Review: The Refraction of Light
Light moves at different speeds through
different media. When it travels from one
medium into another, the change in speed
causes the ray to bend (refract) towards or away
from the normal.
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Something else:
The angle of refraction is related
to the different speeds:
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Refraction
(bending light)
Refraction is when light bends as it
passes from one medium into another.
normal
air
θi
When light traveling through air
passes into the glass block it is
refracted towards the normal.
glass
block
θr
When light passes back out of the
glass into the air, it is refracted away
from the normal.
Since light refracts when it changes
mediums it can be aimed. Lenses are
shaped so light is aimed at a focal
point.
θi
θr
air
normal
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Ray Tracing for Lenses
Lenses are used to focus light and form images.
There are a variety of possible types; we will
consider only the symmetric ones, the double
concave and the double convex.
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Concave Lenses
Concave lenses are thin in the middle and make
light rays diverge (spread out).
•
F
optical axis
If the rays of light are traced back (dotted sight lines),
they all intersect at the focal point (F) behind the lens.
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Concave Lenses
•
F
optical axis
Light rays that come in parallel to the optical axis diverge from the focal point.
The light rays behave the same way if we ignore the thickness of the lens.
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Concave Lenses
•
F
optical axis
Light rays that come in parallel to the optical axis still diverge from the focal point.
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Lenses
The first telescope, designed and built by Galileo, used lenses to focus light from
faraway objects, into Galileo’s eye. His telescope consisted of a concave lens and a
convex lens.
light from
object
convex
lens
concave
lens
Light rays are always refracted (bent) towards the thickest part of the lens.
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Convex Lenses
Convex lenses are thicker in the middle and focus light rays to a focal point in front of
the lens.
The focal length of the lens is the distance between the center of the lens and the
point where the light rays are focused.
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Convex Lenses
•
F
optical axis
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Convex Lenses
•
F
optical axis
Light rays that come in parallel to the optical axis converge at the focal point.
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Ray Tracing for Lenses
The three principal rays for lenses are similar to
those for mirrors:
•
The incident ray that runs parallel to the PA will
refract through the principle focus
• A incident ray that goes through the principle focus
will refract parallel to the PA
•
The incident ray that goes through the optical centre
does not refract, assuming the lens is thin enough, it
will not be deflected.
• This is the thin-lens approximation.
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Ray Tracing for Lenses
These diagrams show the principal rays for
both types of lenses:
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Ray Tracing for Lenses
As with mirrors, we use these principal rays to
locate the image:
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Ray Tracing for Lenses
The convex lens forms different image types
depending on where the object is located with
respect to the focal point:
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Text work
Read P 556-560 for extra support
Complete worksheets
Do#5-7
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Thanks/Further Info
• Faulkes Telescope Project: Light & Optics by Sarah Roberts
• Fundamentals of Optics: An Introduction for Beginners by Jenny
Reinhard
• PHET Geometric Optics (Flash Simulator)
• Thin Lens & Mirror (Java Simulator) by Fu-Kwun Hwang
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