Материал: Андрианова English Thesaurus of Student-Physicist 2013

P A R T V

Light and Optics

UNIT 22

REFLECTION AND REFRACTION OF LIGHT

Read the following texts. Study the thesaurus on fig. 22, answer the questions after the texts

Light has a dual nature. In some experiments it acts like a particle, while in others it acts like a wave. In this part of the book, we concentrate on the aspects of light that are best understood through the wave model. First we discuss the reflection of light at the boundary between two media and the refraction (bending) of light as it travels from one medium into another. We use these ideas to study the refraction of light as it passes through lenses and the reflection of light from mirrored surfaces. Finally, we describe how lenses and mirrors can be used to view objects with telescopes and microscopes and how lenses are used in photography. The ability to manipulate light has greatly enhanced our capacity to investigate and understand the nature of the universe.

The nature of light

Until the beginning of the 19th century, light was modeled as a stream of particles emitted by a source that stimulated the sense of sight on entering the eye. The chief architect of the particle theory of light was Newton. With this theory, he provided simple explanations of some known experimental facts concerning the nature of light-namely, the laws of reflection and refraction.

Most scientists accepted Newton's particle theory of light. During Newton's lifetime, however, another theory was proposed. In 1678, the Dutch physicist and astronomer Christian Huygens (1629-1695) showed that a wave theory of light could also explain the laws of reflection and refraction. The wave theory didn't receive immediate acceptance, for several reasons. First, all the waves known at the time (sound, water, and so on) traveled through some sort of medium, but light from the Sun could travel to Earth through empty space. Further, it was argued that if light were

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some form of wave, it would bend around obstacles; hence, we should be able to see around corners. It is now known that light does indeed bend around the edges of objects. This phenomenon, known as diffraction, is difficult to observe because light waves have such short wavelengths. Even though experimental evidence for the diffraction of light was discovered by Francesco Grimaldi (1618-1663) around 1660, for more than a century most scientists rejected the wave theory and adhered to Newton's particle theory, probably due to Newton's great reputation as a scientist.

The first clear demonstration of the wave nature of light was provided in 1801 by Thomas Young (1773-1829), who showed that under appropriate conditions, light exhibits interference behavior. Light waves emitted by a single source and traveling along two different paths can arrive at some point and combine and cancel each other by destructive interference. Such behavior couldn't be explained at that time by a particle model, because scientists couldn't imagine how two or more particles could come together and cancel each other. The most important development in the theory of light was the work of Maxwell, who predicted in 1865, that light was a form of high-frequency electromagnetic wave.

Although the classical theory of electricity and magnetism explained most known properties of light, some subsequent experiments couldn't be explained by the assumption that light was a wave. The most striking of these was the photoelectric effect, discovered by Hertz. Hertz found that clean metal surfaces emit charges when exposed to ultraviolet light.

In 1905, Einstein published a paper that formulated the theory of light quanta ("particles") and explained the photoelectric effect. He reached the conclusion that light was composed of corpuscles, or discontinuous quanta of energy. These corpuscles or quanta are now called photons to emphasize their particle-like nature. According to Einstein's theory, the energy of a photon is proportional to the frequency of the electromagnetic wave associated with it, or: E=hf, where h - is Planck's constant.

Reflection and refraction

An important property of light that can be understood based on common experience is the following: light travels in a straight-line path in a homogeneous medium, until it encounters a boundary between two different materials. When light strikes a boundary, it is either reflected from that

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boundary, passes into the material on the other side of the boundary, or partially does both.

The preceding observation leads us to use what is called the ray approximation to represent beams of light. For example, a beam of sunlight passing through a darkened room traces out the path of a light ray. We will also make use of the concept of wave fronts of light. A wave front is a surface passing through the points of a wave that have the same phase and amplitude. The rays, corresponding to the direction of wave motion, are straight lines perpendicular to the wave fronts. When light rays travel in parallel paths, the wave fronts are planes perpendicular to the rays.

When a light ray traveling in a transparent medium encounters a boundary leading into a second medium, part of the incident ray is reflected back into the first medium. The reflected rays are parallel to each other. The reflection of light from such a smooth surface is called specular reflection. Reflection from any rough surface is known as diffuse reflection.

You may have noticed a common occurrence in photographs of individuals: their eyes appear to be glowing red. This occurs when a photographic flash device is used and the flash unit is close to the camera lens. Light from the flash unit enters the eye and is reflected back along its original path from the retina. This type of reflection back along the original direction is called retroreflection.

When light passes from one transparent medium to another, it's refracted because the speed of light is different in the two media. The index of refraction, n, of a medium is defined as the ratio c/v, where с – is speed of light in vacuum, v – is speed of light in a medium. If we make carefu l measurements, however, we find that the index of refraction in anything but vacuum depends on the wavelength of light. The dependence of the index of refraction on wavelength is called dispersion.

When light strikes a prism, a ray of light of a single wavelength emerges bent away from its original direction of travel by an angle, called the angle of deviation. Prisms are often used in an instrument known as a prism spectrometer. The dispersion of light into a spectrum is demonstrated most vividly in nature through the formation of a rainbow, often seen by an observer positioned between the Sun and a rain shower.

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Fig. 22. Thesaurus for Unit 22

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Huygens' principle

The laws of reflection and refraction can be deduced using a geometric method proposed by Huygens in 1678. Huygens assumed that light is a form of wave motion rather than a stream of particles. He had no knowledge of the nature of light or of its electromagnetic character. Nevertheless, his simplified wave model is adequate for understanding many practical aspects of the propagation of light.

In Huygens' construction, all points on a given wave front are taken as point sources for the production of spherical secondary waves, called wavelets, which propagate in the forward direction with speeds characteristic of waves in that medium. After some time has elapsed, the new position of the wave front is the surface tangent to the wavelets.

Total internal reflection

An interesting effect called total internal reflection can occur when light encounters the boundary between a medium with a higher index of refraction and one with a lower index of refraction. Total internal reflection occurs only when light attempts to move from a medium of higher index of refraction to a medium of lower index of refraction.

Interesting application of total internal reflection is the use of solid glass or transparent plastic rods to "pipe" light from one place to another. Such a light pipe can be quite flexible if thin fibers are used rather than thick rods. If a bundle of parallel fibers is used to construct an optical transmission line, images can be transferred from one point to another. Very little light intensity is lost in these fibers as a result of reflections on the sides. Any loss of intensity is due essentially to reflections from the two ends and absorption by the fiber material. Fiber-optic devices are particularly useful for viewing images produced at inaccessible locations. Physicians often use fiber-optic cables to aid in the diagnosis and correction of certain medical problems without the intrusion of major surgery.

1.Why does light have a dual nature?

2.What was the contribution of the Dutch physicist and astronomer Christian Huygens into the theory of light?

3.Describe the first clear demonstration of the wave nature of light provided in 1801 by Thomas Young.

4.What is the energy of a photon according to Einstein's theory?

5.What is retroreflection?

6.Where the effect of total internal reflection is used?

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