We demonstrate that light propagation in strongly-modulated 2D/3D PhCs becomes refraction-like in the vicinity of the photonic bandgap, which is contrary to the fact that light propagation in weakly-modulated photonic crystals is very different from refraction and thus the definition of refraction index becomes meaningless. Such a crystal behaves like a material having an effective refractive index controllable by the band structure. This situation is analogous to the effective-mass approximation in electron-band theory. The propagation states having a negative effective index exhibit unusual properties, such as mirror-like imaging effect, image-transfer effect. These properties are confirmed by finite-difference time domain simulations.