Transcript c. Section 2.3 Human Vision

6.1 Human Vision
I. Parts of the Eye
• The pupil is the dark transparent region in the
centre of the eye where light enters.
• The iris is the coloured circle of muscle
surrounding the pupil.
– The iris controls the amount of light entering the
eye.
• The sclera is the white part of the eye
surrounding the iris.
b
a
• The cornea is the transparent tissue
covering the iris and the pupil.
• Behind the pupil is a flexible convex
lens.
• The lens focuses light onto the retina
located in back of the eye.
– The retina is covered with light sensitive cells
that convert light energy into electrical energy.
• Electrical signals are sent to the brain
by the optic nerve.
c
d
II. The Cornea-Lens-Retina System
• Light rays first entering the eye are
refracted by the cornea so that they
converge toward the retina.
• Light then passes through the lens
which “fine-tunes” the focus.
• The image that forms on the retina
is inverted.
• The area where the optic nerve
enters the retina is called the blind
spot.
– This area has no light-sensing
cells.
III. Black and White vs. Coloured Vision
• The retina contains two types
of light-sensitive cells called
rods and cones.
• Rod cells are cylinder-shaped
cells that allow us to see
images in shades of light and
dark when the light is dim.
rod
cone
• Cone cells are cone-shaped
cells that allow us to see colour
in bright light.
• http://faculty.washington.edu/chudler/after.html
Near-Sighted Vision
• Can not clearly focus on distant objects.
• Occurs because the lens converges the light
rays to form an image in front of the retina.
• A concave lens is used to correct near-sighted
vision.
Far-Sighted Vision
• Can not clearly focus on nearby objects.
• Occurs because the lens converges the light
rays to form an image behind the retina.
• A convex lens is used to correct far-sighted
vision.
Astigmatism
• Blurred vision due to a irregular shaped
cornea.
• Causes the image to focus on more than one
point on the retina.
• Corrected by using eyeglasses, contact lenses,
or laser surgery.
Blindness
• Blindness is any vision impairment that keeps people
from carrying out important life functions.
• Most people who are legally blind can perceive some
light.
– May be able to see a tiny part of the middle of the whole
scene (tunnel vision).
– May be able to only see the edges but not directly ahead.
– May be able to see light and dark but not clearly, even with
visual aids.
Other Types of Blindness
• Snow blindness is a temporary blindness caused by overexposure to
the glare of sunlight.
• Night blindness is a condition in which it is difficult to see in dim
light.
• Colour blindness is the ability to see only in shades of grey.
• Colour vision deficiency is the inability to distinguish certain
colours.
– The most common is the inability to distinguish between red
and green.
Section 6.2
Extending Human
Vision
Extending Human Vision
• With light, mirrors, and lenses, we have been
able to make microscopes and telescopes
• Microscopes and telescopes have allowed us
to see and learn new things
I. How to Bring an Image into Focus
• to focus properly, the screen that is receiving
the image must be the correct distance from
the lens (where the light rays converge)
• if the screen is too close or too far from the
lens, the image will appear blurry
• this is similar to near-sightedness and farsightedness
II. Microscopes
• a compound microscope uses
two convex lenses
• focuses on small objects
• light reflects off the mirror,
through the object, though the
objective lens and eyepiece
lens, into the eye
• the objective lens has a fixed focal point
• if an object is between 1-2 focal lengths away
from the lens, the image is enlarged
• the eyepiece lens magnifies the image (10X),
and the objective lens magnifies it again
(usually 4X, 10X, 40X)
III. Telescopes
• it is difficult to see far away images because
further away objects reflect less light into your
eye. This makes far away objects appear dim.
• telescopes mirrors or lenses that is bigger to
gather more of the light from distant objects.
This makes far away objects appear much
brighter.
• the brighter image is then enlarged
a) Refracting Telescopes
• Uses convex lenses
• the objective lens collects light and focuses
the image inside the telescope; the image is
then magnified by the eyepiece lens
Problems with Refracting Telescopes
• It needs very large lenses! Sometimes, they
are too big:
• Heavy
• Distort image
• Expensive
• Difficult to make
b) Reflecting Telescopes
• most large telescopes today are
reflecting telescopes
• uses a concave mirror, a plane
mirror, and a convex lens to
collect and focus light
• light enters one end of the
telescope and strikes a concave
mirror at the opposite end of the
telescope.
• the light reflects off the
concave mirror and strikes a
plane mirror at an angle
before it converges to its focal
point
• the light reflects into the
eyepiece, which is a convex
lens that magnifies object
• some telescopes collect light
rays using several mirrors and
then combine the rays into a
single image (e.g. The Keck
Telescope)
c) The Hubble Space Telescope
• Earth’s atmosphere blurs the image of distant
objects in space, just like water blurs your vision
• to overcome this blurry effect, some telescopes
are placed in space
• The Hubble Space Telescope was launched into
outer space in 1990- it is a reflecting telescope
that uses two mirrors.
V. Binoculars
• made of 2 telescopes mounted side by side
• the telescopes are shortened by putting prisms
inside that act like plane mirrors
• the prisms make it so the light does not have to
travel through a long tube
• the thumbscrews on binoculars are used to
change the focal length in order
to focus on the object being
viewed
VI. Lasers
• In laser light, all of the light rays move in the
same direction, have the same wavelength
and the crests and troughs line up
• laser light can travel great distances without
spreading out and can contain a lot of energy
• laser light contains only one wavelength- it
does not make a rainbow
• lasers are used to remove cataracts, re-attach
retinas, stop bleeding, and reshape corneas
VII. Optical Fibres
• optical fibres are transparent glass tubes that
can transmit light from one place to another
• light is reflected from side to side down the
optical fibre like water moving through a pipe
• TOTAL INTERNAL REFLECTION: a type of
reflection in which light strikes a boundary
between to materials and is completely
reflected
• optical fibres are used in medicine to transmit
images of the inside of a person’s body from a
tiny camera at one end of the cable and a
monitor at the other end.
• in telecommunications, optical fibres are used
with laser light to transmit telephone, video, and
internet signals.