In earlier experiments concerning reflection and refraction of light at plane surfaces and image formation by lenses and mirrors, it was not necessary to consider the wave nature of light. Instead, rays of light were used to show the direction light traveled. The direction changed abruptly whenever light struck a boundary between two different materials. The laws of reflection and refraction were used to describe these changes in direction. If, however, a beam of light is transmitted through a very small opening, it does not continue as a narrow beam but instead, spreads out (diffracts) as it diverges from the opening. In addition, if light passes through more than one slit, the diffracted light from each slit overlaps and produces interference. Such effects can be understood by considering the wave nature of light. THEORY Diffraction The Huygens-Fresnel principle is of fundamental importance in understanding the phenomenon of diffraction. This principle states that every point on a wavefront acts as an emitter of secondary spherical wavelets. At a later time, the new position of the wavefront is determined by the superposition of these secondary wavelets