Final answer:
The 'oe' in 'roentgen' is pronounced as 'ay', similar to the long 'e' sound in 'say'. The digraph comes from Greek, and while some words have simplified spelling in English, 'roentgen' retains the original pronunciation.
Step-by-step explanation:
The pronunciation of 'oe' in the word 'roentgen' is "ay" as in 'ay' (the same as the long 'e' sound in 'say'). This follows the standard pronunciation for the 'oe' digraph in many English words that have roots in Greek or Latin. As you may notice, several words that originally used the 'oe' digraph now have more English-looking spellings with just a plain 'e', such as 'ameba' instead of 'amoeba' or 'subpoena' instead of 'subpoena'.
The roentgen unit, named after Wilhelm Röntgen who discovered X-rays, is an example of a scientific term that retains the original digraph pronunciation. Remember that in English, pronunciation rules can have many exceptions, so this approach of learning specific examples is very useful.