Optical properties of photonic crystals - New Jersey Institute of ...

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As seen from the above figure, when a ray of light is incident on a slab of
material, part of it is reflected back, a part of it is absorbed and the rest is transmitted
through the material. In general such an event is governed by the equation
Here 'R' is the reflectance, 'T' is the transmittance and 'A' is the absorptance of
the material under consideration.
3.1 Reflectance, Absorptance and Transmittance of Radiation
These are the three main properties that are of concern in the domain of optics. They can
be described in a number of ways.
3.1.1 Reflectance
It is an abrupt change in the direction of propagation of a wave that strikes the boundary
between different media. At least part of the oncoming wave disturbance remains in the
same medium. Regular reflection, which follows a simple law, occurs at plane
boundaries. The angle between the direction of motion of the oncoming wave and a
perpendicular to the reflecting surface (angle of incidence) is equal to the angle between
the direction of motion of the reflected wave and a perpendicular (angle of reflection).
Reflection at rough, or irregular, boundary will cause the ray to diffuse. The reflectivity
of a surface material is the fraction of energy of the incoming wave that is reflected by it.
3.1.2 Absorptance
In wave motion, transfer of energy from wave to matter occurs as the wave passes
through it. The energy of an acoustic, electromagnetic, or other wave is proportional to