# The angle between the incident beam and the mirror plane is equal to the angle between the incident beam and the reflected one.

The angle between the incident beam and the mirror plane is equal to the angle between the incident beam and the reflected one. What is the angle of incidence?

The angle of incidence is equal to the angle of reflection.
The problem describes three corners.
The angle of incidence is 90 degrees – the angle between the incident beam and the plane mirror as determined by the angle of incidence.
The angle between the incident beam and the mirror is equal to the angle between the reflected beam and the mirror according to the law of reflection. The same value is the angle between the incident ray and the reflected one – according to the problem statement. The sum of these three angles is 180 degrees – the mirror is flat. The three angles are equal and together make up 180 degrees, therefore, the magnitude of each of them is 180/3 = 60 degrees.
90-60 = 30 degrees – the value of the angle of incidence.

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