Ans: Newton's law of viscosity is for Newtonian fluids for whom, the rate of deformation of fluid element (rate of shear strain) is linearly proportional to the shear stress.

As shown in the figure below, the x-axis represents the rate of deformation, the y-axis represents the shear stress, and the slope of each line represents the dynamic viscosity. [There is a linear relationship between shear stress and rate of deformation for each of the given fluid]