Law of Reflection
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Describes the behavior of light after it strikes a smooth surface such as a mirror.
Incident ray
Beam of light striking a surface.
Reflected ray
Beam of light bouncing back after striking a surface.
Normal
Line drawn perpendicular to surface where the incident ray strikes, from
which all angles are measured.
Angle of incidence (θi)
Angle measured from the incident ray and the normal.
Angle of reflection (θR)
Angle measured from the reflected ray and the normal.
The law of reflection states that when a ray of light reflects off a surface,
the angle of incidence is equal to the angle of reflection:
θ(i) = θ(r)
Normal
•
Eye
θ(r)
Reflected Ray
Object
θ(i)
Incident Ray
Virtual Image
How to Draw a Ray Diagram (Plane Mirrors)
1. Draw a ray from the base of the object perpendicular to the mirror.
Extend this ray behind the mirror using a dashed line.
2. Draw a ray from the top of the object perpendicular to the mirror.
Extend this ray behind the mirror using a dashed line.
3. Draw a ray from the top of the object to the point where the first ray meets the
mirror.
4. Draw the reflected ray according to the law of reflection.
5. Extend the reflected ray behind the mirror using a dashed line until it intersects
the ray from the top of the object. This is the top of the image .
O
O’
Object
Image
Reference line
Mirror
More about images & plane mirrors…
• Each sight line can be traced backwards beyond the mirror.
• All sight lines intersect at the image location.
• The image is formed at the single point on the opposite side of
the mirror from where all the light rays appear to have
diverged.
Viewer 1
Object
Viewer 2
Viewer 3
Virtual
Image
Location