I am not a native speaker, nor do I know the proper grammatical name of this kind of words to look up this question. So please excuse the possible duplication or simplicity. But as far as I know, the words mentioned in the subject cannot be used in plural, although they have a grammatical plural form. So you don't say
These two parts have different reflectances.
This document describe radiations reflected from different parts of the machine.
but rather reflectance and radiation.
- Am I correct?
- What is the proper explanation for this? I need to tell it to a colleague.
- How can one determine if this rule applies to a specific word or not, besides the gut feeling?