6.4. Transmission.   

Obeying the laws of physics, transmission also has two basic forms, regular and diffuse. Transparent glass exhibits regular transmission, whereas frosted glass exhibits diffuse transmission. However, unlike reflection, mixed transmission is very rare. The model used to implement this system includes only regular transmission. 

     
Refraction.

    
                     Fig. 6.2 : Refraction.
Figure 6.2 demonstrates the refraction of the light ray as it passes through a transparent object.

In that figure the following holds: 

                                I       =       incident ray

                                N       =       surface normal

                                T1       =       1st refracted ray

                                T2       =       2nd refracted ray

                                        =       angle of incidence

                                |-|    =       angle of refraction

Every medium (e.g. air,glass) has associated with it a constant known as the 'index of refraction', which is the ratio of the speed of light in a vacuum to the speed of light in the medium. ([RICH 69] lists the refractive index of some common substances -- e.g. glass E 1.46-1.96.) On entering a transparent object, the light ray will be deflected towards the surface normal if the refractive index of the object being entered is greater than the refractive index of the object being exited, or away from the surface normal if the reverse is the case. Note that T is parallel to I as both are travelling through the same medium.