Bionanophotonics is a consequence of complicated interaction
between light and minute structure. Before entering into its details,
it will be meaningful to build up an image of what light is. First we
have to create an image of the wave nature of light. We sometimes
notice that the sun’s ray peeps throughwisps of cloud. In such a case,
we feel that light is a kind of ray that travels straight through a space.
When light illuminates a matter of a size of 1 μm or less, or reﬂects
from a thin layer, however, the wave nature of light will deﬁnitely
come out. For example, when light passes through a narrow hole,
the light does not travel straight, but its direction will be slightly
uncertain, the degree of which is dependent on the size of a hole.
When the hole is small enough, the uncertainty of the propagation
direction will be prominent, whereas it will not be so when light
passes through a large hole. This phenomenon is called diﬀraction of light.