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Part I the nature of light






Home reading, Second Term

II. Text 1.

ELECTROMAGNETIC RADIATION

Electromagnetic Radiation - Energy resulting from the acceleration of electric charge and the associated electric fields and magnetic fields. The energy can be regarded as waves propagated through space (requiring no supporting medium) involving oscillating electric and magnetic fields at right angles to each other and to the direction of propagation. In a vacuum the waves travel with a constant speed (the speed of light) of 2.9970 x 108 metres per second; if material is present they are slower. Alternatively, the energy can be regarded as a stream of light, each

photon having an energy E=hυ =h/cλ, where h is the Plank constant, and λ is the wavelength of the associated wave. The characteristics of the radiation depend on its wavelength. (See electromagnetic spectrum.)

Electromagnetic Spectrum - The range of wavelength over which electromagnetic radiation extends. The longest waves (105 – 1013 metres) are radio waves, the next longest (10-3– 10-6 m) are infrared waves, then comes the narrow band (4– 7 x 10-7m) of visible light, followed by ultraviolet waves (10-7– 10-9m), X-rays (10-9– 10-11m), and gamma rays (10-11– 10-16m).

Ionizing Radiation - Radiation of sufficiently high energy to cause ionization in the medium through which it passes. It may consist of a stream of high-energy particles (e.g. electrons, protons, alpha-particles) or short-wavelength electromagnetic radiation (ultraviolet, X-rays, gamma-rays). This type of radiation can cause extensive damage to the molecular structure of a substance either as a result of the direct transfer of energy to its atoms or molecules or as a result of the secondary electrons released by ionization. In biological tissue the effect of ionizing radiation can be serious. This effect has been and is being studied and monitored.

Laser - an acronym for light stimulated emission of radiation. It is a light amplifier usually used to produce non-chromatic coherent radiation in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum.

П. Text 2.

PART I THE NATURE OF LIGHT

Light is defined as that portion of the electromagnetic spectrum that is visible to the human eye. Light varies in wavelength and thus appears as different colours to the human eye, from the longer wave lengths of red to the shorter wavelengths of violet. Red light has the lowest frequency, and the other colours increase in frequency through orange, yellow, green, blue, and violet. Red light has the longest wavelength at 0.7 micron (a millionth of a metre) and violet has the shortest wavelength at 0.4 microns. (Line 6)

Light travels through a vacuum at about 300, 000 kilometres per second. According to Einstein's Special Theory of Relativity, the speed of light is finite, nothing can ever travel faster. Understanding that the speed of light is the speed limit of the universe is basic to understanding Einstein's concept of the universe. (Line 10)

Despite our great scientific progress, we still do not fully understand the nature of light. Light is not the easiest of natural phenomena to describe. For many centuries, scientists have argued and disagreed over its nature. Sir Isaac Newton, following the then- contemporary view, theorized that light beams were travelling streams of corpuscles (particles). Others, such as Christian Huygens and Thomas Young, argued that light travelled in waves rather than particles. It was Albert Einstein who was able to synthesize these two views with his theory of the dual nature of light. (Line 17)

Today, light is considered to be both wave and particle; this perspective is called the wave-particle duality. Whether light needs to be described as a particle or wave depends on the experiment being performed. No matter which description is used, the answer will be the same. The debate regarding the nature of light is one of the most interesting in the history of science. It continued even while important discoveries concerning light were being made. Let us look at the historical, Newtonian perspective of light as summarized in the following reading. (Line 23)


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