Concave Mirrors
Spherical Mirrors
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A spherical mirror has the shape of a section of a sphere.
If the outside is mirrored, it is __________.
If the inside is mirrored, it is _________. (Fig. 1)
Concave & Convex Mirrors
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For a concave mirror, both the focal point (F) and the centre of
curvature (C) are in front of the mirror.
For a convex mirror, both the focal point (F) and the centre of curvature
(C) are behind the mirror.
Incoming light ray (for both)
Figure 2. Concave and convex mirrors.
Focal Length – Concave Mirrors
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Parallel rays hitting a spherical mirror come together (intersect) at
the __________
Focal point, F, is _____________ the mirror
The distance from the focal point to the vertex of the mirror is the
_______________, f
Terminology
Constructing Ray Diagrams
Figure 1. Concave and
convex mirrors.
1. P ray: A light ray parallel to the principal axis is reflected
through the ___________. This is how the focus is defined.
2. C ray: A light ray through the centre of curvature is
reflected back ___________________. This rule makes
sense because any line through the centre of curvature is a
radius of the circle formed by the mirror. A radius is always
at 90° to the mirror. A ray along the normal has an angle of
incidence of 0°. This means that the angle of reflection is
also 0°. The reflected ray will return back on the same path.
3. F ray: A ray through F will reflect ____________ to the
principal axis. This rule uses the fact that the angle of
incidence is always ____________ to the angle of reflection. Even if you switch the incident and
reflected rays, the light will still follow the same path; only the direction will change. This principle
is called the _______________ of light.
4. V ray: A ray aimed at the vertex will follow the law of _____________. Because the principal axis
is perpendicular to the surface of the mirror, the angle of incidence can be easily measured (not
widely used as it requires the use of a protractor).
Summary:
Incident Ray
Reflected Ray
Parallel to principal axis
Through the focal point
Through C
θ
Through vertex
How do we see an image?
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A point object emits (or reflects) light in all _____________
All the rays reflected by the mirror meet at one ___________
Eye detects the light that hits the __________
Ray Diagrams for a Point Source
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O’
O
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No need to draw that many rays to
find the image
Draw any two out of four ________ rays originating from the point
object
Image is located at the intersection of their ______________ rays
Any other reflected rays will ______________ at this point as well
Ray Diagrams for a Non-Point Source
O’
O
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For non-point objects, use ____________ points (as in plane mirrors)
IMPORTANT - Any point on axis will form image on ___________.
Concave Mirror Image Properties Summary
Object
location (do)
Size
Attitude
Location
Type
Beyond C
At C
Between F
and C
At F
Between F
and the
Mirror
Scale Ray Diagrams
A pencil 15 cm high is located 60 cm in front of a concave mirror with a focal length of 20 cm.
Construct a scale diagram to correctly determine the location of the image and its height. [Thinking]
(indicate the scale used)
Ray Diagram Practice
1. Mark and label the focal point and the centre of curvature of the mirror in this diagram.
2. What is the orientation of the image when an object is located between a concave mirror and its
focal point?
3. What is the orientation of the image when an object is located between the focal point and the
centre of curvature of a concave mirror?
4. Is it possible for the image to be upright when an object is 30 cm away from a concave mirror with
a focal length of 25 cm? Explain your reasoning.
5. A concave mirror has a focal length of 5 cm. An object 2 cm high is 11 cm from the mirror. Draw a
scale ray diagram. Measure the image height and image distance.
Homework:
Complete the Concave Ray Diagram sheet, along with the above questions.