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In this paper, the author discusses both theoretically and experimentally, the conditions for which the tip can tune the resonance frequency of a microcavity without introducing a substantial line broadening. In particular, whispering-gallery modes in silica microspheres and modes of photonic crystal cavities were investigated and show that a dielectric nano-object acts as a dipolar Rayleigh scatterer, where the ratio of its polarizability to the cavity mode volume determines its influence on the cavity spectrum. It was concluded that a suitable tip can be used for mechanical tuning of a microresonator. However, it was also shown that scanning near-field optical microscope (SNOM) can be invasive and one has to take into account the tip influence on very high-Q modes.