Experiment
# 12
Experimentr l,
THE DIFFRACTION GRATING SPECTROMETER
Purpose:
1. To becomefamiliar with theuseof a diffiactiongratingspecftometer
2. To measurethe wavelengthsof certain lines in the spectnrmof the
mercury arc lamp.
Apparatus:Spectrometer,
gating andholder,mercuryarc lamp, sodiumtarop(optional),
variousdischarge
tubesandaccessories.
References:
Discussion:Theuseof thegratingequation
N],_ dsin0
(r)
implies the following experimental conditions:
(i) the spectrometeris in proper adjusfinen! as specified in Part I
(ii) the grating rulings are parallel to the slig i.e. , vertical
(iii) the plane of the grating is perpendicular to the axis of the collimator
Angular measurementson the circle must be made with respect to the stationary
zero point. A single zero line would enableus to read only to the nearest one-half degree
and estimateto a smaller fraction thereof. By meansof a vernier, however, it is possible
to read with precisionto a small fraction of a subdivision on a circle. With the vernier on
your instmment , it is possibleto read the position of the telescopeto the nearestminute.
There is, however, the possibility of a slight eccentricity existing between the center of
the circle and the center of the vernier scale. This eccentricity error may be rnade
negligible by reading the circle by means of two verniers 180" apart and determining the
average reading. The reading of verniers is facilitated by means of trvo magnifiers
attached to a plate covering the circle for the purpose of keeping dust from the engraved
scales.
Procedure:
lnspectthe spectrometerand identify its parts.
Arange your data sheetas shown on page52.
Part I. Adjusfinent of the Specrrometer
(1) Adjust the grating to satisff the condition of the gating equafion.This is done
by placing the telescopein such a position that rhe image of the stit illuminated by
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mercurylight is locatedat the intersectionof the crosshairs. Thenput the gating into
the gating holder so that the grating faces the collimatorobjectiveand is as nearly
perpendicular
to the opticalaxis aspossible.
(2) Adjustthetelescopeasfollows: Placethe telescopesothatit is in line with the
collimator.Focusthe eyepieceon the crosshairs,illuminatedby the light from the slig by
sliding the eyepiecelens in and out unfil the cross hairs appearsharpest.Slide the
eyepiecebackandforth pastthe best positiona numberof times,until you are surethat
you know what the sharpestimagelooks like. Nex! thewholeeyepiece
tube is movedin
and out by rotatingthe knurled collar rurtil the imageof the collimatorslit is in focus,i.e.,
appetrsclearand sharp.When the adjusfinents
are completelysatisfactory
thereshould
para[a:q
be no
or relativemotion of the crosshairs and slit imageas the eyeis moved
slightly to and fro acrossthe eyepiecelens.If thereis parallaxfor smallmotionsof the
eye, touchup the last adjusfrnentuntil parallaxdisappears.
Finally,nrurowdown the slit
as much as possibleand set the telescopeso that the point of intersectionof the cross
hairs is symmetricallyplacedwithin this very niurow slit image.Recordthe readingsof
both verniers( A andB ) with the telescopein this position.Usethe sampledatasheet.
Now shift the telescopeslightly,andagainbringbackthe intersection
of the crosshairsto
the centerof the slit image,andrecorda secondsetof vernierreadings.
PartII. Wavelengths
of the MercuryLines.
(1) Turn the telescope(approximatelyI9') until the bright greenmercuryline of
the first order appearson the crosshairs. Make two independentsettingson this line and
recordyour resultson the datasheet alwaysreadingboth verniers.
(2) Repeat(1) withthe first orderonthe othersideof the centralimage.
(3) Repeat(1) and(2) for all othervisiblemercurylines,takingonly onesettingon
eachsideof the cental image.
(a) Repeat(l) and(2) for the secondorderbright greenmercuryline.
(5) Checkfor any visiblelinesin the third order.
Pafi III. Specfiaof VariousElernents.
As time permits,examinethe spectraof variousdischarge
tubes,includingsodium
andhydrogen,andrecordyour qualitativeobservations.
Do not measure
the wavelengths.
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ExperirnentiT i2
Experirnent# 12
Calculations:
Expressall wavelengttrsin Angshom units ( 1 A _ 10-tcm _ l0-r0m ).
Pafi II From the data of procedures(l), (2) and (3), obtain averagevalues of 0 for each of
the mercury lines observedin fust order.
Using 0 as obtained above and the value of the grating spacing obtained from the
number of nrlings per mm statedon the grating (it should be about 600 rulings/mm), use
equation (l) to determine the wavelengths of each of the lines observed in the rnercury
spectnrm.
From the data of procedure(4), again determine the wavelength of the bright green
mercury line, now basedon the second-orderobservation.
Compareyour resultswith the acceptedwavelengthsof mercury lines given below.
Tabulateyour results along with the acceptedvalues.
Questionsto Be Answered in the Report:
1. Computethe longest wavelengthfor which your grating will form a third-order
image.
2. You made two determinations of the wavelength of the bright green mercury
line, one based on the first order interferenceand the other based on the second order
interference. Which one do you think is more precise? Explain why
Wavelengthsof SpectralLines
Mercury
uA)
Color
lntensity
4047
4078
4358
49t6
5461
s770
579t
Violet
Violet
Blue-Violet
Blue-Green
Green
Yellow
Yellow
VeryBright
Weak
Bright
Weak
Very-Bright
Bright
Bright
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Experiment
Hydrogen
4861
Blue-Green( Hp)
Weak
6s63
Red(HJ
Bright
Sodium
Yellow (DJ
Yellow (D,)
5890
5892
Bright
Bright
DATA SHEET
1oOrder - Hg Green,Right Side
Central Image
Mean
Mean
2"d Order - Hg Greeq Right Side
1"' Order - Hg Greeq Left Side
Vemier B
Vernier A
Vernier B
Vernier A
Vernier B
Vernier A
Vernier B
Vernier A
Mean
Mean
ETC
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