Reflection occurs when light does not pass through a medium or surface.  This can be compared to throwing a baseball at a wall.  Instead to going through the wall, the baseball bounces back.  Whether light approaching a different medium will be reflected depends on the angle at which it hits the medium.  Before I explain this, let me first introduce some common terms.

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Figure 4:  The Concept of Reflection

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The incident ray is just the light which is approaching a different medium.  In Figure 4, it is clear that the incident ray is approaching water (medium2) from air (medium1).  The normal is an imaginary plane which is perpendicular to the boundary of the two media.  That is, it always forms a 90o angle with the surface of the water.  The reflected ray is what bounces back when the light hits the water.  Notice that the angle at which the incident ray hits the surface of the water is the same as the angle at which it is reflected off the surface (they both equal 60o in this case).  This is because the angle of incidence is always equal to the angle of reflection

When all of the incident ray is reflected (that is, nothing gets through the air/water boundary), we can say that the light underwent a total internal reflection.

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