Alabama Science in Motion
Light: Emission Spectra
Emission Spectra
Purpose: To use the emission spectra of two gases to determine diffraction grating spacing,
frequency of light, and energy associated with different colors of light.
Equipment Needed:
Equipment
Qty
Equipment
Qty
Mercury spectral tube
1
Metersticks with stands and markers
2
Mystery spectral tube
1
Black out material
1
Power supply
1
Diffraction grating and spring support
1
SAFETY CONCERN: Caution! You should not look directly at the mercury discharge
coming from the slit in the mercury lamp. When you observe the spectra, you will be
looking at an angle to the slit, but you should not stare directly at the slit.
Procedure Part I:
Determination of the Grating Spacing
When light is diffracted as it passes through a grating, the relationship between the wavelength of
the light, λ, the angle of diffraction, θ, and the spacing between the lines on the grating, d, is given
by
nλ= d sin θ
For this experiments you can only see the first order of diffraction, so n = 1 in this formula.
1. Setup the two meter sticks on their metal supports and be sure they are exactly perpendicular to
each other (90 degree angle) and that one stick is at the 50 cm mark of the other.
2. Put the grating in the spring support holder and place it on the meter stick so that it is 100 cm
from the Mercury discharge slit. The top of the grating is marked on the grating, and it should be
positioned so that the top is highest above the meter stick.
3. The two movable metal markers should be placed on either side of the intersection point of the
two meter sticks and on the meter stick closest to the Mercury lamp. These will be used to mark
the position of the image.
4. The mercury lamp should be positioned with the slit as close as possible to the intersection of
the meter sticks.
5. One lab partner will view the emission spectrum of Mercury/ Mystery by looking through the
diffraction grating and observing the yellow, green and violet or (red, blue-green or blue for
Mystery ) lines at a position on either side of the slit. The other partner will stand behind the
mercury lamp and move a pencil along the meter stick to the position described by the observer.
One of the metal markers will be placed at the position where the yellow, green or blue image
appears. This procedure should be repeated for each of the spectral lines by each of the partners,
and the lines should be measured on both sides of the slit position. Thus there should be two
measurements of each spectral line by each observer.
Revised 03/10
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Alabama Science in Motion
Light: Emission Spectra
6. The measured values should be entered in the Data Table 1 and all the X values for each
spectral line should be averaged.
7. Use the values of X and Y and determine the angle θ for each of the three spectral lines using
X
the formula
Sin θ =
X 2 Y 2
Fill in the values for wavelength and angle for each spectral line in Data Table 2, and then use
these values to determine three values of d from the diffraction formula nλ= d sin θ
Calculate an average value of d and record on Data Table 2. If you wish to compare d to the value
on the diffraction grating, remember that d is the distance between slits or 1 divided by the
number of slits per meter.
Procedure Part II:
8. By using the wavelength of three of the mercury emission lines, you will be able to calculate
the frequency of the radiation, using the relationship between the wavelength λ, the frequency f,
and the speed of light c, in the equation fλ= c. Record on Data Table 2.
9. You can also calculate the energy (E) associated with different colors of light by using the
relationship,
E = hf or E = hc/λ, since fλ= c.
The constant h is called Planck’s constant, and the value of h is 6.62 x 10-34 Joule-seconds, so
your energy will be expressed in Joules. Put these values on Data Table 2.
Revised 03/10
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Alabama Science in Motion
Light: Emission Spectra
Student Data Sheet
Name:_______________________________
Partner’s Name (s) :___________________
Period: _______________________________
Date: _______________________________
Observer
1
Data Table 1 for Mercury Spectra
Yellow Line
Green Line
XXX
XXX
Violet Line
XXX
2
Average
Data Table 2 for Mercury Spectra
λ (nm)
Y (cm) X (cm)
578 (yellow) 100
546 (green)
100
436 (violet
100
d Average
XXX
XXX
Revised 03/10
Θ (deg) d(mm) f (Hz)
XXX
XXX
Energy (J)
XXX
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Alabama Science in Motion
Light: Emission Spectra
Procedure Part 3: To be completed the following day:
Now that you have determined the grating spacing, you can use that to determine the wavelength
and energy of the emission spectra of an unknown gas and identify that gas.
Your teacher has replaced the mercury tube with an unknown gas. Following the procedure used
in the earlier experiment, complete the following data tables and use the chart to identify the
unknown gas.
Analysis:
1. Using the chart provided, what is your mystery gas?
Violet
Blue
Green
Yellow
Orange
Helium
XXXX
450 nm
510 nm
585 nm
XXXX
Hydrogen
420 nm
490 nm
XXXX
XXXXX
XXXX
Mercury
436 nm
XXXXX 546 nm
578 nm
XXXX
Oxygen
440 nm
490 nm
525-565nm
XXXXX
XXXX
Argon
460 nm
XXXX
495-570nm
595 nm
XXXX
A range of numbers indicates there are multiple bright lines within that region.
Red
690-750 nm
670 nm
XXXX
615-665 nm
610-720 nm
2. The emission spectra of a gas is often compared to a fingerprint. Explain this comparison.
3. How would this experiment change if two of the gases were mixed together?
Revised 03/10
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Alabama Science in Motion
Light: Emission Spectra
Data Tables for Unknown Spectra
First Bright Line
Observer
d
1
Y
100 cm
2
100 cm
3
100 cm
Average
100 cm
Second Bright Line
Observer
d
1
Y
100 cm
2
100 cm
3
100 cm
Average
100 cm
Third Line
Observer
1
d
Y
100 cm
2
100 cm
3
100 cm
Average
100 cm
Fourth Line (if visible)
Observer
d
Y
1
100 cm
2
100 cm
3
100 cm
Average
100 cm
Revised 03/10
X
Sin θ
λcalculated
Frequencycalculated
X
Sin θ
λcalculated
Frequencycalculated
X
Sin θ
λcalculated
Frequencycalculated
X
Sin θ
λcalculated
Frequencycalculated
λaccepted
nm
%error
Energy (J)
λaccepted
nm
%error
Energy (J)
λaccepted
nm
%error
Energy (J)
λaccepted
nm
%error
Energy (J)
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