Explain internal reflection and total internal reflection.

(N/A) When light travels from an optically denser medium to a rarer medium at the interface,it is partly reflected back into the same medium and partly refracted into the second medium. This reflection is called internal reflection.
When a ray of light enters from a denser medium to a rarer medium,it bends away from the normal.
In the figure,for the incident ray $AO_{1}$,the light is partially reflected $(O_{1}C)$ and partially transmitted $(O_{1}B)$ or refracted.
The angle of refraction $(r)$ is larger than the angle of incidence $(i)$.
As the angle of incidence increases,so does the angle of refraction,until for the ray $AO_{3}$,the angle of refraction becomes $\frac{\pi}{2}$.
The refracted ray is bent so much away from the normal that it grazes the surface at the interface between the two media. This is shown by the ray $AO_{3}D$.
The angle of incidence for which the angle of refraction becomes $\frac{\pi}{2}$ is called the critical angle '$i_{C}$'.
The deviated ray obtained after incidence at the critical angle is called the critical ray.
If the angle of incidence is increased further,e.g.,for the ray $AO_{4}$,refraction is not possible and the incident ray is totally reflected. This is called total internal reflection.
When light gets reflected by a surface,normally some fraction of it gets transmitted. The reflected ray,therefore,is always less intense than the incident ray,no matter how smooth the reflecting surface may be. In total internal reflection,on the other hand,no transmission of light takes place.