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*r*_{1} - *r*_{2} = (*m* + ) *m* = 1,2,.... *constructive*

*r*_{1} - *r*_{2} = (*m*) *m* = 1,2,... *destructive*

- The above formulas are valid whenever an odd number of phase changes occur. If an even number of phase changes occur then the original, unmodified formulas must be used.
- The phase change does not happen when light is reflected off a less dense medium, i.e. for total internal reflection.

10/9/1997