The Diffraction Grating THE DIFFRACTION GRATING · The Diffraction Grating 2 Since the angle of...
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The Diffraction Grating 1 THE DIFFRACTION GRATING THE DIFFRACTION GRATING WARNING: DO NOT TOUCH THE TERMINALS OF THE POWER SUPPLY AND DISCHARGE TUBE WHEN IN OPERATION. KEEP HANDS, FINGERS, AND WHATEVER ELSE OFF THE TUBE. HIGH VOLTAGE!!! Objective : Identify a light source by means of its emission spectrum. Apparatus: Discharge tubes and power supply, meter sticks and pointers, diffraction grating and holder. Introduction: When monochromatic light passes through a number of closely-spaced slits, virtual images of the light appear on each side of the central bright line. The direction in which these images are seen depends upon the distance between the slits (d - the grating spacing) and the wavelength of the light (l). Analysis of the situation can be made with the help of the figure above. Light comes from the left and strikes the grating. Each slit diffracts the incoming light and the diffracted waves interfere with each other. For constructive interference, the light waves must be in phase. This condition is realized only when the path length difference of light travelling from adjacent slits is a whole number of wavelengths, i.e. (Eq. 1) The first set of images on each side of the center is referred to as first order (n = 1). The second set of images on each side of center is called the second order (n = 2) and so forth. d sinq = nl n = 1,2,3,4,5,...
Transcript of The Diffraction Grating THE DIFFRACTION GRATING · The Diffraction Grating 2 Since the angle of...
The Diffraction Grating
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THE DIFFRACTION GRATINGTHE DIFFRACTION GRATINGWARNING: DO NOT TOUCH THE TERMINALS OF THE POWER SUPPLY AND DISCHARGETUBE WHEN IN OPERATION. KEEP HANDS, FINGERS, AND WHATEVER ELSE OFF THETUBE. HIGH VOLTAGE!!!
Objective: Identify a light source bymeans of its emission spectrum.
Apparatus: Discharge tubes and powersupply, meter sticks and pointers, diffractiongrating and holder.
Introduction: When monochromatic lightpasses through a number of closely-spacedslits, virtual images of the light appear oneach side of the central bright line. Thedirection in which these images are seendepends upon the distance between the slits(d - the grating spacing) and the wavelengthof the light (l).
Analysis of the situation can be made with the help of the figure above. Light comes from the left andstrikes the grating. Each slit diffracts the incoming light and the diffracted waves interfere with eachother. For constructive interference, the light waves must be in phase. This condition is realized onlywhen the path length difference of light travelling from adjacent slits is a whole number of wavelengths,i.e.
(Eq. 1)
The first set of images on each side of the center is referred to as first order (n = 1). The second set ofimages on each side of center is called the second order (n = 2) and so forth.
†
dsinq = nl n = 1,2,3,4,5,...
The Diffraction Grating
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Since the angle of diffraction (q) depends on the wavelength, if we use a light source emitting severalwavelengths we shall find the images of these separated in space. By measuring the angles of diffractionfor the various images, we can use the above relation, Eq. 1, to compute the corresponding wavelengths,which are characteristic of that source.
Procedure:
(1) Set up the apparatus as shown to the right.
(2) Sight through the grating and observe theimages of the source. The images will appearoff to the side when viewed through thegrating as shown below.
(3) Have your partner slide the pointers along the meter stick to mark the locations of the firstorder image of a particular color on both sides of the source.
(4) The distance between the two pointers is 2x. From the value of x and the measured value ofD, the distance between the grating and the meter stick, compute q for that particular color.
(5) Calculate the wavelength of this color using Equation 1.
(6) Repeat steps 3-5 for each differently colored line in the first order spectrum of the sourceused.
(7) Change to a different discharge tube and repeat steps 3-6. Do this for all of the dischargetubes provided.
(8) Compare the wavelengths found to the wavelengths of some common gases 'and identify the gases in thedischarge tubes you used. Refer to Table 1, and the chart on the laboratory wall for help in identifying thegases.
diffractiongrating
dischargetube
rulers
pointer
2x
D
q
The Diffraction Grating
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Table of Wavelengths
6678.2
6562.8
6402.2
6149.5
5965.5
5875.6
5870.9
5852.5
5790.7
5789.7
5769.6
5764.4
5656.7
5570.3
5460.7
5400.7
5015.7
4921.9
4861.3
4713.1
4702.3
4502.4
4471.5
4463.7
4376.1
4358.3
4340.5
4319.6
4300.1
4266.3
4259.3
4200.7
4191.0
4101.7
4046.6
4044.4
4026.2
He
H
Ne
Hg
Ne
He
Kr
Ne
Hg
Hg
Hg
Ne
Ne
Kr
Hg
Ne
He
He
H
He
Ar
Kr
He
Kr
Kr
Hg
H
Kr
Ar
Ar
Ar
Ar
Ar
H
Hg
Ar
He
Source Wavelength (Å)Wavelength (Å)Source
