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Practical Laboratory #2: Emission Spectra
2 .1 Objectives
- measure the emission spectrum of a heated gas usingthe digital spectrometer.
- record a number of the bright lines in the spectrum.
- compare the measured spectrum with the known spectra for specific gases
- identify the unknown gas.
2 .2 Introduction
We see very differently than we hear. With sound, we are able to pick out many different frequencies, i.e. different pitches. For example, if we listen to music, we can pick out the drums and voice separately, even though they are happening at the same time. We don’t have that capability with light. Instead, we end up seeing one individual color, which most likely is made up of many different wavelengths of light. The electromagnetic spectrum, shown in Fig. 2.1, covers a huge range of wavelengths, from gamma rays at 10 −^14 m to AM radio waves at 104 m. In this lab we are going to be concerned with the narrow band of wavelengths, ∼ 400 − 750 nm (a nm = 10 −^9 m), that make up visible light. In order to
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- Emission Spectra
Figure 2 .1: Theelectromagnetic spectrum with the visible light regionblown up.
know very accurately what wavelengths are being emitted by a source of light, we will use a digital spectrometer.
2. 3 Theory
In last week’s lab you saw evidence of light behaving as a wave. In this lab we will explore light acting as a particle, called a photon. Sunlight and incandescent light (such as from a lightbulb) are sometimes called “white” light as they are made up of many wavelengths of light (all the colors mixed together make white). If you shine white light through a prism it spreads the light out into a rainbow because different wavelengths of light have slightly different indexes of refraction, called dispersion. If you heat up a gaseous element, such as hydrogen, until it glows and send that
- Emission Spectra
Figure 2 .3: Model of an atom showing absorption and emission of photons.
The periodic table of the elements is also explained by the atomic model, where every element is uniquely identified by the number of protons in its nucleus. Quantum mechanics explains the energy states of the electrons in an atom. Every element in the periodic table has a distinct set of electron energy levels, so for a given element only photons of specific energies can be emitted. Therefore, when you are measuring the emission spectrum of an element, only certain wavelengths of light are allowed and the “pattern” that is produced is unique f or that substance. The spectrum of various gases and phosphors are shown in Fig. 2 .4.
The energy of an emitted photon and its wavelength are related. For example, the color of a laser pointer (e.g. red or green) is determined by the energy of the emitted light. The energy of a photon is described by the equation:
E =
hc λ
where λ is the wavelength in meters, h is the Planck constant (h ≈ 6. 63 × 10 −^34 J s), and c is the speed of light (c ≈ 3. 00 × 108 m/s). When dealing with visible light, wavelengths are usually given in nanometers (nm) so it
- Equipment
Figure 9.4: Emission spectra for various elements and phosphors
is convenient to convert the quantity hc into units of electron volts (eV)^2 and nm. Equation 2 .1 can now be written as:
E =
hc λ
1240 eV nm λ
where λ is the wavelength in nm and the energy of the photon is in eV.
(^2) An electron volt (eV) is the amount of energy it takes to move an electron through one volt of potential so 1 eV = U = qV = (1. 6 × 10 −^19 C)(1 V) = 1. 6 × 10 −^19 J.
2. 4 Equipment
- Digital spectrometer with optical fiber (Ocean Optics USB650 Red Tide) (see Fig. 2 .8)
- Logger Pro software
- (^) Gas discharge tube ( see Fig. 2 .5a) in a box labeled with a letter from A to E.
- Emission Spectra
Figure 2 .8: Thedigital spectrometer with a white USB cable and blueoptical fiber plugged in.
2. 5 Procedure
Setup
- Connect the SpectroVis Plus spectrometer to the USB port of a com- puter. The white cable in Fig. 2 .8 is the USB cable. Start the Logger Pro data collection program, and then choose New from the Filemenu.
- Connect a SpectroVis Optical Fiber (the blue cable being held in Fig. 2 .8) to the cuvette holder of the spectrometer (which is located on its bottom).
- To prepare the spectrometer for measuring light emissions do the fol- lowing in Logger Pro: Open the Experiment menu and select Change Units I Spectrometer:1 I Intensity.
- To set an appropriate sampling time for collecting your emission data, in Logger Pro: open the Experiment menu and choose SetUp Sensors I Spectrometer:1. In the small dialog box that appears, change the Sample Time to 60 ms, change the Wavelength Smoothing to 0, and change the Samples to Average to 1.
- Procedure
Observing the gas discharge tube lightsource
You will investigate the spectrum of ONE unknownlight source, in the gas discharge tube box labeled with a letter from A to E.
Gas discharge tube — The gas may take some time to heat up and emit its complete spectrum; so after turning it on, wait about 3 minutes before making your observations. Do not leave the tube on after making your measurements.
Last updated January 10, 2015 169
Safety Tip
- Gas discharge tubes get very hot. Do not touch them unless cooled
2. 6 Questions
1. List the wavelength and uncertainty of the peaks you found in thecolor spectrum.
Color:
λ±δλ (nm): " :
Light Source
hydrogen gas^ Color:
Wavelength (nm):
Violet Blue ^ Green^ Yellow Orange^ Red
helium gas^ Color:
Wavelength (nm):
Violet Blue ^ Green^ Yellow Orange^ Red
neon gas Color: Violet Blue _ Green (^) Yellow Orange Red_
Wavelength (nm): " : 540 588
mercury gas Color: Violet Blue _ Green (^) Yellow Orange Red_
Wavelength (nm): " :
krypton gas Color: Violet Blue _ Green (^) Yellow Orange Red_
Wavelength (nm): " :
2. From the Table below which gas has peaks that are closest to your measurments? ANS: ______________________________
Letter on the Discharge Tube you measure ____
unknown gas Violet Blue _ Green (^) Yellow Orange Red_
Table of brightest lines in the spectrum of various gases
(^3). List peaks inconsistent with your gas choice (NONE if there are none) ________________
Phy25 2 Spring 2015 Practical Laboratory 2: Emission Spectra
4. (^) For a line with a wavelength of (λ±δλ) nm, calculate the energy and its uncertainty for
the photon emitted, in electron-volts and in joules (Ref. Eq. 2.2) using scientific notation.
Show calculations below. _______________eV ________________J
