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EXPERIMENT 6 ELECTRON DIFFRACTION {| GOALS Physics--Demonstrate that accelerated electrons have an effective wavelength, A, by diffracting them from parallel pianes of atoms in a carbon fiim. Measure the spacings between two sets of parailel planes of atoms in carbon. Technigues--Contro! the wavelength of the electron beam by varying the accelerating voltage. Use the De Broglie expression for the wavelength of the electrons and the Bragg condition for analyzing the diffraction pattern. Error Analysis--Catculating the uncertainties in the data points on your graph gives you a good opportunity to use the principles of error propagation. I) QUESTION (work out in lab book before class) Derive an expression for the extrapolated value of the diffraction ring of diameter D’ shown in Fig. 12. It REFERENCES Wehr, Richards, and Adair; 6.9, 7.1-7.4. Halliday and Resnick; 43.1, BACKGROUND Several of your laboratory experiments show that light can exhibit the properties of either waves or particles. The wave nature is evident in the diffraction of light by a tuled grating and in the interferometer experiments. in these experiments, wavelength, phase angle, and coherence length of wave trains were investigated--all features of wave phenomena. However, the photoelectric effect cannot be explained by a wave picture of radiation. It requires a model in which light consists of discrete bundles or quanta of energy cailed photons. These photons behave like particles. There are other exampies illustrating this duat nature of light. Generally, those experiments involving propagation of radiation, e.g. interference or diffraction, are best described by waves. Those phenomena concerned with the interaction of radiation with matter, such as absorption or scattering, are more readily explained by a particle model. Some connection between these models can be derived by using the principal of tha equivalence of mass and energy introduced by Einstein in 1905 in his special theory of relativity, namely, E = me? 
