When a light ray strikes a surface, it makes a certain angle with the surface. This angle is measured with reference to a normal drawn to the surface and is known as angle of incidence. This ray is reflected from the surface, at an angle to the normal, known as the angle of reflection. By convention, the angle of incidence is assumed to be equal to the angle of reflection in case of plane mirror. That is,
Angle of incidence = angle of reflection = 15 degrees.
Reflection involves a change in direction of incident ray and this convention is used to determine the direction of the reflected ray. This relationship between incident and reflected light rays can be used for various applications, such as periscope.
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