MP1_Manual - General Physics II Lab MP1 The Balmer Series...

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General Physics II Lab MP1 The Balmer Series of Hydrogen General Physics II Lab MP1 The Balmer Series of Hydrogen Purpose This experiment has two goals: (i) to observe the emission spectrum of hydrogen and (ii) to verify the relationship between the discrete emission lines in the Balmer series of hydrogen. Equipment and components Hydrogen spectrum lamp, spectrometer, two diffraction gratings (300 lines/mm and 600 lines/mm), convex lens with a holder and magnifying glass. Background The development of quantum theory was stimulated by experimental observations of several striking phenomena. One of the phenomena was that the emitted light by atoms is only at discrete wavelengths, . If a prism or grating is used to disperse the emitted light, there will be a line spectrum of light called the atomic spectrum . Figure 1 below indicates the spectrum of hydrogen, which consists of a series of lines designated as λ αβ H, H γ and so on. Figure 1 Observation of the visible spectrum of hydrogen An important feature of the hydrogen spectrum is that the discrete lines become increasingly closer at smaller wavelengths. In 1885 Johann Balmer, a Swiss school teacher, published an empirical equation that described the wavelengths of the known hydrogen lines within the limits of experimental error. Balmer’s equation can be written in the form 2 2 11 1 ( - ) 2 = R N λ , (1) where N is an integer greater than or equal to 3 and R , now known as the Rydberg constant, was adjusted to match the data. In Eq. (1), N = 3 corresponds to the α line, N = 4 corresponds to the β H line, and so on. It was later discovered that the emission spectrum of hydrogen has several other series of lines in the ultraviolet and infrared ranges of wavelengths. These spectra can be more generally described by the equation H 2 2 1 - N M ) , (2) with M 1 and N > M. The Layman series is predicted by this equation with M = 1, the Balmer series corresponds to M = 2, the Paschen series corresponds to M = 3, the Brackett Revised: 3 January 2011 1/10
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General Physics II Lab MP1 The Balmer Series of Hydrogen series corresponds to M = 4, and the Pfund series corresponds to M = 5. Equation (2) can be derived using Bohr's model, which is a quantum theory of atoms. Procedure I. Apparatus 1. Light source – A sealed tube containing hydrogen gas will be used as a spectrum light source. A high voltage (HV) power supply is used to provide an electric field between the two electrodes at the end of the tube. Under such a strong applied electric field, some gas molecules are ionized and the resulting ions and electrons are accelerated. These energetic ions and electrons will excite the gas molecules by collision. The excited gas molecules emit light with a characteristic line spectrum. 2. Diffraction grating – Diffraction grating is an optical component consisting of a large number of equidistant parallel slits or rules on a glass or metal surface. When a plane wave of wavelength λ is incident normally on the grating, wavelets from each slit will
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MP1_Manual - General Physics II Lab MP1 The Balmer Series...

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