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BC Science 8
Chapter 6
6.1 Human Vision
 Pupil
- is an opening that appears dark
because light passes through it
without reflecting back
 Iris
- is the coloured circle of muscle
surrounding the pupil, and
controls the amount of light
entering the eye (in dim light, the
iris dilates, or expands, the pupil
to reduce the amount of light
entering the eye…in bright light,
the iris contracts the pupil to
reduce the amount of light
entering the eye)
 Cornea
- is a transparent tissue made of
cells that are transparent enough
to let light pass through, but tough
enough to hold the eye together
 Sclera
- the opaque tissue surrounding the
cornea; we see it as the white area
surrounding the iris.
 Retina
- the inner lining of the back of the
eye containing light-sensitive rods
& cones that convert light images
into electrical signals for
interpretation by the brain
 Optic Nerve
- the nerve that connects the eye to
the brain
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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Figure 6.3
figure 6.4
The Cornea-Lens-Retina System
1. Light rays are refracted by the cornea so that they converge
toward the retina. (the cornea does most of the focusing of the
light rays)
2. The refracted rays enter the eye through the pupil, where the iris
changes the size of the pupil. If light levels are dim; the pupil
gets larger to let in more light. If light levels are bright; the
pupil gets smaller to let less light in.
3. Light rays then pass through the lens where they converge on the
retina. The lens gets thicker to help focus lights rays from objects
that are closer, and gets thinner to help focus light rays from
objects that are farther away.
4. The image formed on the retina is upside down. Cells in the retina
change the image into electrical signals.
5. The electrical signals are sent to the brain along the optic nerve,
and as the brain interprets the signals, it changes the image so that
it is upright.
6. The blind spot is the area where the optic nerve enters the retina
that does not have any light-sensing cells.
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BC Science 8 – Chapter 6
Human Vision
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Diagram 6.5
 Rod Cells
- is called our black–and-white
vision system
- cylinder-shaped cells, located in
the retina, that absorb light and
allow us to see images in shades
of light and dark and movement
when the light is dim
 Cone Cells
- cone shaped cells, located in the
retina that absorb light, and allow
us to see colour in bright light
Correcting Vision Problems
 Normal Vision
- When light rays from a distant
object enter the eye, the rays are
nearly parallel. The lens, which is
convex, causes the rays to converge
at the retina, making a sharp image.
- When light rays from a nearby
object enter the eye, the rays are
diverging. The muscles in the eye
cause the lens to change shape,
making the lens thicker, giving
the lens a greater ability to
converge the light rays to form a
clear image.
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BC Science 8 – Chapter 6
Human Vision
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 Near-Sighted Vision
- when you can see nearby objects
clearly but distant objects look
blurred
- it happens when the light rays
converge before they reach the
retina.
- A concave lens is used to diverge
- the parallel rays before they reach
the cornea. Then, as the light rays
pass into the eye, they converge on
the retina to form a clear image.
 Far-Sighted Vision
- when you can see objects far from
the eye clearly, but nearby objects
look blurred.
- it happens when the light rays
do not converge by the time they
reach the retina.
- A concave lens is used to diverge
the parallel rays slightly so that the
image forms farther back, on the
retina to form a clear image.
 Astigmatism
- when the shape of the cornea is
irregular, causing an image to
focus an image on more than one
point on the retina.
- can be corrected using glasses,
- contact lenses, or with laser
surgery to reshape the cornea
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BC Science 8 – Chapter 6
Human Vision
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 Blindness
- any vision impairment that keeps
people from carrying out important
life activities; ability to detect very
little or no light
- many people who are legally blind
can perceive some light or even
have a limited amount of vision
- In developing countries, blindness
Is most often a result of disease or
malnutrition; of the 40 million
people who are blind in the world
today, about 80% could have
some or all of their sight restored
through treatment.
Other Types of Blindness
 Snow Blindness
- is a painful condition of temporary
partial or complete blindness
caused by overexposure to the
glare of sunlight (on snow fields at
high altitudes)
- treatment includes resting your
eyes in a dark room for several
days to allow the inflammation to
decrease
 Night Blindness
- is a condition in which it is difficult
or impossible to see in dim light
- most common cause is the rod
cells losing their ability to respond
to light
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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 Colour Blindness
- is the ability to see only in shades
of grey, and occurs in about 1
person in every 40 000.
 Colour Vision Deficiency
- is quite common, occurring in
about 8 % of males, and 1 % of
females
- is an inability to distinguish certain
- colours (1, 2, or all 3 kinds of
cone
cells can be involved)
- the most common involves the
inability to tell red and green
apart
6.2 Extending Human Vision
Pg. 92 diagrams in workbook
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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Microscopes – How They Work
1.
A compound light microscope uses 2 convex lenses with short focal
lengths to magnify small, close objects
2.
The object to be viewed is placed on a transparent slide and
illuminated from below
3.
Light passes by or through the object on the slide and then travels
through the objective lens (a convex lens)
4.
