When is the term "photon" valid? This question seems to have taken on different forms in a separate post: a) Do photons exist in empty space (vacuum)? b) Do photons exist in the absence of interaction (with matter)? c) Are photons called photons only when they are exhibiting particle-like behavior? d) Are electromagnetic waves photons? e) ... There may be subtleties to these questions, but lets try to cover the straightforward answers first. Please review marlon's and ZapperZ's FAQ topic "Is light a particle or a wave"