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.
Fibre Cable
CORE
STSE:
“Fibre
Optics”
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...