Physics 4C
Chapter 34: Images
Real and Virtual Images
Specular and Diffuse Reflection
Plane Mirrors
Spherical Mirrors
Ray Tracing
Concave Mirrors
Convex Mirrors
Thin Lenses
Ray Tracing
Converging Lenses
Diverging Lenses
Lenses in Combination
The Human Eye
Real and Virtual Images
Real and Virtual Images
The Ray Model of Light ⇒ light travels in
straight-line paths called light rays.
⇒ Actually, a light ray is an idealization; it is
meant to represent an extremely narrow beam of
light.
⇒ When we see an object, according to the ray
model, light reaches our eyes from each point on
the object.
Real and Virtual Images
⇒ Although light rays leave each point in many
⇒ Your brain always assumes that light traveled
different directions, normally only a small bundle
of these rays can enter an observer’s eye, as shown
in the figure.
in a straight line path in reaching your eyes.
⇒ Because of this, you often see objects in a
different location than where they really are. We
have already seen one example of this with
refraction.
⇒ If the person’s head moves to one side, a
different bundle of light rays will enter the eye
from each point.
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Real and Virtual Images
⇒ Another example is the image seen from a
plane mirror.
⇒ The reflected light looks as though it came
from a point behind the mirror called the image
point.
Real and Virtual Images
⇒ This type of image is
called a virtual image
because the light rays do
not actually emanate from
the image point.
⇒ A virtual image can never be formed on a
surface or projected onto a screen.
⇒ In a sense, a virtual image only exists within
the brain. However, in physics, a virtual image is
said to exist at the perceived location.
Real and Virtual Images
Real and Virtual Images
⇒ A real image, such as one formed by a
⇒ A mirage is one type of virtual image that
concave mirror, is an image from which light rays
actually do emanate.
arises from the refraction of light.
⇒ A real image can be projected onto a screen
and exists in space whether or not someone is
there to view it.
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Real and Virtual Images
Real and Virtual Images
air of uniform density
air near the ground is less
dense so the speed of light
is slightly greater
⇒ Light travels slightly faster through warmer air
where the density is less.
⇒ Because of this, light bends towards cooler air.
Specular Reflection
Diffuse Reflection
⇒ Reflection from a smooth surface is called
specular reflection.
⇒ Reflection from a rough surface is called
diffuse reflection.
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Specular and Diffuse Reflection
Plane Mirror
⇒ Almost
everything we see
is because of
diffuse reflection.
⇒ If only specular
reflection occurred,
we could only see
objects from
certain angles.
⇒ The image from a plane mirror is virtual, upright,
the same size as the object, and as far behind the
mirror as the object is in front of it.
Plane Mirror
Spherical Mirrors
⇒ To view one’s full
length in a mirror, only a
half-length mirror is
needed.
⇒ A spherical mirror has the shape of a
section from the surface of a sphere.
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Concave and Convex Mirrors
Ray Tracing
Concave Mirror
Convex Mirror
Ray Tracing
Ray 2: A ray passing through the focal point
will be reflected parallel to the central axis.
Ray 1: A ray initially parallel to the central
axis will be reflected through the focal point.
Ray Tracing
Ray 3: A ray that travels along a line that
passes through the center of curvature C be
be reflected upon itself.
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Images from a Concave Mirror
Images from a Concave Mirror
⇒ A object placed between F and the mirror
⇒ A object placed between C and F will
will produce a virtual, enlarged, upright image.
produce a real, enlarged, inverted image beyond
the center of curvature C.
Images from a Concave Mirror
Images from a Convex Mirror
⇒ A object placed beyond C will produce a
real, reduced, inverted image between C and F.
⇒ The image produced by a convex mirror is
always virtual, reduced, and upright.
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Thin Lenses
Thin Lenses
Thin Lenses
Ray Tracing: Converging Lens
⇒ If nlens > nsurrounding medium:
Lenses thicker in the
middle than at the
edges are converging.
Lenses thinner in the
middle than at the
edges are diverging.
