tailieunhanh - English for students of Physics_Unit 8

We mentioned compounds of calcium, lithium, and strontium without specifying which compounds we were talking about. This may have given you the impression that only the spectrum of one of the elements in a compound can be observed. It is true that the flame of your alcohol burner is hot enough to produce the spectra of sodium, lithium, calcium, copper, and a few other elements, but that is not hot enough to produce the other spectra of elements, such as oxygen and chlorine. However, if we heat a sample of a compound to a sufficiently high temperature (for example,. | 33 Unit Eight OPTICS READING PASSAGE Spectral analysis We mentioned compounds of calcium lithium and strontium without specifying which compounds we were talking about. This may have given you the impression that only the spectrum of one of the elements in a compound can be observed. It is true that the flame of your alcohol burner is hot enough to produce the spectra of sodium lithium calcium copper and a few other elements but that is not hot enough to produce the other spectra of elements such as oxygen and chlorine. However if we heat a sample of a compound to a sufficiently high temperature for example by putting it in an electric arc the spectra of all the elements in the compound will be observed. Under such conditions the resulting spectrum is no longer simple. It will most likely contain complicated patterns of many closely spaced lines. Yet each element gives out its own spectrum which is different from that of any other. It takes accurate measurements of the positions of spectral lines to identify an element. Once this has been done however the presence of that element has been definitely established. With a good instrument it is observed that the yellow of the sodium flame is not just any yellow. It is a very specific color indeed which has its own special place in the spectrum. It is a yellow made by no other element. The presence of this particular pair of lines always means that sodium is present in the light source. Even if the yellow color is hidden from the unaided eye by many colors the spectroscope will show the presence of sodium. Although calcium lithium and strontium give flame tests of nearly the same color each gives its own set of characteristic spectral lines when viewed through a spectroscope. The spectroscope thus enables us to distinguish one element from another. Spectral analysis or spectroscopy can be done on tiny quantities of matter such as very small sample of a rare mineral or of a biological material. Spectroscopy can even be .