An object in front of a curved mirror gives off
light in all different directions. Most miss the
mirror so don’t make an image. Many, many,
many light rays do hit and follow the law of
reflection. A well-shaped mirror directs all of
those reflected light rays so that they can be
traced back to a single point.
Since all of the light rays point to the
same place, we don’t have to use all
of them, we just need to find three
light rays that are easy to draw and
find the point they seem to come
from. We then know that any
reflected ray will look like it comes
from that point and that is where the
image appears to us.
To understand how these special rays work, we need to know two
things about a curved mirror’s geometry.
First, every curved mirror has a central axis that “cuts” directly
through the middle of the mirror and indicates where the mirror
faces.
Axis
Second, there is a special point on the axis where all light rays
coming directly into the mirror (parallel to the axis) cross the axis
after they are reflected – known as the focal point. The focal point
is located at the midpoint between the surface of the mirror and its
center of curvature (the center of circle that defines the mirror’s
shape.
Focal
Point
R
Center of
Curvature
F
The first special ray leaves the object traveling parallel to the axis
and reflects off the mirror heading through the focal point.
.
R
F
The second special ray leaves the object traveling through the focal
point and reflects off the mirror heading parallel to the axis.
.
R
F
The third special light ray that passes through the center of
curvature hits the mirror directly and bounces directly back, along
the same line through the center of curvature.
.
R
F
Drawing a Concave Mirror Ray Diagram
Step #1: Incoming light ray parallel to axis –
Outgoing on trajectory through focal point
.
R
F
Object beyond focal point
Drawing a Concave Mirror Ray Diagram
Step #2: Incoming light ray through focal point
– Outgoing parallel to axis
.
R
F
Object beyond focal point
Drawing a Concave Mirror Ray Diagram
Step #3: Incoming light ray through center of
curvature – Outgoing through center of curvature
.
R
F
Object beyond focal point
Drawing a Concave Mirror Ray Diagram
Step #4: Draw image where reflecting lines
(real or imaginary) meet.
.
R
F
Object beyond focal point
IMAGE IS:
• Inverted
• Real
• Smaller than object
•Closer to mirror
Drawing a Concave Mirror Ray Diagram
Step #1: Incoming light ray parallel to axis –
Outgoing on trajectory through focal point
R
F
Object between center of curvature and focal point
Drawing a Concave Mirror Ray Diagram
Step #2: Incoming light ray through focal point
– Outgoing parallel to axis
.
R
F
Object between center of curvature and focal point
Drawing a Concave Mirror Ray Diagram
Step #3: Incoming light ray through center of
curvature – Outgoing through center of curvature
.
R
F
Object between center of curvature and focal point
Drawing a Concave Mirror Ray Diagram
Step #4: Draw image where reflecting lines
(real or imaginary) meet.
.
R
F
IMAGE IS:
• Inverted
• Real
• Larger than object
• Further from mirror
Object between center of curvature and focal point
Drawing a Concave Mirror Ray Diagram
Step #1: Incoming light ray parallel to axis –
Outgoing on trajectory through focal point
R
Object within focal point
.
F
Drawing a Concave Mirror Ray Diagram
Step #2: Incoming light ray “through” focal
point – Outgoing parallel to axis
R
Object within focal point
.F
Drawing a Concave Mirror Ray Diagram
Step #3: Incoming light ray “through” center of
curvature – Outgoing through center of curvature
R
Object within focal point
.F
Drawing a Concave Mirror Ray Diagram
Step #4: Draw image where reflecting lines
(real or imaginary) meet.
R
Object within focal point
.F
IMAGE IS:
• Upright
• Virtual
• Larger than object
• Further from mirror
Drawing a Convex Mirror Ray Diagram
Special Ray #1: Incoming light ray parallel to axis
– Outgoing on trajectory through focal point
F
R
Drawing a Convex Mirror Ray Diagram
Special Ray #2: Incoming light ray toward focal
point – Outgoing parallel to axis
F
All objects
R
Drawing a Convex Mirror Ray Diagram
Special Ray #3: Incoming light toward center of
curvature – Outgoing “through” center of curvature
F
All objects
R
Drawing a Convex Mirror Ray Diagram
Step #1: Incoming light ray parallel to axis –
Outgoing on trajectory through focal point
F
All objects
R
Drawing a Convex Mirror Ray Diagram
Step #2: Incoming light ray toward focal point –
Outgoing parallel to axis
F
All objects
R
Drawing a Convex Mirror Ray Diagram
Step #3: Incoming light ray toward center of
curvature – Outgoing through center of curvature
F
All objects
R
Drawing a Convex Mirror Ray Diagram
Step #4: Draw image where reflecting lines (real
or imaginary) meet.
FF
All objects
R
IMAGE IS:
• Upright
• Virtual
• Smaller than object
• Closer to mirro