Transcript Chapter 30 Lens

Refraction and Lens
Refraction
Refraction: the change in
direction of a wave as it crosses
the boundary b/w 2 media in
which a wave travels different
speeds
 the bending of waves b/c one part of
each wave is made to travel slower
(or faster) than another part.
 Ex: Your legs appearing shorter in a
swimming pool; a pencil appearing bent
when in a glass of water
• When light rays enter a medium in which
their speed decreases (more dense
medium), as when passing from air to water,
the rays bend toward the normal.
• When light rays enter a medium in which
their speed increases (less dense medium),
as when passing from water to air, the rays
bend away from the normal.
Lenses
•
•
A lens is a piece of glass or plastic that
bends parallel rays of light so they cross
and form an image.
Lenses function off the principle that light
refracts thru glass

•
Light bends b/c it is changing speed going
from one medium into another
There are 3 types of lenses:
1. Plane (flat)
2. converging (aka Convex)
3. Diverging (aka Concave)
Converging Lenses
• A converging lens is thicker in the middle than the
edges.
 Convex refers to the shape of the lens
• Rays of light that are initially parallel are made to
converge (come together)
 Converging refers to the behavior of the light
• Rays that move
through the center
move slower
than rays moving
through the edges
Diverging Lens
• A diverging lens is thinner in the middle than the
edges.
 Concave refers to the shape of the lens
• Rays of light that are initially parallel are made to
diverge (spread apart)
 Diverging refers to the behavior of the light
• Rays that move
through the center
move faster
than rays moving
through the edges
Focal Points
• The focal point is the point where light
converges.
• In a converging lens, light passes through
the focal point.
• In a diverging lens, light appears to go
through the focal point. Light looks like it
originates from a single point.
Lens Image Formation
Lens
Converging
(convex)
Diverging
(concave)
Outside the
focal point
Real
Smaller
Inverted
Inside the
focal point
Virtual
Larger
Upright
Virtual
Smaller
Upright
Virtual
Smaller
Upright
Check Your Understanding
What type of lens is used in a magnifying
glass?
A converging lens. It will create a larger
(magnified), upright image. Many
instruments used for magnification, such
as a microscope or telescope, use multiple
converging lenses.
Check Your Understanding
Why are diverging lenses not used on
overhead or LCD projectors?
B/c diverging lenses only produce virtual
images. Only real images can be
projected onto a screen.
The Eye
• The eye is similar to the camera.
• The amount of light that enters the eye
is regulated by the iris which surrounds
the opening called the pupil. Light
enters through the transparent cornea
through the pupil. The lens focuses the
light on a layer of tissue on the back of
the eye called the retina.
• The eye uses a converging lens to
focus light on the retina.
• In both a camera and the eye, the image
that is formed is up side down. Our brain
turns the image right side up for us
• Adjustments in focusing is called
accommodations.
• Accommodation is accomplished by
changing the thickness and shape of the
lens.
– Our eye does this by contracting or releasing
eye muscles
Check Your Understanding
Why does your brain have to “flip” the
image projected onto the back of our
eye?
Because our eye uses a converging lens,
which inverts images outside the focal
point (most objects). Our brain has to flip
the image so as not to confuse our
balance.
Some Defects in Vision
• The eyes of a farsighted person forms images
behind the retina. The eyeball is too short.
• A person with farsighted vision can see objects
that are far but not near.
• A converging lens corrects farsighted vision.
• The eyes of a nearsighted person forms
images in front of the retina. The eyeball is
too long.
• A person with nearsighted vision can see
objects that are near but not far.
• A diverging lens will correct nearsighted
vision
Summary of Vision Defects
Vision Type
CAN See
CANNOT
See
Corrective
Lens
Farsighted
Far
Near
Converging
Nearsighted
Near
Far
Diverging
Astigmatism: a defect of the eye
caused when the cornea is curved more
in one direction than in another
Total Internal Reflection
• When light travels from a more dense
medium to a less dense medium, there is a
certain angle to which no light will be
refracted and all light will be reflected. This
is called the critical angle.
• Total internal reflection is the 100% reflection
of light that strikes the boundary between 2
media at an angle greater than the critical
angle.
 Ex: The critical angle for glass is 43°, the critical
angle for water is 48°, and the critical angle for a
diamond is 24.6°.
• The dotted line represents the critical
angle. The green line represents total
internal reflection.
• Optical fibers are transparent fibers that
pipe light from one place to another using
total internal reflection.
• Optical fibers are important in
communications.
Check Your Understanding
What type of lens does one where to
correct farsightedness?
A converging lens because it can make
objects appear larger (closer) than they
actually are.