The wave nature of light becomes apparent when there are obstacles or apertures with sizes comparable to the wavelength of light (several hundred nano-meters), or when we look within comparable distances at the edges of shadows. In these cases, interference leades to effects that geometric optics cannot explain, such as the double-slit phenomena (diffraction). Diffraction is another manifestation of interference phenomena.
The term can be used as an alternative to the term interference, but it usually refers specifically to the deviation of wave fronts from straight-line behavior. we are already familiar with the bending of ocean waves around obstacles, and the circular spread of water waves passing through an opening narrower than their wavelength.
These phenomena are both diffractive effects. So is the pattern of maxima and minima that spreads across a screen in Young's double-slit experiment. The term "diffraction" also refers to interference between waves that emanate from a large number, or even a continuous set, of sources. Diffraction gratings have many slites or sources of coherent light and can be treated as a simple generalization of double slits.
These gratings have important applicatins in the study of atomic systems and crystalline materials. We shall see that even a single, narrow slit produces characteristic and striking interference patterns that result from diffraction. Holography is spectacular application of diffraction. Interference and diffraction effects are both consequences of the super-position of waves.
By the 1820s, serious attempts were underway to understand the consequences of the wavelike nature of light. Young's double-slit experiment had shown that there are clear interfence effects associated with light. Although the bending of water waves around obstacles or around the edges of apertures, it is not part of our everyday experience for light.
In addition to his double-slit experiment, Young performed experiments that showed that light can pass around a single small obstacle, just as water waves can. Young firmly believed in the wave theory and publicy supported it despite considerable resistance in the scientific community. Augustin Fresnel, francois Arago, and others produced further experiments and theories to establish even more firmly the reality of diffraction phenomena in light, wich refers to situation in wich light bends around obstacles by virute of its wave properties. Frensel carried his work sufficiently far that by 1821 he was able to use a primitive version of an interferometer to make the first quantitative measurement of the wavelength of light.
