Measurement of the Balmer Lines of the Hydrogen Spectrum
References: Halliday,Resnick, Walker , Fundamentals of Physics ,John Wiley,2013. Young and Freedman, University Physics with Modern Physics, Addison Wesley 2004 Equipment: Spectrometers, Diffraction gratings, hydrogen spectral tubes, Gas Discharge Tube Power Supply. Introduction: Diffraction Grating. If plane waves of light fall at normal incidence on an opaque “wall” containing two arrow parallel slits a distance d apart, (Fig. 1); the light spreads out by diffraction upon passing through the slits. On a distant screen the overlapping beams from the two slits undergo interference, to produce a pattern of dark and bright fringes. At C, equidistant from the slits, all wave lengths of the light arrive in phase and interfere constructively to produce a “central image” or “zero-order interference pattern” having the same color as the original light.
P L L+ nλ Light
θ d
C
P’ “Wall”
Screen
Figure 1 140520
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PHYS 0060/0160 LAB D - 420
At some other point P which is at a distance L from one slit and L + nλ from the other (λ is some specific wavelength present in the light beam; n is an integer) there is also constructive interference, and a bright fringe appears with the color pertaining to that specific wavelength. At intermediate points distant L and L+ (2n + 1) (
λ ), destructive interference occurs for that 2
wavelength, λ. If the original light beam contains only a number of well separated spectrum lines, each with its own λ (as happens in atomic spectra), the pattern on the screen is a set of lines for each λ, repeated below C and also repeated for numerous values of the integer n. The pattern corresponding to n = 1 is the first-order spectrum (one above and one below point C); n = 2 gives the second-order spectrum, and so on.
It can be seen from Fig. 1 that the governing equation is nλ = d sin θ
