Fakultät für Physik und Geowissenschaften Physikalisches Grundpraktikum

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O12e “Atomic Spectra” Tasks 1. Determine the grating constant of a reflection grating. To this end measure the reflected light (0. order) as well as the first three orders of the green line (λ=546.1nm) of a Hg-lamp at a constant angle of incidence (about 60°). 2. Determine the wavelengths of four doublets of the spectrum of a sodium lamp using the first order diffraction spectra. Use the same grating arrangement as in Task 1. 3. Calculate the resolving power of the reflection grating. The effective width of the grating is 50 mm (0.5%). Literature Physics, P. A. Tipler, 3rd Edition, Vol. 2, Chap. 33-11, 30-4 University Physics, H. Benson, Chap. 38.5, 38.7 Physikalisches Praktikum, 13. Auflage, Hrsg. W. Schenk, F. Kremer, Optik, 2.0, 2.3 Instruments and Accessories Precision goniometer-spectrometer, reflection grating, atomic spectroscopic lamps Keywords for Preparation - Diffraction, interference, coherence - Constructive and destructive interference - Reflection and transmission grating - Resolving power of an optical grating - Origin of line spectra Remarks The precision goniometer-spectrometer is a high resolution instrument and correspondingly delicate. The instrument is already adjusted. At the start of the experiment you will be introduced in using the spectrometer by the supervisor. Work carefully! Do not touch the grating and cover it with a protective cap when not in use. The measurement uncertainty is specified by the manufacturer to 10’’. If there is sufficient time left observe various light sources (light bulb, fluorescent tube, sun light) with a pocket spectrometer and characterize the spectra. The spectral lamps have a warm-up time of about 10 min.

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Diffraction at a reflection grating

Phase difference:

(sin sin )2

g α βδ πλ−=

α : angle of incidence

β : angle of diffraction

λ : wave length g : grating constant Resolving power:

k Nλλ

=Δ

k : diffraction order N : number of slits Fig. 1. Schematic optical path.

Precision goniometer-spectrometer

Fig. 2. Spectrometer

1 Collimator 3b Ocular (focusing of intermediate image) 1a Chamber for light source 4 Microscope for circular scale readings 1b Adjustable slit 4a Fine adjustment of circular scale readings 2 Grating 4b Ocular (focusing of circular scale) 2a Locking screw for grating 5 Pivot arm 3 Observing telescope 5a Fine drive for pivot arm (5b – fixed) 3a Adjustment of focal point 5b Locking screw for pivot arm

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The autocollimation telescope generates a bundle of parallel rays from the light passing the slit. For focusing a slit with adjustable width and a collimating lens is used with the slit positioned in the focal plane of the lens. From the collimator the light is incident on the grating and with the rotatable telescope pair (1, 3) the reflected (or transmitted) diffraction maxima are observed relative to the

incident beam direction (angle ϕ00). For adjusting the angle of incidence α to ca. 60° the white light of

zeroth order is used. Fig. 3. Image in the autocollimation telescope The telescope streak plate has a central double cross that is used for the slit image adjustment. Fig. 4. Microscope image for the reference circle reading The image of the reference circle in the microscope (Fig. 4) shows two fields. In the larger one readings of degrees and multiples of 10‘ are made, in the smaller one minutes up to 10‘ (left side) and seconds (right side) can be read off. Before taking a reading the vertical line in the degree field has to be adjusted by the screw (5b) (Fig. 2) to the center of a division mark pair. Angle readings can be estimated down to 0.5′′.

The reading in Fig. 4 is 134° 30′ + 2′ 55′′ = 134° 32′ 55′′ (134.55 °)

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Na Term Scheme and Na Spectrum

Hg Term Scheme und Hg Spectrum