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8 Unit 2 NEW Chapter 6

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					Grade 8 Science
Unit 2: Optics

Chapter 6:
Lenses refract light to
form images.
Lenses
A  curved piece of
 transparent material that
 refracts light in a
 predictable way.
Usually made from glass or
 plastic.
Lenses
There are two types of
 lenses:
1. Convex
 Centre of the lens bulges
    out
 Causes light rays to bend
    toward each other
    (converge)
2. Concave
Centre of the lens is curved
 in
Causes light rays to bend
 away from each other
 (diverge)
Convex vs. Concave Lenses
Convex Lenses
Can   act as a magnifying
 glass
Each lens has its own focal
 length (the distance from
 the centre of the lens to
 the focal point)
The greater the curvature
of the lens, the shorter the
focal length. (pg. 217)
Lenses  have focal points on
 either side because light
 shines through either side.
The line through the centre
 of the lens is called the
 principle axis.
Optical centre is where the
 principle axis meets the
 lens centre.
Optical
Centre




 Principle
 Axis
Concave Lenses
To find the focal point, you
 must extend the refracted
 rays back.
Lenses  with the greater
 curvature have the shortest
 focal length. (pg. 221)
Corrective Lenses




            The Eye (pg. 229)
Vision
Near-sighted Vision
See objects up close but
 not at a distance.
Concave lenses are used to
 correct this vision.
Near- sighted Vision
Far-sighted Vision
See objects at a distance
 but not up close.
Convex lenses are used to
 correct this vision.
Far-sighted Vision
Determining Focal Length...   You must include
                              the following
                              incident rays:
                              1. Travelling
                                  parallel to
                                  the p.a.
                              2. Travelling
                                  through the
                                  o.c.
                              3. Travelling
                                  through the F.
Ray Diagrams: Convex Lenses
Object between the lens and
 focal point.
Object is between the focal
 point and 2X the focal
 length.
Object is more than 2X the
 focal length.
Ray Diagrams: Concave Lenses




The above is true for an object in all
positions.
              CORE
              STSE:
              “Fibre
              Optics”
Fibre Cable
Optical Technologies...
1. Microscopes
 Uses two convex lenses
    with relatively short focal
    points to magnify
 Magnified twice to
    increase enlargement
The Microscope
2. Telescopes
The objective lens has a
 longer focal length than the
 microscope.
Can have either a refracting
 or reflecting telescope
Refracting Telescope (pg. 245)
The  lenses bend light to
 focus it.
The objective lens must be
 as large as possible to view
 distant galaxies (makes it
 heavy)
Reflecting Telescope (pg. 246)
Uses   a concave mirror,
 plane mirror and a convex
 lens to collect and focus
 light from objects at a great
 distance.
Most large telescopes are
 this type.
The Hubble Space Telescope
                  Launched in 1990
The Reflecting
Telescope
3. Cameras (pg. 248)
4. Binoculars (pg. 247)
5. Face shields
6. Magnifying glasses
7. Contact lenses
8. Flashlights
9. Eye glasses
The Nature of Science
The  development of new
 technologies involve many
 individuals and groups of
 people.
These technologies can
 alter what we know about
 the nature of science.
                    Individuals...


      1602




Galileo      1672




                      Newton
Groups...

				
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posted:4/8/2011
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