where λ λ is the wavelength of the probing wave, n is the refractive index of the medium the wave is travelling in and the wave is converging to a spot with a half-angle of θ θ . While it is possible to overcome the diffraction limit on resolution by using a near-field technique, it is usually quite difficult. Since the denominator of the above equation for the Abbe diffraction limit will be approximately two at best, the wavelength of the probe is the main factor in determining the minimum resolvable feature, which is typically about 1 μm for optical microscopy.