With diffuse reflection the beam is weakened because of ______.

• A. Absorption
• B. Incoherence
• C. Diffraction
• D. Scattering

Answer: (B)

Diffuse reflection happens because a rough surface reflects light from many tiny facets at different angles. Those facets also introduce slight differences in the path length and, consequently, in the phase of the waves they reflect. When these many reflected waves combine, they do not stay in a fixed, collective phase relationship, so the light becomes incoherent. In any particular direction, the random phases mean the contributions interfere in a complex, largely non-constructive way, spreading energy over many directions and making the beam in a given direction appear weaker. Absorption would simply remove energy, diffraction isn’t the main effect here, and while scattering describes spreading, the specific weakening of the beam in a single direction comes from the loss of coherence due to the surface roughness.