Why does the Becke Line appear?
When a ray passes from one medium into another with a lower refractive index, it is refracted and diverges from the its optic normal towards their line of contact. This occurs with rays 1 and 2 in diagram 1. Ray 3 shows a very high angle of incidence - higher than the limiting angle - and is reflected instead of refracted.
The same happens in diagram B, which represents the vertical cut of a preparation with contact between two minerals. The rays represented are reflected and cause an accumulation of light along the side of the line of contact of the mineral with the higher index (they would be on the other side in medium "n" if they had been refracted). These reflected beams produce the Becke Line.
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