# Lenses

Document Sample

```					Lenses

Chapter 30
Converging and Diverging Lenses
   Lens – a piece of glass which bends parallel rays so
that they cross and form an image
   Converging Lens – a lens which is thicker in the
middle, makes parallel light rays converge
   Diverging Lens – a lens which is thinner in the
middle, makes parallel light rays diverge
   Principle Axis – line joining the centers of curvature
of a lens’ surface
   Focal Point – the point at which a beam of light
converges
   Incident parallel beams that are not parallel to the
principle axis focus at points above or below the focal
point
   Focal Plane – all possible points from incident
beams
   A lens has two focal points and two focal planes
Converging and Diverging Lenses
Image Formation by a Lens
   Magnification occurs when an image is observed
through a larger angle with the use of a lens than
with out the lens, and allows more detail to be seen
   A converging lens will only magnify when the object
is between the focal point and the lens
   Virtual Image – an image formed through reflection
or refraction that can be seen by an observer but
cannot be projected on a screen because light from
the object does not actually come to a focus
   Real Image – an image that is formed by
converging light rays and that can be displayed on a
screen
   When a diverging lens is used alone, the image is
always virtual, right-side up, and smaller than the
object
Image Formation by a Lens
Constructing Images Through Ray
Diagrams
    Ray Diagrams – show the principle
rays that can be used to determine that
size and location of an image
    One end of the object is always placed
right on the principle axis
    3 useful rays in ray diagrams
1.   Ray parallel to the principle axis that
passes through the focal point after
refraction by the lens
2.   A ray through the center of the lens that
does not change direction
3.   A ray through the focal point in front of
the lens that emerges parallel to the
principle axis after refraction by the lens
Constructing Images Through Ray
Diagrams
Image Formation Summarized
   If an object is one focal length away from
a converging lens, the image is virtual,
magnified, and right-side up
   If an object is beyond one focal length, a
converging lens produces a real, inverted
image
   If the object is close to the focal point,
the image appears far away; if it is far
from the focal point, the image appears
nearer
   When an object is viewed through a
diverging lens, the image is virtual,
reduced, and right-side up
Some Common Optical Instruments

   The Camera – consists of a lens and sensitive
film mounted in a lighttight box, the amount of
light which gets to the film is regulated by a
shutter and a diaphragm (which varies the size of
the opening)
   The Telescope – a simple telescope uses a lens to
form a real image of a distant object
   The Compound Microscope – uses two converging
lenses of short focal length, forming a real and a
virtual image (so we see the object right-side up)
   The Projector – An arrangement of converging
lenses is used
Some Common Optical Instruments
The Eye
   The amount of light that enters is
regulated by the iris, the colored part of
the eye that surrounds the opening called
the pupil
   Light enters through the transparent
covering called the cornea, passes
through the pupil and lens, and is focused
on a layer of tissue at the back of the
eye—the retina—extremely sensitive to
light
   In both the camera and the eye, the
image is upside down, our brain flips the
image right-side up for us
The Eye
Some Defects in Vision

   Farsighted – person’s eyes form
images behind the retina, the
eyeball is too short
   Nearsighted – person’s eyes form
images in front of the retina, the
eyeball is too long
   Astigmatism – the cornea is
curved more in one direction than
the other, the eye does not form
sharp images
Some Defects in Vision
Some Defects in Lenses
   Aberrations – the distortions in an
image
   By combining lenses in certain ways,
aberrations can be minimized
   Spherical aberration results when light
passes through the edges of a lens and
focuses at a slightly different place from
light passing through the center of the
lens
   Chromatic aberration is the result of the
different speeds of light of various colors
and hence the different refractions they
undergo
Some Defects in Lenses

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 7 posted: 6/25/2011 language: English pages: 16