Optics Lesson 4
Reflection In Curved Mirrors
Terminology
Centre of curvature (C) – Middle of the “circle”
Focus (focal point) (F) – Where reflected rays all
intersect
Principal Axis (PA) – The line drawn through the
centre of the mirror (and meeting the mirror at 90°)
Vertex (V) – The exact point where the PA meets the mirror.
Recall
 Light rays always have an Angle of Reflection = Angle of
Incidence
 This equal angle is between the ray and the normal with is
a90° to the mirrors surface at the point where the ray hits the
mirror
Angle of Incidence
Normal (90° to mirror surface)
Angle of Reflection
Terminology
 Concave Mirrors – are also known as converging mirrors
as the light converges at the focus in front of the mirror and
creates a real image.
 Mirror is “caved” in
 Convex Mirrors – are also known as diverging mirrors as
the focus is found behind the mirror (virtual image) and is
found by tracing back the reflected rays in a straight line.
Focal Point – Concave Mirror
We can use parallel rays to PA to find
the focal length of a concave mirror.
All parallel lines will reflect into the
focal point
*note the normal is 90° to the point
on the mirror where the ray strikes
The focal point for a concave
mirror is to the left
Concave Mirror Rules
Concave Mirror Rules
Concave Mirror Rules
Concave Mirror Rules
Ray Diagrams – Concave Mirrors
You need 2 rays to locate an image.
The image is located where the rays intersect.
If the rays do not intersect, you need to trace them back to the point
where they intersect.
You need to include 2 basic rays.
1. A parallel ray is reflected to the mirror then through the “focal point”
2. A ray is sent through the focal point is reflected parallel to the principal
axis.
F
Ray Diagrams
Describing Images
S – Size (Larger, Smaller, Same)
A – Attitude (Upright or inverted)
L – Location (How far from the mirror?)
T –Type (Real or Virtual)
Small than object
Inverted
Between C and F
Real
F
Object beyond C
Object at C
Object between C and F
Object at F
Object between F and mirror
Properties of a Converging Mirror
p. 499
Uses of Concave Mirrors
 Searchlights, car headlights and flashlights all use
concave mirrors to create parallel or almost parallel beams of
light (Why?)
 Telescopes and satellite dishes use concaves mirrors to
focus electromagnetic waves
at the focal point (Why?)
Uses of Concave Mirrors Con’t
We can also use
concave mirrors to
magnify a reflection
close up. Why does it
give this type of
reflection?
End of Day 1
Optics Lesson 4
Reflection In Curved Mirrors
Recall
 Light rays always have an Angle of Reflection = Angle of
Incidence
 This equal angle is between the ray and the normal with is
a90° to the mirrors surface at the point where the ray hits the
mirror
Angle of Incidence
Normal (90° to mirror surface)
Angle of Reflection
Terminology
 Concave Mirrors – are also known as converging mirrors
as the light converges at the focus in front of the mirror and
creates a real image.
 Mirror is “caved” in
 Convex Mirrors – are also known as diverging mirrors as
the focus is found behind the mirror (virtual image) and is
found by tracing back the reflected rays in a straight line.
Focal Point – Convex Mirror
 The focal point for a
convex mirror is to
the right.
 It can be found by drawing
lines parallel to the PA
towards the mirror
 Extend the reflected lines
past the mirror
 Where the imaginary lines
intercept is the focal
point
Centre of sphere – Convex Mirror
 The C for a convex mirror
is to the right.
 It can be found by drawing
a line perpendicular to
the mirror at any
point
 Extend the reflected lines
past the mirror
 Where the imaginary lines
intercept the PA is the C
Convex Mirror Rules
 The difference between a concave mirror and a convex
mirror is that in a convex mirror, F (now called a virtual
focus) and C are behind the mirror and light rays
seem to come from an apparent light source
behind the mirror
Convex Mirror Rules
Convex Mirror Rules
Convex Mirror Rules
Ray Diagrams – Convex Mirrors
The image is located where the extended reflected lines intersect behind
the mirror
You need to include 2 basic rays.
1. A parallel ray that extends from the top of the object to the
mirror and is reflected back to the viewer
2. A ray is directed towards the focal point and is reflected parallel to
the principal axis.
3. Extend the reflected rays past the mirror until they intersect
4. The point of intersection is the top of the image. The image extends
down to the Principal axis
Images in a Convex Mirror
 The rays reflected off a convex mirror
always diverge
 Reflected rays from an object never
cross in front of the mirror to form a
real image
 Instead it ALWAYS creates a
smaller, upright, virtual image
Uses of Convex Mirrors
 Convex mirrors show a
wide range of view with
their smaller virtual
image

.
 Used in:
 Security mirrors
 Side-view mirrors in cars
 “Objects in mirror are closer
than they appear” ??????
One More Demonstration
Concave vs. Convex Curved Mirrors
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