  We provide a systematic study of the optical models for thin layers: the 3D
model, the 2D model and the linearized 2D model. We show that the 2D model is
applicable for layers with small optical thicknesses. Excellent agreement of
the 2D model with the 3D model is demonstrated over broad spectral ranges from
DC to near UV for representative van der Waals atomic layers and thin metal
layers. The linearized 2D model requires additionally weak optical response.
Analytical expressions for the applicability and accuracies of the optical
models are derived. We discuss the advantages and limitations of the models for
the purpose of measuring their optical response functions. Further, we
generalize the theory to take into account in-plane anisotropy,
heterostructures, and stratified substrates. Implications of the 2D model for
the correct analysis of transmission attenuation and implementations of half-
and total-absorption layers are discussed.
