The characteristics, basic principle, experimental system and measured examples of a non-contact photo-acoustic method are introduced in order to measure the thermophysical properties of thin solid materials on the basis of my recent studies. The relation among light, heat and sound in relation to the photo-acoustic method is described briefly. The phase lag of the photo-acoustic signal for the incident modulated light expressed as a function of some properties such as the thermophysical properties of sample is derived from the RG (Rosencwaig and Gersho) theory. The thermal diffusivity of sample is obtained from the measured phase lag versus the modulated frequency of incident light, even if the thermophysical properties of backing material are unknown. The relation between the modulated frequency of incident light and the obtained vertical information of sample, and the analytical point in accordance with the transparency of sample for the incident light are also explained to some extent.