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Lenses
| 1.
Preparation: |
a) |
Make sure
you understand the terms: focal length, real image,
virtual image. |
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b) |
The
equation relating object distance (do), image
distance (di), and focal length (f) is 1/f = 1/do
+ 1/di
What will be the
shape of a graph of 1/do against 1/di?
How could such a
graph be used to find the focal length, f? |
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c) |
Learn the
equation which relates the focal length (f) to the refractive
index (n) and radii of curvature of the lens surfaces (r). This
equation is often called the "lens-maker’s
equation". |
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d) |
See the
section concerning measuring the radius of curvature of a lens
surface in part 2. |
| 2. |
Experiment
to verify the "lens-maker’s equation" using a convex
lens. |
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By measuring image
distances (di) for as many different object distances
(do) as possible, find the focal length of a convex
lens.
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When
obtaining real images (on screens) there is often quite
a wide range of positions for the screen which all give
fairly clear images. When trying to measure di
you will have to think of the best way to minimise the
errors due to this fact. |
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The value of f should be taken from a
graph of 1/do against 1/di.
To verify the "lens-maker’s
equation", you will also need to measure the radius of
curvature of the lens surfaces, r (the lenses used are symmetrical).
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The diagram below
might help you to see how to measure r. |
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|
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In this diagram, d is the diameter of the lens
and x is half the thickness of the lens.
See if your values of f and r verify the "lens-maker’s
equation" assuming that the refractive index of the material
of which the lens is made is 1·47.
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| 3. |
Experiment to
measure the focal length of a concave lens
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Your report on this
experiment should start with a brief explanation of why
it is a little more difficult to measure the focal length of a concave
lens than to measure the focal length of a convex lens.
The method proposed here involves first obtaining a real image
of an object using a convex lens (diagram 1) and then
observing the change in the image distance when the concave
lens put is put in place (diagram 2). |
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....................................................diagram
1
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.....................................................diagram
2
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With the object and
convex lens in the same positions, place the concave lens
as shown and move the screen and/or the concave lens to obtain a
clear image. We say that, at the point where the screen was
there is now a virtual object of which the concave lens
can form a real image.
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When the image is clear measure do
(= X - Y) and di |
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Use the equation 1/f = 1/do
+ 1/di to calculate the focal length of the concave
lens. |
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Remember that, in this case, do
will be a negative quantity (why?). |
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Repeat the experiment with different values
of X and find an average value for the focal length. |
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© David
Hoult 2008 |
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Open Door Web Site