5.11
Refracting Light in Lenses
Your eyes depend on refraction. They make use of a special
optical device. A lens is a curved, transparent device that
causes light to refract as it passes through. As you read,
light reflects off the page, travels to your eyes, and refracts
when it enters the lens of each eye. A magnifying glass
(Figure 1), contact lenses, the lenses in eyeglasses, and
camera lenses are all examples of useful lenses.
Why Does Light Refract?
You have seen that light refracts when it travels from one
material into another. Why does this happen? Using careful
measurements, scientists have discovered that the speed of
light differs in different transparent materials. The denser
the material is, the slower light travels as it passes through.
When light travels from air into denser glass, it slows down.
This change in speed causes the light to change direction.
The same thing happens, for the same reason, if you ride a
bike from pavement into sand, as shown in Figure 2. The
new medium causes a change in speed and direction.
Figure 1
Lenses have a variety of uses, depending on
their size, shape, and other factors. As light
passes through this lens it refracts to create
enlarged images.
Exploring Lens Combinations
How do designers decide which lenses to
use and how to set up the best combination
in microscopes and other devices? You can
explore this question using several glass
lenses.
•
Examine several single lenses. Look
through them from both sides, from near
and far, and at objects far away and nearby.
4. Does the distance between your eye and
the lens affect the image?
•
Combine lenses by putting one in front
of the other and looking through them.
5. What is the best combination for viewing
nearby objects?
Do not look at any bright light sources through the
lenses.
6. What combination is most useful for
viewing objects far away?
Handle lenses carefully to avoid breakage.
7. When using a concave/convex
combination, what arrangement works
for viewing objects near or far?
1. How do concave and convex lenses
compare?
2. How does the thickness of a lens affect
the image?
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3. How does the distance between the lens
and the object viewed affect the image?
Unit 5
•
Try combining three lenses.
8. Did you discover any useful
combinations of three lenses?
Understanding Concepts
refracted
ray
air
1. Explain why light bends as it
travels from air into water.
pavement
2. Light speeds up when it
passes from glass to air. Redraw
Figure 2, showing what would
happen if light were travelling
from glass into air.
incident
ray
glass
(slower speed)
sand
(slower speed)
Figure 2
Light refracts when its speed changes, just as a bicycle
changes direction as it slows down when moving from
pavement to sand.
(a) a convex lens
(b) a concave lens
Lens Designs
Generally, lenses are convex or concave, as shown in
Figure 3. A convex lens is thicker in the middle than at
the outside edge. A concave lens is thinner in the middle
than at the outside edge. These different designs cause
different effects and images when light passes through
them. However, notice that in Figure 3 a light ray through
the middle of the lens does not refract, because it meets
the surface at a 90° angle. Would this happen with the
bicycle?
Combining Lenses
Devices often use more than one lens. A microscope, for
example, has two lenses—the objective lens close to the
object viewed, and the eyepiece lens that you look through.
lens
principal
focus
principal axis
principal focus: the
position where parallel
light rays come together
focal length: the distance
from the principal focus
to the centre of the lens
light is refracted at
both surfaces of the lens
focal
length
a A convex lens bulges outward. It causes light rays to come together.
principal focus
principal focus: the
position where parallel
light rays appear to
come from
lens
principal axis
light is refracted at both
surfaces of the lens
focal length: the distance
from the principal focus
to the centre of the lens
focal length
b A concave lens is caved inward. It causes light rays to spread apart.
SKILLS HANDBOOK: 4A Research Skills
3. Describe the attitude and
approximate size of the image
when the object is very close
to and far from
4. Explain, using a diagram, whether
a thick or a thin lens would have a
greater focal length in
(a) a convex lens
(b) a concave lens
5. Is a convex lens more like a
convex mirror or a concave mirror
in the way it produces images?
Explain your answer.
Making Connections
6. Light refracts more when passing
from air to diamond than to any
other common material. (This
is one of the reasons diamonds
sparkle.) What can you conclude
about the speed of light in
diamond?
7. Make a list of devices that use
at least one lens.
Exploring
8. Not all lenses are concave or
convex. For example, one type
of lens has one side concave
and the other convex. Find some
examples of other shapes of
lenses and report on interesting
combinations of lenses that you
discover.
4A
Figure 3
Convex and concave lenses. The
characteristics of these lenses determine
how they are used in different optical
devices.
Optics
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