Reflection and Mirrors

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Reflection and Mirrors
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The Law of Reflection

• When light strikes a surface it is reflected.
• The light ray striking the surface is called the
  incident ray.
• A normal (perpendicular) line is then drawn at
  the point where the light strikes the surface.
• The angle between the incident ray and the normal
  is called the angle of incidence.
• The light is then reflected so that the angle of
  incidence is equal to the angle of reflection.
• The angle of reflection is the angle between the
  normal and the reflected light ray.

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Mirror
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The incident ray, normal, and reflected ray are
all in the same plane.
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Regular reflection occurs when light is reflected
from a smooth surface. When parallel light rays
strike a smooth surface they are reflected and
will still be parallel to each other.
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Diffuse reflection occurs when light is reflected
from a rough surface. The word rough is a
relative term. The surface is rough at a
microscopic level. For example, an egg is a
rough surface. When parallel light rays strike a
rough surface, the light rays are reflected in
all directions according to the law of reflection.
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• Light reflected from the surface of a painted
  wall would be an example of ____ reflection.
• A. Regular
• B. Diffuse

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Types of Mirrors
Convex mirrors are made from a section of a
sphere whose outer surface was reflective. Convex
mirrors are also known as diverging mirrors since
they spread out light rays. They are typically
found as store security mirrors.
Concave mirrors are made from a section of a
sphere whose inner surface was reflective. Concave
mirrors are also known as converging mirrors
since they bring light rays to a focus. They are
typically found as magnifying mirrors
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Plane Mirrors have a flat surface. The mirror
hanging on the wall in your bathroom is a plane
mirror.
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Real images are images that form where light rays
actually cross. In the case of mirrors, that
means they form on the same side of the mirror as
the object since light can not pass through a
mirror. Real images are always inverted (flipped
upside down).
Virtual images are images that form where light
rays appear to have crossed. In the case of
mirrors, that means they form behind the
mirror. Virtual images are always upright.
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image
Original

• What type of image is shown above?
• A. Real
• B. Virtual

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Plane Mirror
In a plane mirror the object is the same size,
upright, and the same distance behind the mirror
as the object is in front of the mirror.
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Images in a plane mirror are also reversed left
to right.
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Original
Image

• What type of mirror created this image?
• Plane
• Convex
• Concave

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Curved Mirrors
The center of curvature also known as radius of
curvature (C) of a curved mirror is located at
the center of the sphere from which it was made.
The focal point (f) is located halfway between
the mirrors surface and the center of curvature.
C 2f
The principle axis is a line that passes through
both the center of curvature (C) and the focal
point (f) and intersects the mirror at a right
angle.
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• A concave mirror has a radius of curvature of 15
  cm. What is the focal length of this mirror?
• A. 15 cm
• B. 30 cm
• C. 7.5 cm

