Maxwell's equations are arguably the most accurate physical theory developed by man. Unfortunately, they are rather hard to deal with, both analytically and numerically. Over the centuries many approximations to these equations have been developed to allow us understand electromagnetic phenomena and develop optical devices. All these approximations are based on the assumption that the light wave length is "small" with respect to the natural scale of the system under study. This, of course, begs the question "How small is small?". Nowadays it is possible to routinely build micro-cavities just a few microns large: are they "small" or do we have to give up all approximations and use unadulterated Maxwell's equations?
In this talk I will present an experimental and theoretical analysis of the mode structure of micro-cavities and try to answer this question.