In this work, we show how an electric and magnetic dipolar particle has its polar- izability that is changed in a non-vacuum environment. This result is used to calculate the signal obtained by an apertureless Scanning Near-Field Optical Microscope (SNOM) when the tip is a dipolar particle. This signal only depends on the field without tip and on the particle and surface optical properties. We apply our theory results to the case of a thermally excited surface. We show that the measure obtained is a local surface spectroscopy.