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In his arrangement the reference and object beams were incident on the emulsion from opposite directions. To achieve this he mounted the photographic emulsion between the light source and the subject matter, so that the portion of the reference beam not absorbed by the emulsion passed through it, and was reflected back from the object. , more like the pages of a book than a venetian blind (Fig. 5). 4 The off-axis principle. (a) When part of the incident beam is deflected by the thin prism P to form an off-axis reference beam RB, this interferes at an angle with the object beam OB from the transparency T at the photographic plate H.

By converting all the developed silver back into silver bromide (which has a higher refractive index than gelatin, the main constituent of the emulsion), we can raise the diffraction efficiency considerably. Methods of doing this are described in later chapters. 3 The information in a hologram The Fourier model tells us that all the information about the subject matter of a hologram is coded in its diffraction field. By looking at its far-field pattern you can obtain the following information about an object: The phase of the beam tells us how much farther the object wavefront (at a particular point) has traveled than the corresponding reference wavefront at that point.

Doppler broadening also occurs when the atoms in solids are excited so that fluorescence occurs. However, not all the lines in solids are broadened in this manner. With some elements, notably chromium and some lanthanides, a few energy levels are not spread at all. This forms the first clue to laser operation. Stimulated emission Protons (carrying unit positive charge) and neutrons (carrying no charge) form the nucleus of the atom and account for nearly all its mass. Electrons have unit negative charge and very much less mass; however, they collectively take up nearly all the space in an atom.