MHS AP Chemistry
Young's Experiment

PART 1: Calibration for Young's Experiment

Observe and record the identity and color of the gas in the glowing Geissler tube.  These tubes contain gas at low pressure (about 0.001 atm).  A high voltage electric discharge ionizes the gas, and the recapture of the electrons to characteristic energy levels results in a unique line spectrum.  Place one meter stick parallel to the line of sight and another behind the tube as diagrammed below.

Using diffraction glasses, observe the line spectrum of helium.  Twist your head until the colored lines are "standing" on the meter stick.  Your partner should stand behind the meter stick and use a pencil to help you find the exact distance from the tube to each line above the meter stick (marked x in the diagram).  Record this distance in centimeters.

Use Table 1 below to determine the wavelength of each line (as an alternative, you may use the spectroscope with the internal wavelength scale and read it directly).  Use the known wavelengths to determine the diffraction grating width (d) as follows:

The tube should be exactly 1.00 meters from your eyes.  Use 100.0 cm as L. L = 100.0 cm

The distance from the tube to the line is x. Record x for each line

Calculate the distance from the diffraction grating to the line, using the Pythagorean Theorem.  This distance is the hypotenuse (H). L2 + x2 = H2

The spacing of diffraction grooves, angle of diffraction, and wavelegth of the color line are related by the equation l = dsinq

From trig, you may recall that
 sin = opp = x hyp H

Substitute and solve for d:
 d = ìHü l î xþ
Calculate an average value for d.

Calibration Data Table (Helium)
[wavelengths from CRC 83rd edition, p 10-28]
 Color l x H d (nm) (cm) (cm) (units: _____) purple 388.9* __________ __________ __________ blue 447.2 __________ __________ __________ blue 471.3* __________ __________ __________ blue/green 492.2* __________ __________ __________ green 501.6 __________ __________ __________ yellow 587.6 __________ __________ __________ red 667.8 __________ __________ __________ red 706.5* __________ __________ __________

PART 2: Application of Young's Experiment

Observe the line spectrum of hydrogen gas.  Measure and record the distance of each line from the glowing tube. Use the equations from PART 1 to determine the wavelength of each line. From the wavelength, determine the associated frequency and energy for each electron transition in excited hydrogen.

Once you have determined the energy of each line in the hydrogen spectrum, use the Rydberg equation to determine from which energy levels the electrons "fall" (find ni; nf = 2).

Spectrum Data Table (Hydrogen)
 Color d x H l n E nf ni (_____) (cm) (cm) (_____) (_____) (_____) ( -- ) ( -- ) __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________ __________

Table 1
Helium Spectrum
Table 2
Constants & Relations
Table 3
Colors & Information
 color l (nm) purple 388.9* blue 447.2 blue 471.3* blue/green 492.2* green 501.6 yellow 587.6 red 667.8 red 706.5* * may be too faint to see.
h = 6.63x10-34 J/Hz
c = 3.00x108 m/s
c = ln
E = hn
RH = 2.18x10-18 J/photon
1 m = 102 cm = 109 nm
 DE = hn = -RH é1 __ 1ù ënf2 ni2û

Be very careful about the signs: What is the sign for the frequency of light?  What is the sign for the energy change of the electron?
 color wavelength (l) frequency (n) [nm] [x1014Hz] violet 400 to 420 7.5 to 7.1 blue 420 to 490 7.1 to 6.1 green 490 to 580 6.1 to 5.2 yellow 580 to 590 5.2 to 5.1 orange 590 to 650 5.1 to 4.6 red 650 to 700 4.6 to 4.3

Attach one sheet with orderly calculations for d (be very careful about the labels for d).

Attache one other sheet with orderly calculations for one of the values of nf.