength: above a certain wavelength, it does not happen, not even at higher intensities.
1.2.3 Photons
This photoelectric effect can be explained by assuming that light consists of particles, so-called photons. A photon has a certain energy depending on its wavelength. If this energy exceeds the energy needed to break the bonding of an electron to its atom, the electron can be released from the atom when the photon is absorbed by the atom. This effect is hard to explain by ray or wave optics. So in this case, it is useful to assume that light consists of photons.
In general, it cannot be said that a certain model of light is the best model for all purposes. It depends on the application and/or the effect that shall be explained which model is the most useful.
1.3 Light Propagation
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