⇒ Ray 1: a ray initially parallel to the central
axis is refracted through the focal point on the
right side of the lens.
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Ray Tracing: Converging Lens
⇒ Ray 2: a ray through focal point on the left
side of the lens is refracted parallel to the central
axis.
Converging Lens
⇒ An object between F
and the lens will create a
virtual, upright, and
enlarged image.
Ray Tracing: Converging Lens
⇒ Ray 3: a ray directly through the center of
the lens is not refracted.
Converging Lens
⇒ An object between F
and 2F will create a real,
inverted, and enlarged
image.
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Converging Lens
⇒ An object beyond 2F
will create a real,
inverted, and reduced
image.
Ray Tracing: Diverging Lens
⇒ Ray 2: a ray traveling towards the focal point
on the right side of the lens is refracted parallel to
the principle axis
Ray Tracing: Diverging Lens
⇒ Ray 1: a ray initially parallel to the central
axis appears to have originated from the focal
point on the left side of the lens
Ray Tracing: Diverging Lens
⇒ Ray 3: a ray directly through the center of
the lens is not refracted
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Diverging Lens
⇒ A diverging lens always creates a virtual,
upright, and reduced image.
The Human Eye
Lenses in Combination
⇒ When lenses are used in combination, the
image from the first lens becomes the object for
the second lens.
The Human Eye
⇒ Without a doubt, the human eye is the most
remarkable of all optical devices.
⇒ The fovea is the spot on the center of our field of
view where our vision is most distinct. Greater detail can
be seen at the fovea that at the side parts of the eye.
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Rods and Cones
⇒ There are two kind of “antennae” in the retina for
detecting light : rods and cones (named because of their
different shapes).
Rods and Cones
⇒ In the human eye, there are about 6-7 million cones
and about 75-150 million rods.
rods ⇒ sensitive to
lightness or darkness
(but not color)
⇒ Cones are concentrated in the
fovea. The number of cones
decreases as you move away from
the fovea.
cones ⇒ sensitive to color
(there are three different
types of cones)
⇒ Rods are concentrated toward
the periphery of the retina.
Rods and Cones
⇒ It takes more energy (or more light) to
“activate” the color-sensitive cones than it does
the rods.
⇒ Because of this, if the intensity of light is very
low, the cones will not respond and the things we
see will have no color. (This is why stars appear
white)
⇒ The rods on the periphery of our retina can not
see color, but they are very sensitive to motion.
Our peripheral vision is poor, but we are sensitive
to anything moving in our periphery.
The Human Eye
⇒ To see any object, the lens must produce an
image of the object at the retina.
⇒ This means that the image distance must
always be the same – the distance between the
lens and the retina.
⇒ The only way for the
image distance i to be the
same for all object distances
p is for the focal length f of
the lens to change.
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The Human Eye
⇒ The process in which the lens changes its focal
length to focus on objects at different distances is
called accommodation.
The Human Eye
⇒ The near point is the closest an object can be
from the eye and still produce a sharp image on the
retina.
⇒ For people in their early twenties with normal
vision, the near point is about 25 cm. It increases to
~50 cm at age 40 and to ~500 cm at age 60.
⇒ The far point is the farthest an object can be
from the eye and still produce a sharp image on the
retina.
⇒ For people with normal vision, the far point is
infinity.
The Human Eye
⇒ Nearsightedness (myopia) refers to an eye that
can only focus on nearby objects. The far point of
the eye is less than infinity.
The Human Eye
⇒ Nearsightedness can be corrected by a diverging
lens that creates a virtual image at the far point of
the nearsighted eye.
⇒ Nearsightedness is usually caused by an eyeball
that is too long.
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The Human Eye
The Human Eye
⇒ Farsightedness (hyperopia) refers to an eye that
can only focus on far away objects. The near point
of the eye is greater than 25 cm.
⇒ Farsightedness can be corrected by a converging
lens that creates a virtual image at the near point of
the farsighted eye.
⇒ Farsightedness is usually caused by an eyeball
that is too short.
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