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Optics, Refraction and Reflection

  The science of optics has its roots in the 17th century, when scientists like Galileo, Huygens, Snell and Newton worked to understand the nature of light. Newton\'s Opticks was considered to be the definitive work upon the subject, but it contained one major flaw. Newton believed that light was made of many tiny particles, whereas Huygens believed that light was a wave. Such was Newton\'s standing that the wave picture of light was not considered for another century, when Young and Fresnel showed that light exhibited wave-like properties. Scientists now believe that both the wave and the particle pictures are valid, but the particles that we know today bear little resemblance to those that Newton envisaged.

Optics in the 20th century has been revolutionized by quantum mechanics. Classical optics reached a triumphant conclusion with the work of James Maxwell, who showed that light was an electromagnetic wave and correctly predicted its velocity. Thereafter the work of Einstein, Planck, Bohr, de Broglie and Heisenberg produced the quantum theory of light. This work led to the development of the laser in the 1960s, which has become a powerful new optical tool.

Two of the more important optical phenomena are refraction and reflection. Refraction occurs when light passes into a medium in which it travels at a different speed. Light travels fastest in a vacuum; this speed cannot be exceeded. In other media, such as air or glass, light travels more slowly. The ratio of its vacuum velocity to its velocity in another medium is known as the refractive index of the medium. Thus we may say that the refractive index of glass is about 1.5, because light travels 1.5 times more slowly in glass than in vacuum. When light passes at an angle into glass, its path is bent, due to the refraction of its wavefront. The diagram on page 530 illustrates what happens. Refraction is not unique to light. All wave-like phenomena, like sound or water waves, can refract under the right circumstances.

Reflection is a very different process. The simplest type of reflection is from a flat or plane, smooth surface, in which case the angle of incidence is always equal to the angle of reflection. Reflection occurs from two types of surface: conducting (usually metallic) and dielectric (usually transparent). With a conducting surface, the reflection is almost total, that is, very nearly all of the incident energy of the wave is contained in the reflected wave. With a dielectric material, the proportion of light that is reflected depends upon the angle of incidence and the refractive index of the material. Thus if the right angle is used, it is possible to have a transparent material reflecting nearly all of the incident light and transmitting none.

Both reflection and refraction also depend upon the frequency of the light. Thus some materials are transparent to one wavelength, but opaque to others. A good example is window glass—it is transparent to visible light but opaque to ultraviolet. JJ



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