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Rules for Locating Reflected Images
1. Light rays that travel through the center of
curvature (C) strike the mirror and are reflected
back along the same path.
2. Light rays that travel parallel to the
principle axis, strike the mirror, and are
reflected back through the focal point (f).
3. Light rays that travel through the focal
point (f), strike the mirror, and are reflected
back parallel to the principle axis.
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All three of these light rays will intersect at
the same point if they are drawn carefully.
However, the image can be located by finding the
intersection of any two of these light rays.
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Locating images in concave mirrors
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Concave Mirror with the Object located beyond C
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Concave Mirror Object beyond C
Light rays that travel through the center of
curvature (C) hit the mirror and are reflected
back along the same path.
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Concave Mirror Object beyond C
Light rays that travel parallel to the principle
axis, strike the mirror, and are reflected back
through the focal point (f).
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Concave Mirror Object beyond C
Light rays that travel through the focal point
(f), strike the mirror, and are reflected back
parallel to the principle axis.
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Concave Mirror Object beyond C
Image Real Inverted Smaller Between f and C
The image is located where the reflected light
rays intersect
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Concave Mirror with the Object located at C
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Concave Mirror Object at C
Light rays that travel parallel to the principle
axis, strike the mirror, and are reflected back
through the focal point (f).
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Concave Mirror Object at C
Light rays that travel through the focal point
(f), strike the mirror, and are reflected back
parallel to the principle axis.
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Concave Mirror Object at C
Image Real Inverted Same Size At C
The image is located where the reflected light
rays intersect
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Concave Mirror with the Object located between f
and C
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Concave Mirror Object between f and C
Light rays that travel through the center of
curvature (C) hit the mirror and are reflected
back along the same path.
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Concave Mirror Object between f and C
Light rays that travel parallel to the principle
axis, strike the mirror, and are reflected back
through the focal point (f).
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Concave Mirror Object between f and C
Light rays that travel through the focal point
(f), strike the mirror, and are reflected back
parallel to the principle axis.
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Concave Mirror Object between f and C
Image Real Inverted Larger Beyond C
The image is located where the reflected light
rays intersect
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Concave Mirror with the Object located at f
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Concave Mirror Object at f
Light rays that pass through the center of
curvature hit the mirror and are reflected back
along the same path.
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Concave Mirror Object at f
Light rays that travel parallel to the principle
axis, strike the mirror, and are reflected back
through the focal point (f).
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Concave Mirror Object at f
No image is formed. All reflected light rays are
parallel and do not cross
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Concave Mirror with the Object located between f
and the mirror
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Concave Mirror Object between f and the mirror
Light rays that travel through the center of
curvature (C) hit the mirror and are reflected
back along the same path.
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Concave Mirror Object between f and the mirror
Light rays that travel through the focal point
(f), strike the mirror, and are reflected back
parallel to the principle axis.
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Concave Mirror Object between f and the mirror
Light rays that travel parallel to the principle
axis, strike the mirror, and are reflected back
through the focal point (f).
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Concave Mirror Object between f and the mirror
Image Virtual Upright Larger Further away
The image is located where the reflected light
rays intersect
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Locating images in convex mirrors
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Convex Mirror with the Object located anywhere in
front of the mirror
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Convex Mirror Object located anywhere
Light rays that travel through the center of
curvature (C) hit the mirror and are reflected
back along the same path.
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Convex Mirror Object located anywhere
Light rays that travel parallel to the principle
axis, strike the mirror, and are reflected back
through the focal point (f).
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Convex Mirror Object located anywhere
Light rays that travel through (toward) the focal
point (f), strike the mirror, and are reflected
back parallel to the principle axis.
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Convex Mirror Object located anywhere
Image Virtual Upright Smaller Behind mirror,
inside f
The image is located where the reflected light
rays intersect
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Image
Original

• What type of mirror could have created the image
  shown above?
• A. Plane
• B. Concave
• C. Convex

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image
Original

• Where would the object need to be placed to
  create this image?
• A. At C
• B. Between f and C
• C. Beyond C
• D. Between f and the mirror

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image
Original

• Where would this image form?
• A. At C
• B. Between f and C
• C. Beyond C
• D. Between f and the mirror

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Mirror Equation
(1/f) (1/do) (1/di)
f focal length do object distance di image
distance
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Mirror Magnification Equation
M -(di / do) (hi / ho)
M magnification di image distance do object
distance hi image height ho object height
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Mirror Sign Conventions
F for Concave Mirrors - for Convex
Mirrors di for images in front of the
mirror - for images behind the mirror do
always hi if upright image - if inverted
image ho always M if virtual - if
real image Magnitude of magnification lt1 if
smaller 1 if same size gt1 if larger
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• If the focus of a convex mirror is 60 cm from the
  mirror, what is the radius of curvature?
• A. 120 cm
• B. -120 cm
• C. 30 cm
• D. -30 cm

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• A concave mirror of radius 60 cm is placed so
  that a luminous object is 35 cm in front of the
  mirror. Where does the image form?
• A. 0.0048 cm
• B. 210 cm
• C. -0.012 cm
• D. -84 cm

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• A man 2 m tall stands 10 m in front of a convex
  mirror which has a radius of curvature of 5 m.
  How tall is the image?
• A. 0.4 m
• B. -0.7 m
• C. 0.7 m
• D. 2 m