  In a recent paper [Sens. Actuator. A Phys. 301 (2020) 111749] Shkir et al.
have studied the structural, optical and photo-electrical properties of the CdS
thin films as a function of Eu doping concentration. The authors have used a
wrong equation to calculate the refractive index and obtained values are
incorrect. Consequently, the optical properties obtained based on the
refractive index such as real and imaginary parts of dielectric constant,
dielectric loss, optical conductivity, nonlinear refractive index (n2), linear
and third-order non-linear optical susceptibility are wrong.
