When \(\psi=0\) on impact on the surface of incidence, the photon fully manifest itself as a mass \(m_p\) with velocity \(c\). When it passes the exit surface in a very thin medium, it is not going to regain \(\psi\) as \(\psi\) has completely disappeared.
What is this particle that was previously a photon? It has a small mass \(m_p\), but no energy density, \(\psi=0\). Does it exist?
If this happens the medium will actually cool when bombarded with photons.
