Atomic Spectroscopy Using Constant Deviation Sprectormeter

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Atomic Spectroscopy using a Constant Deviation Spectrometer with Fabry-Perot Etalon

A constant deviation spectrometer with Fabry-Perot etalon has been used to investigate the various atomic emission characteristics from a Zn-Cd-Hg lamp, a low pressure Hg lamp and a Ne lamp. The Fabry-Perot etalon of Free Spectral Range 0.010 cm-1 was used to resolve the fringes of the Hg 546.0731nm spectral line, and the 594.4834nm, 614.3063nm, 640.2246nm and 650.6528nm spectral lines of the Ne lamp, and study them at very high dispersion. Using the method of exact fractions, an extremely accurate value for the etalon spacing of d=15.077582±0.0000006mm was obtained. The spectral line widths (their full widths at half maximum intensity) of afore mentioned lines were examined, and factors affecting their line broadening discussed. An appreciation has been obtained by the author for the optical resolving power of the Fabry-Perot etalon – despite its simplistic appearance, its ability to explore the atomic structure of atoms so profoundly is testament to its usefulness even in modern science.

1. Introduction
It is well known that electrons can be characterized by a unique set of discrete energy levels [1]. These levels have well defined energies and electrons moving between them must absorb or emit radiation with energy equal to the difference between them. Atomic emission lines arise from an electron returning to a lower state of excitation. A photon with energy equal to the energy difference between the levels is released in the process. The frequency f at which the spectral line occurs is related to the photon energy E by Planck's law �� = ℎ�� where h is Planck's constant = 6.626068 × 10-34 m2 kg / s. (1)


Atomic Spectroscopy using a Constant Deviation Spectrometer with Fabry-Perot Etalon

Thus the wavelength of the light emitted from a source can be used to determine various physical parameters about the source. Spectroscopy is the physical separation of light according to its wavelengths. This ability to analyze a spectrum and characterize its source has been a very important tool in science, and has led to many important discoveries including contributing to quantum mechanics [2]. In this experiment a constant deviation spectrometer has been used to study, at low dispersion and hence low resolution, the spectral emissions from a Zn-Cd-Hg lamp, a low pressure Hg lamp and a Ne lamp. When individual spectral lines were to be analysed, the Fabry-Perot Etalon was employed. This instrument is used to study small wavelength ranges at high dispersion, and hence because of its spectral resolving power it optical interferometer of choice.

2. Experimental Set-up
Figure 1 shows a schematic diagram of the constant deviation spectrometer.

Figure 1: Schematic Diagram of the constant deviation spectrometer showing the location of Fabry-Perot Etalon.

The spectrometer consists of two perpendicular arms, holding a telescope and collimator respectively. The constant deviation prism is set in line with both arms. The final deviation of light rays from the source through the prism is always 90° and rotating the prism allows different wavelengths to be observed. The prism is rotated by a screw to which a rotatable wavelength drum is attached, displaying the wavelength being viewed. Attached to the telescope was an eyepiece for direct viewing of the output. The eyepiece contained a fixed set of cross-wires and a micrometer driven cursor which was used to make quantitative measurements on spectra observed. Figure 1 also shows the location where the Fabry-Perot etalon is inserted along the optical axis.


Atomic Spectroscopy using a Constant Deviation Spectrometer with Fabry-Perot Etalon

3. The Fabry-Perot Etalon
(Please note that only principal equations have been quoted here - the detailed mathematics of the Fabry-Perot etalon are rather tedious and have been relegated to Appendix 1.)

This experiment made use of...
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