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Lenses
Reflecting and Refracting
Lenses use the idea of refraction:
Telescopes
When light enters a
MORE DENSE
medium it slows
down…
•
You need to be able to describe
what the two types of lenses do to
light
Lenses refract light.
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Convex lenses and converging lenses
•
Concave lenses are diverging lenses
Ray diagrams for lenses
A “distant
object”
Focal length
The rays of light are
refracted INWARDS and
meet at the focus, F.
F
10/19/15
The image formed is
REAL – in other words, it
can be seen on a screen
You have to be able to state what is
meant by a focal point and how to find
the focal length
•
Focal pint is where the rays of light
meet
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The focal length is the distance
between the lens and the focal point.
Now lets look at the two types of
telescopes you need to know
about...
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Reflecting Telescopes
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Refracting Telescopes
Refracting telescope
•
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They consist generally of two lenses.
The first is the objective lens and the
second is the eye piece.
What do the 2 lenses do?
•
•
Objective lens – collects light
creates a real image within a
telescope of a distant object.
Eyepiece lens – magnifies the
image of a distant object.
Problems with the
Refracting Telescope
•
What problems might there be with the
refracting telescope, think about
reflection.
Refracting telescopes
•
•
Advantage
Because of their
simple design,
they are generally
easy to use and
more reliable as
their optics are
permanently fixed
and aligned.
•
•
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Disadvantages
They can be heavy,
especially larger
aperture telescopes
, because of their
need for large
lenses.
They can also have
a longer body which
may impact on
Reflecting
Telescope
•
•
The first reflecting telescope that
worked well was produced in 1668
by Sir Isaac Newton.
Reflecting telescopes do no have
the problems that refracting
telescopes do.
How do Reflecting Telescopes
Work?
•
Reflecting telescopes use curved
mirrors instead of objective lenses.
Reflecting telescope
•
•
In a reflecting telescope the image is
formed by reflection from a curved
mirror.
Reflecting telescopes
•
•
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Advantage
Fairly compact and
portable which is great
for transportation and
storage. Generally more
affordable. Mirrors can
produce less
optical aberrations.
Collect more light then
refracting telescopes
•
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Disadvantage
Mirrors need regular
re-alignment as they
can easily slip out of
alignment due to knocks
and bumps or
temperature changes.
They can be susceptible
to
spherical aberrations.
Any Difference
•
•
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Similarities
Both have eye piece
lens that magnifies
the image.
Both have an
objective lens/ curved
mirror that focuses
the light.
•
•
•
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Differences
One focuses the
light via reflection
using a mirror.
The other focuses
light through
refraction using a
lens.
Refracting
Questions
1.
Write a short definition of refraction.
2.
What does focal length mean?
3.
Explain how lenses work and how they
are used in refracting telescopes.
Answers
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1.Write a short definition of
‘refraction’.
A: Light/waves change direction
when they cross the boundary
between two media.
Answers
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2 What does ‘focal length’ mean?
A:The distance from a lens to the
point where rays of light from a
distant object meet.
Answers
•
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3 Explain how lenses work and how they are used
in refracting telescopes.
A: Lenses change the direction of light entering
and leaving them. The shape of convex lenses
bends rays of light so that a beam of parallel rays is
brought to a focus. In a refracting telescope an
objective lens brings parallel rays from distant stars
to a point. Another convex lens is used as an
eyepiece lens, which magnifies this image.
6 mark past paper question
Answer