The convex lens makes an enlarged image inside the microscope tube
5.
The light rays from that image then pass through the eyepiece lens,
which is another convex lens, and magnifies the image even more
6.
This final image can be hundreds of times larger than the actual
object, depending on the focal lengths of the 2 lenses.
Telescopes – How They Work
1.
A telescope uses a lens or a concave mirror that is much larger
than your eye to gather more of the light from distant objects.
2.
The largest telescopes can gather more than a million times more
light than the human eye so objects such as distant galaxies appear
much brighter.
3.
The image can be magnified to a greater extent to reveal more detail
because the image formed by a telescope is so much brighter.
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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Refracting Telescopes
1.
A telescope, like a microscope, has an objective lens and an
eyepiece, but the objective lens in a telescope has a longer focal
length because the objects viewed are far from the lens.
2.
In a refracting telescope, light from a distant object passes through
an objective lens and an eyepiece where an image is formed and
never seen directly.
3.
Instead, the image is magnified by the eyepiece lens, and directed
into the eye of the operator or into a camera.
4.
In order to form a detailed image of distant objects, such as planets
or galaxies, the objective lens must be as large as possible, which can
be a problem, because a large lens is heavy, costly, difficult to make,
and can distort the image it forms if the lens sags due to its weight.
figure 6.15
Reflecting Microscopes
1.
Most large telescopes today are reflecting telescopes, due to the
problems with making large lenses.
2.
A reflecting telescope uses a concave mirror, a plane mirror, and a
convex lens to collect and focus light from distant objects.
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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3.
Light from a distant object enters one end of the telescope and
strikes a concave mirror at the opposite end.
4.
The light reflects off this mirror and converges, before it
converges at a focal point, the light strikes a plane mirror that is
placed at an angle within the telescope tube.
5.
The light is reflected from the plane mirror toward the telescope’s
eyepiece.
6.
The light rays converge at the focal point, creating an image of the
distant object (a convex lens in the eyepiece then magnifies this
image)
Figure 6.18
figure 6.19
The Hubble Space Telescope
 The Hubble Space Telescope is a type of reflecting telescope that uses
two mirrors to collect and focus light to form an image.
 The primary mirror in the telescope is 2.4 m across and can collect
visible light, as well as other types of electromagnetic radiation from
planets, stars, and distant galaxies.
 The image from the Hubble Space Telescope is clear, not blurred by the
Earth’s atmosphere producing images much sharper and more detailed
than the largest ground-based telescopes.
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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The Hubble Space Telescope
Diagram pg. 221
Binoculars
 Binoculars are actually two refracting telescopes mounted side by side.
 The telescopes are shortened by placing prisms inside that serve as plane
mirrors. Instead of traveling down the long tube of a telescope, the
prisms reflect the light in binoculars back and forth inside a shorter
tube.
Figure 6.21
Cameras – Wild–Angle, and Telephoto Lenses
 A Digital Camera works by gathering and bending light with a convex
lens.
 The lens projects an image onto a light detector to record a digital
image of a scene.
 When you take a photograph, a shutter opens to allow light to enter the
camera for a specific length of time.
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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 The light reflected off your subject enters the camera through an opening
called an aperture.
 The light then passes through the lens, which focuses the image on the
light detector.
 The image is inverted and smaller than the actual object because a
convex lens is used.
Wide-Angle Lenses
 Wide-Angle Lenses have short focal lengths that produce a small image
Telephoto Lenses
 Telephoto lenses have longer focal lengths; therefore the image through
a telephoto lens seems enlarged and closer than it actually is.
Comparing a Camera to the Human Eye
Human Eye
Camera
Similarities
Human Eyelid
Lens Cap
Both reduce the chance of
accidental damage
Pupil
Lens
Both have an opening to let
light in
Iris
Diaphragm
Both limit the amount of
light entering the eye
Rods and Cones
Pixels
Detect light in our eyes
Retina
CCD (light detector)
Changes light into electrical
signals to produce an image
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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Diagram 6.25
Lasers
 In Laser Light, all of the light has the same wavelength, all the light rays
are moving in the same direction, and all of the crests and troughs of the
light are lined up.
 Laser light travels great distances without spreading out, and can contain
a lot of energy, making it useful as a tool for eye surgery.
 Light does not refract into a rainbow, because lasers only have one
wavelength.
 Laser Surgery is used to remove cataracts, re-attach retinas, stop
bleeding, and reshape corneas.
Figure 6.26A and Figure 6.26B
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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Optical Fibres
 Optical Fibres
-are very thin transparent glass
fibres that can transmit light from one
place to another
- laser light enters one end of the
fibre and keeps reflecting off the
smooth inside walls until it reaches
the other end (total internal
reflection)
- The light in optical fibres can be
used to carry sound, movie, and
internet signals.
- A tiny camera on one end of an
optical fibre can send pictures to a
monitor and let surgeons view the
inside of the body
Figure 6.27
Eric Hamber Secondary
BC Science 8 – Chapter 6
Human Vision
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