Snell's law reveals the propagation and refraction properties of the light when it passes through the interface composed by twomedia with different optical constants.It is the foundation of modern physical and geometrical optics.The effort to realize a perfect lens is trying to find the material with a negative refractive index

which has notyet been found in the natural material in pastyears.In this work

we have fabricated a series of noble-metal-based film samples (Au and Ag) with different incident angles.We quantitatively measured the refraction of the light passing through the metal/air interface

and observed that the refraction can change from negative to positive in the visible range at the simplest naturalmetal/air interface.The mechanisms for causing the negative refraction have been carefully discussed and analyzed

and are notattributed to the effects

such as the plasmon resonance

the negative permeability

the Goos-Hanchen shift and so on.We hope

therefore

the results given in this work will stimulate people to understand more of the physicalorigin of the negative refraction happened in themetal-based artificial materials

and will help its application in the photonics material and device design and fabrication in the future.