The effect of small surface deformations of spherical microparticles on the Q-factor of their natural resonance modes has been studied in detail. Based on the geometric optics representation of a natural mode of a sphere as a congruence of optical rays confined between the particle surface and the internal caustic, a simple equation is derived that relates the Q-factor of a mode with the surface deformation amplitude. The values of the Q-factor of deformed water droplets are compared with similar results obtained by the perturbation method of the wave theory.