Figure 7: Spectroscopic apparatus.
Apparatus
Equipment
Safety. The sodium vapor lamp assembly becomes very hot. Do not disturb the lamp assembly while connected, or immediately after turn-off. Do not touch the glass lamp; oils from the skin may eventually damage the bulb.
Alignment of the Spectroscope
Adjustment of the eyepiece of the telescope should yield sharp images of the slit and the crosshair, without parallax between the two.
The width of the slit may be adjusted as desired.
The spectroscope should already be aligned. Alignment comprises the following steps. The collimator is leveled (using a level; hence this adjustment depends on location of the spectroscope). The telescope is focused on an object several hundred feet distant, which adjusts the telescope to receive collimated light. With the slit illuminated, the telescope leveling adjustments and the collimator focusing adjustment are adjusted to obtain a sharp, centered image of the slit.
Angular Measurements
Familiarize yourself with measurement of angles, using the vernier on the spectroscope scale. The main scale is divided into 1/2 degree increments. The vernier has 30 divisions, which gives a least count of 1 minute of arc. The angular position of the direct beam is denoted φ0. For each spectral line angular positions of the deviated beam, to the right and to the left, are observed, denoted φr and φl, respectively. The angular deviations are then θr = φr - φ0 and θl = φl - φ0.
Sodium spectrum, first order. These observations serve two purposes. In order to avoid the use of messy formulae, the grating must be aligned perpendicular to the light beam from the collimator. This is a trial and error procedure, which you conduct by rotation of the table supporting the grating until the right and left deviations, θr and θl, are equal to within a few minutes.
The grating space, d, is then determined from these data by use of the grating formula
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Eq. (1) |
The order of the spectrum is denoted m; m=1 for first order, etc. Average λ=5893Å for the Na doublet. Our grating is a "replica", which has been reproduced from a ruled grating, but precise knowledge of the grating spacing has been sacrificed in the reproduction. Thus, our determination of d should be more accurate than the nominal value marked on the grating.
Sodium spectrum, second order. measure angular deviations, in second order, for both members of the sodium doublet. With a narrow slit, you should see two distinct yellow lines; in first order the lines may not be separated. Measure the angular separation of the members of the doublet as carefully as you can; the separation is only a few minutes, barely within the resolution of our apparatus.
From these data, estimate the separation in wavelength between the members of the sodium doublet. The following formula follows directly from the basic grating formula given above. Δθ must be in radians.
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Eq. (2) |
This completes work with the sodium vapor lamp, which may be turned off.
Helium spectrum, first order. From measurements of angular deviations to the right and to the left, obtain data to determine wavelengths of the lines which are relatively bright in the helium emission spectrum. Your discussion should address the following matters:
(a) The structure of the helium atom and its relation to our observations. The discussion should indicate familiarity with standard spectral notation and should include an explanation of that notation. Some information about helium is attached. Insofar as feasible, relate parts II and III.
(b) The nature of the fine "structure" observed here in the sodium doublet. Again, indicate familiarity with appropriate notation.
Information from the "American Institute of Physics Handbook":
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