The quoted value applies to the diffuse interstellar medium, but R can be significantly higher in dense molecular clouds, including those where stars form.

A plot of either Ex-V/EB-V or A\/EB-V versus A is known as the interstellar extinction curve. Figure 2.7 shows the standard curve, in which the independent variable is 1/A. This important result, summarizing the extinction and reddening properties of the interstellar medium, was obtained by applying equation (2.14b) empirically, i. e., by comparing the observed color indices of many stars with unreddened objects of the same spectral type. The ratio R is then obtained from equation (2.15b) by measuring the limit of Ex-V/EB-V for long wavelengths, under the assumption that A\ tends to zero in this limit. In practice, observations out to the mid-infrared suffice for this purpose.

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