Experiment 2:  H & He Emission Spectra

Adapted from Cabrillo College

 

Introduction

      In gas discharge lamps, electricity excites particles of a particular substance.  Generally, the substance is a gas filled into a glass tube.  The electrical energy excites the electrons in the material, and the added electronic energy is given off as light.  By using a spectrophotometer, the emitted light can be separated into a spectrum of light lines.  Atoms emit discrete wavelengths of light that correspond to a specific energy, E, that is characterized by the wavelength, l, and the frequency, n.  These are related by Planck’s constant, h, and the speed of light, c:

 

 

 

 

 


      One of the pivotal experiments leading to the quantized description of energy/matter was the measurement of the hydrogen atomic emission spectrum.  Equations describing this spectrum have been derived from experiment and theory.  The Rydberg equation is based on data analysis that resulted in a relationship between the emitted wavelength and some whole number, n:

 

 

 

 

 

 


      Another approach used to understand the origin of these emission lines comes from Niels Bohr’s treatment of the atom.  He assumed that the electrons in an atom could only have certain energies, such that the energies of an electron in an atom are quantized.  He also assumed that an electron could orbit the nucleus without losing energy, a condition that would eventually result in the electron colliding into the center of the atom.  This was in flat contradiction with classical physics of a charged particle. 

In Bohr’s model, the quantum number n related the radius of the electron’s orbit (distance from the nucleus).  From his assumptions and his theoretical treatment of the electron orbiting the nucleus, he successfully derived an equation describing the energy levels of a hydrogen atom:

 

 

 

 

 


In this experiment you will measure the wavelengths of lines emitted from a hydrogen and or helium lamp.  You will use those wavelengths to determine the values of n.  You will calculate energies that could be emitted by using Bohr’s equation and the probable transitions relating to the Balmer (where n=2)  series.  You will compare your empirical results with the theoretical values predicted by the equation or available in the literature.

 

Materials and Procedure

Hydrogen and Helium tubes       meter sticks       diffraction gratings         transformer

 

Set up your experiment according to the illustration below:

                                                                        opposite

                              0 cm-------------------LS------------------------100 cm

                                                            |

                                                            |

                                                            |                hypotenuse

                                                            |

                                                            |

                                                            | sin q

                                                      diffraction slit

 

LS= He or H tubes

 

Using basic trigonometry measure the distances from the light source to the spectral lines, the LS to the DS and compute the hypotenuse and the sin of the angle for each spectral line observed.

 

The wavelength l  can then be calculated using the Bragg Equation:  l = 1.9 x 10 E3  x   sin q

 

This lab will be evaluated for DC, DP and CE