compare transmission, reflection, refraction and absorption of light using different material
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II.8. Reflection, Transmission, and Absorption
Reflection is the process by which electromagnetic radiation is returned either at the boundary between two media (surface reflection) or at the interior of a medium (volume reflection), whereas transmission is the passage of electromagnetic radiation through a medium. Both processes can be accompanied by diffusion (also called scattering), which is the process of deflecting a unidirectional beam into many directions. In this case, we speak about diffuse reflection and diffuse transmission (Fig. II.14). When no diffusion occurs, reflection or transmission of an unidirectional beam results in an unidirectional beam according to the laws of geometrical optics (Fig. II.15). In this case, we speak about regular reflection (or specular reflection) and regular transmission (or direct transmission). Reflection, transmission and scattering leave the frequency of the radiation unchanged. Exception: The Doppler effect causes a change in frequency when the reflecting material or surface is in motion.
Absorption is the transformation of radiant power to another type of energy, usually heat, by interaction with matter.

Fig. II.14 - a-c: Direct, mixed and diffuse reflection d-f: direct, mixed and diffuse transmission

Fig. II.15 - When directly reflected or directly transmitted, an unidirectional beam follows the laws of geometrical optics: direct reflection (left): ain = aout, direct transmission (right): n1 • sin(ain) = n2 • sin(aout) with n1 and n2 denoting the respective medium's index of refraction
II.8.a. Reflectance r, Transmittance t, and Absorptance a
In general, reflection, transmission and absorption depend on the wavelength of the affected radiation. Thus, these three processes can either be quantified for monochromatic radiation (in this case, the adjective "spectral" is added to the respective quantity) or for a certain kind of polychromatic radiation. For the latter, the spectral distribution of the incident radiation has to be specified. In addition, reflectance, transmittance and absorptance might also depend on polarization and geometric distribution of the incident radiation, which therefore also have to be specified.The reflectance r is defined by the ratio of reflected radiant power to incident radiant power. For a certain area element dA of the reflecting surface, the (differential) incident radiant power is given by the surface's irradiance Ee, multiplied with the size of the surface element, thus