experiment 6

# experiment 6 - EXPERIMENT 6 Atomic Spectra INTRODUCTION...

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Experiment 6- Page 1 EXPERIMENT 6 Atomic Spectra INTRODUCTION Visible light is the portion of the electromagnetic radiation spectrum to which the eye is sensitive. Figure 1 indicates that visible light is only a narrow band of electromagnetic radiation. Any portion of this radiation can be characterized by either a wavelength ( λ ) or a frequency ( ν ). The speed of electromagnetic radiation ( c ) is a constant that relates to the wavelength and frequency through the following equation: c = λ ν (1) The value of c is 2.998 × 10 8 m/s. FIGURE 1

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Experiment 6- Page 2 As illustrated in Figure 2, the wavelength is defined as the distance between two successive crests. The frequency is the number of waves passing a given point in one second. FIGURE 2 In 1900, Max Planck discovered that the energy E of a single frequency of electromagnetic radiation ( ν ) is given by the equation: E = h ν (2) where h is a proportionality constant called Planck's constant, which has a value of 6.626 × 10 34 J s/photon. It is a very simple operation to rewrite this equation in terms of wavelength. From Equation (1), ν= c / λ . Therefore: E = λ hc (3) Since both h and c are constants, this new equation shows us that the energy is proportional to the reciprocal of the wavelength, and that short wavelengths have more energy than long ones. Thus, cosmic rays are a very high energy form of radiation. Fortunately because of our atmosphere, the ozone layer in particular, these rays do not reach the surface of the earth. Nor, in fact, does any radiation of wavelengths of approximately 10 7 m or less. On the other hand, radio waves are of low energy and are not likely to be very damaging. The spectroscope was invented in 1859 by Bunsen and Kirchhoff. This instrument passes a narrow slit of radiation through a prism or diffraction grating that refracts the radiation into a spectrum. Depending on what type of incident radiation is used and what sample (if any) is placed in its path, the spectrum will vary. The degree to which the radiation is refracted depends on its wavelength. A wave with a shorter wavelength is refracted to a greater extent than a wave with a longer wavelength. Ordinary white light consists of all wavelengths in the visible region. When a ray of white light is refracted, it is spread out into a wide band called a continuous spectrum . When an element is heated sufficiently to give off light and the emitted light is passed through a prism or grating, the spectrum displayed is in the form of a few very specific lines rather than a continuous spectrum. This means that only certain wavelengths (or energies) of light are emitted. Each element has a unique characteristic line spectrum which can be used in identifying the presence (or absence) of the element in a sample. The intensity of the lines, when compared with calibration standards, can also be used to determine the amount of the element present in a sample. These techniques are routinely used today for both
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experiment 6 - EXPERIMENT 6 Atomic Spectra INTRODUCTION...

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