E6-Atomic Spectra-Rev1 - Nanyang Technological University...

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Unformatted text preview: Nanyang Technological University School of Physical and Mathematical Sciences Division of Physics and Applied Physics PAP 119 Physics Lab Ib Experiment 6: Atomic Spectra Background Emission Spectrum Every atom has a set of discrete energy levels occupied by its electrons. When an electron makes a transition from a higher energy level to a lower energy level, a photon is emitted. The wavelength λ of the photon is related to the change in energy Δ E of the electron by Δ E = hc λ . (1) Because there are many possible transitions between the energy levels in a given atom, photons of di ff erent frequencies will be produced. An atom’s emission spectrum is the set of all these photon frequencies. Since di ff erent elements have di ff erent atomic structures and di ff erent atomic energy levels, the spectrum will be di ff erent for each element. In this way, an emission spectrum can be used to identify an element. However, the intensities of each spectral line will depend on the quantum mechanical probabilities of electrons making transitions between particular states. Therefore, intensities of spectral lines are hard to calculate. Nevertheless, tables that describe the relative intensities of the spectral lines for di ff erent elements are available in this experiment. Spectrometer A spectrometer is an instrument for analyzing the spectra of radiations. Various forms of the spec- trometer are used to study di ff erent parts of the electromagnetic spectrum and for di ff erent purposes. In its simplest form, a spectrometer consists of three basic components: a collimator, a refracting or di ff racting element to separate light into its various components, and a telescope. Sometimes a di ff rac- tion grating is used in place of the prism for studying optical spectra. Figure 1 shows the schematic diagram of a spectrometer. The light to be analyzed enters the collimator through a narrow slit positioned at the focal point of the collimator lens. The light leaving the collimator is therefore a thin, parallel beam. The beam of light passes through the di ff racting element placed on a spectrometer table, which can be rotated, raised and lowered, and leveled. The di ff racting element deviates each component of light by di ff erent amounts, producing a spectrum. With the telescope focused at infinity and positioned at an angle to collect the light of a particular colour, a precise image of the collimator slit can be seen. By rotating the telescope, the slit images corresponding to each constituent colour can be viewed and the angle can be measured. These angles can then be used to determine the wavelengths that are present in the light. 1 Figure 1: Schematic diagram of a spectrometer....
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This note was uploaded on 04/04/2012 for the course PHYSICS FE1001 taught by Professor Yap during the Spring '10 term at Nanyang Technological University.

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E6-Atomic Spectra-Rev1 - Nanyang Technological University...

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