curved mirrors chapter 14 section 3. spherical mirrors a spherical mirror has the shape of a...
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Curved Mirrors
Chapter 14 Section 3
Spherical Mirrors A Spherical mirror has the shape of a
sphere’s surface. It has some kind of curve The mirror is not flat.
Examples: Passenger side rear view mirror Some dressing table mirrors
Used to magnify or shrink the size of a reflected image of an object.
Concave Spherical Mirrors
Concave Spherical Mirror – An inwardly curved mirrored surface that is a portion of sphere and that converges incoming light rays. Concave spherical mirrors are used
whenever a magnified image of an object is needed.
Curvature of the mirror
One factor that determines where the image will appear and how large that image will be is the amount by which the mirror is curved.
This depends on the radius of curvature
Radius of Curvature
The radius of curvature is the same as the radius of the sphere whose curvature would be identical to that of the mirror.
Radius of curvature variable – Uppercase (R) The radius is the distance from the
mirror’s surface to the center of curvature (C)
Principal Axis
The Principal axis is the line that extends infinitely from the center of the mirrors surface through the center of curvature.
Concave Mirror Diagram
Image Point
The image point in the previous picture forms below the principal axis and forms the image in front of the mirror.
If a piece of paper was placed at the image point, a clear image would form on the paper of the pencil. Move the paper back and forth and the
image will become blurry and out of focus.
Real Image
Real Image – An image formed when rays of light actually intersect at a single point.
Real images can be displayed on a surface Example:
Movie screen
Image Location Can Be Predicted
The object’s distance, image distance and radius of curvature are all interdependent.
Change one of the three can affect one of the others.
Mirror Equation
p – Object Distance
q – Image Distance
R – Radius of Curvature
Rqp
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Focal Point
Focal Point - Point where rays from lens or curved mirrors converge. Denoted by capital letter (F)
At this point on a curved mirror the light rays converge to a single point. The reflected rays of light from a source
emitted at the focal point will emerge parallel to each other.
Focal Point Diagram
Focal Length
Focal Length – The distance from the curved mirror or lens to the focal points. Denoted as lower case (f)
The focal length is half the distance of the radius of curvature. Or the radius of curvature is double that
of the focal length.
Mirror Equation
p – Object Distance
q – Image Distance
f – focal Length fqp
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Front and Back of a Concave Mirror
In order to use the mirror equation, a set of sign conventions for the three variables must be established. The region where light rays reflect and real
images form is called the front side of the mirror.
The region where light rays do not exist and imaginary images are formed is called the back side of the mirror.
Mirror images are usually drawn so the front side is to the left of the mirror’s surface.
Positive and Negative Concave Mirror
Object and image distances can be positive and negative. The distance has a positive sign when
measured from the center of the mirror to any point in front side of the mirror.
Distances for images that form on the back side of the mirror will always be a negative distance.
Since the focal point is in front on the mirror, the focal length will always be positive.
Positive and Negative Heights
The object and image heights are positive when they are above the principal axis.
The object and image heights are negative when they are below the principal axis.
Magnification
Unlike flat mirrors, curved mirrors form images that are not the same size as the object.
To measure how much larger or smaller the image is compared to the object's size is called the magnification of the image.
Magnification is defined as the ratio of the height of the pencil’s image to the pencil’s actual height. Magnification is denoted by an
uppercase letter (M) It is also defined as the negative of
the ratio of the image distance over the object distance.
If the image is smaller then the object, M should be less than 1.
If the image is greater then the object, M should be greater than 1. Magnification is a unitless quantity.
Magnification Equation
M – Magnification h’ – Image height h – Object height q – image distance p – object height
p
q
h
hM
'
Magnification Clarification
For an image in front of the mirror, M is negative and is inverted.
For an image in the back of the mirror, M is positive and the image is upright.
Sign Convention for Magnification
Orientation of image with respect to
object
Sign of M Types of image this applies to
Upright + Virtual
Inverted - Real
Ray Diagrams
Ray diagrams can be used for concave spherical mirrors.
The rules for making a ray diagram of a flat mirror are the same for making ray diagrams of concave spherical mirrors. Measure all distances along the principal
axis and mark the center of curvature (C) and focal point (F).
Must be drawn to scale.
Ray Diagram for a Spherical Mirror
For a spherical mirror, three reference rays are used to find the image point. The intersection of any two rays locates
the image. The third ray should intersect the same
point and is used to check the diagram.
Rules For Drawing Reference Rays
Ray Line drawn from object to mirror
Line drawn from mirror to image after reflection
1 Parallel to principal axis
Through focal point (F)
2 Through focal point (F)
Parallel to principal axis
3 Through center of curvature (C)
Back along itself through (C)
Object Distance Greater Than Focal Length
Object Distance Equal to Focal Length
Object Distance Less Than Focal Length
Example Problem
When an object is placed 30.0cm in front of a concave mirror, a real image is formed 60.0cm from the mirror’s surface. Find the focal length.
Example Problem Answer
f = 20.0 cm
Convex Spherical Mirror Convex Spherical Mirror – An
outwardly curved, mirrored surface that is a portion of a sphere and that diverges incoming light rays.
Examples: Mirrors to dangerous driveway
entrances. Passenger Side rearview Mirror
“Objects in the mirror are closer than they appear”
Diverging Mirror
A convex spherical mirror is sometimes called a “Diverging Mirror” The incoming rays of light diverge after
reflecting off the surface as though they were coming from some point behind the mirror.
Virtual Image
The image that is produced on a convex spherical mirror (diverging mirror) will always be a virtual image. Imaginary image.
Convex Spherical Mirror Terminology
The image distance (q) will always be negative.
The focal length (f) will always be negative cause the mirrored surface is on the opposite side of the radius.
Ray Diagrams for a Convex Mirror
Drawing Ray Diagrams for a convex mirror are slightly different then for a concave mirror. The three rays are still needed going to
the focal point, center of curvature and one parallel to the principal axis
The focal point and center of curvature are located behind the mirror. Dotted lines are extended along the
reflected reference rays to points behind the mirror.
An upright imaginary image forms where the three rays intersect.
Ray Diagram of a Convex Spherical Mirror
Magnification for a Convex Spherical Mirror
The magnification for a convex spherical mirror will always be less than 1. The image always appears smaller than
the object. This gives the appearance that the object is
further away from the mirror. Hence the warning sticker on the bottom of
the mirror on the passenger side rearview mirror.
Convex Mirror Applications
Convex spherical mirrors take the objects in a large field of view and produce a small image.
They are well suited for providing a fixed observer with a complete view of a large area.
Different Applications of Convex Spherical Mirrors
Symbol Situation Sign
p The object is in front of the mirror +
q The image is in front of the mirror (real image) +
q The image is behind the mirror (virtual image) -
R,f The center of curvature is in front of the mirror (Concave Spherical Mirror)
+
R,f The center of curvature is behind the mirror (Convex Spherical Mirror)
-
R,f The mirror has no curvature (Flat Mirror) ∞
h’ The image is above the principal axis +
h’ The image is below the principal axis -
Example Problem
The radius of curvature of a convex mirror is 12.0cm. Where is the focal point?
Example Problem Answer
6.00 cm behind the mirror, therefore the value needs to be negative.
f = -6.00cm
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