Light physical nature

Creams are semisolid emulsion systems having a creamy appearance as the result of reflection of light from their emulsified phases. This contrasts them with simple ointments, which are translucent. Little agreement exists among professionals as to what constitutes a cream, and thus the term has been applied both to absorption bases containing emulsified water (w/o emulsions) and to semisolid o/w systems, which are physicochemically totally different, strictly because of their similar creamy appearances. Logically, classification of these systems should be based on their physical natures, in which case absorption bases would be ointments and the term cream could be reserved exclusively for semisolid o/w systems, which in all instances derive their structures from their emulsifiers and internal phases. 

In this chapter we are concerned primarily with the physical nature of light and the mechanism of light absorption by molecules. We will discuss... 

We saw in Chapter 3 that light from the Sun was broken into a spectrum and that a new element—helium—was discovered, identified by the dark lines in that specttum. It is essential to learn at least a little about the physical nature of light in order to understand how the lines in the specUnm can tell us about energy levels in the atoms. 

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