Light propagates in a straight line through space. This is one of

the well-known features of light. However, this description is true

only when the space is uniform and there is no obstacle to hinder

its propagation. If there is an obstacle, light will go round it and

will diffusely propagate with extending propagation direction. This

phenomenon is known as diffraction, which is one of the typical wave natures of light. The diffraction of light is familiar even in our

daily life: When the sunlight goes though a narrow crack of a wall,

one will notice that the size of a light spot casting on a floor is

not the same as that of the crack, but is much larger than it. If one

holds two pencils in contact up to the light, a few dark lines will be

noticed between them. These phenomena are typical indications of

the diffraction of light.