A technique for the determination of size changes of gas bubbles by means of Fabry-Perot resonances in elastic light backscattering and optical trapping is presented. A theoretical description of the interaction of a strongly focused laser beam with gas bubbles is also given. The differences in elastic light scattering for particles with a relative refractive index larger than one and for gas bubbles with a relative refractive index smaller than one are discussed in detail. The results of elastic light scattering experiments verify the theoretical considerations.