Plane Mirror Reflection
• We describe the path of light as straight‐
We describe the path of light as straight‐line rays
• Reflection off a flat surface follows a simple rule:
– angle in (incidence) equals angle out (reflection)
– angles measured from surface “normal”
(perpendicular) surface normal
Optics
incident ray
same
angle
exit ray
reflected ray
Curved Mirrors
Reflection Vocabulary:
Real and Virtual Images
• Real Image –
Real Image –
Reflection Vocabulary:
Real and Virtual Images
• Virtual Image –
Virtual Image –
– Image location is the result of actual light rays that converge at a real focal point so the image is REAL
– Can be projected onto a screen because actual light rays are passing through that point.
– Image location is the where light rays appear to converge, but no light rays actually exist there.
– Cannot be projected onto a screen.
– "Virtually": the same as if
Virtual Images in Plane Mirrors
Rays seem to come from behind
the mirror, but, of course, they
don't. It is virtually as if the rays
were coming from behind the mirror.
If light energy doesn't flow from the image, the image is "virtual".
As far as the eye‐brain system is
concerned, the effect is the same
as would occur if the mirror were
absent and the chess piece were actually located at the spot labeled "virtual image".
Curved mirrors
• What if the mirror isn’
What if the mirror isn’t flat?
– light still follows the same rules, with local surface normal
Curved Mirrors
Two Types of Curved Mirrors:
How can we explain this?
Focal point
Focal point
Concave Mirror
(converging mirror)
Convex Mirror (diverging mirror)
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Concave Mirror Ray Diagram:
Ray Tracing Rules for Mirrors
• Horizontal light ray reflects through the focal point.
• Light ray through the focal point reflects horizontally
• OPTIONAL: Light ray hitting center of mirror reflects at an equal angle
image
Object outside focal point
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Object Moved Closer to Focal Point
Focal point
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Concave Mirror Ray Diagram:
Focal point
Focal point
image
No image formed...
Image is further and larger, still inverted
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Object at focal point
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General Characteristics for Concave Mirror Images:
Concave Mirror Ray Diagram:
image
Object past the focal point:
real, inverted image in front of mirror
Object at focal point:
no image formed
Focal point
Object inside focal point
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Object inside focal point:
virtual, upright image behind mirror.
Here, the image distance is negative. 14
Large Concave Mirror
Curved Mirrors
Large concave mirrors can collect and concentrate large amounts of sunlight. This can produce intense heat which could be utilized...
Did we explain this?
mirror
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Convex Mirror Ray Diagram:
Optical Illusion:
Rays appear to originate from this location. Image appears here.
image
Focal point
Object
Always leads to virtual, upright, smaller image
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Application of Convex Mirrors:
Security mirrors in department stores allow people to get a wide field view of shoplifters…
Mirror Equation:
To be more quantitative, an equation can be applied to mirrors to find the location of object, image, and focal point:
1 1 1
 
f do di
f = focal length
do = object distance (from mirror)
Additional mirrors on car side view mirrors also 19
provide a wider field of view. Example:
Imagine you have a concave mirror with a focal length of 10 cm. If you hold an object 15 cm from the mirror, describe the image:
1 1 1
 
f do di
1
1 1
 
10 15 di
1 1 1
 
10 15 di
3
2 1


30 30 di
1 1

30 di
di  30 cm
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di = image distance (from mirror)
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