## Proof of Equality of Angles of Incidence and Reflection

The equality of the angles of incidence and reflection can be proved using Fermat's principle: that light takes the least time to travel between two points.

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The time taken for light to travel the routein the diagram above is

For the time to be a minimummust be equal to zero andmust be greater than zero.

and

Putto obtain

hence is a minimum.