The normal is a line drawn at right angles (90 degrees) to a surface at the point where an incident ray meets the surface. It is used as the reference line for measuring angles of incidence, reflection and refraction.
In reflection
The angle of incidence is measured between the incident ray and the normal. The angle of reflection is measured between the reflected ray and the normal. The law of reflection states: angle of incidence = angle of reflection.
In refraction
The angle of incidence and the angle of refraction are both measured from the normal, not from the surface. This is a frequent source of error.
Drawing the normal
Draw the normal as a dashed line, perpendicular to the surface. Mark the angles of incidence and refraction clearly.
In exam questions
Always draw the normal in ray diagrams. Measure angles from the normal, not from the surface. Forgetting to include the normal or measuring angles from the surface are common errors that lose marks.
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