A method utilizing a visible-infrared interferometric technique has been developed to measure the thickness of ultra-thin films, the small spacing between two separated surfaces and the variations in the spacing. Successful measurements for the thickness of a thin magnetic film on a metallic substrate, the spacing between a magnetic head slider and a quartz disk, and the magnetic head slider fluctuations above a quartz disk are demonstrated. Specially designed single bounce reflectance attachment is used for the wavelength range from 400 nm to 700 nm and from 700 nm to 2500 nm. The wide range of the wavelength allows a vast variety of film material to be measured. Based on an intensity analysis of the reflection interference beams from the top and the bottom surfaces of the film over a wide spectrum, a relation of the intensity maximums and minimums vs. wavelength is obtained to calculate the film thickness, the spacing and the slider fluctuation. An optical thickness of L/10 (L is the wavelength) is obtained by a careful numerical analysis of the measured data which is significantly less than the limitation, L/4, by the conventional method for the first time.'