| a. |
Explain
what is the normal adjustment for an optical instrument. Illustrate your
answer with two different examples. |
2
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An optical instrument is in normal
adjustment when the final image is formed at a position where
the user expects to see. |
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A microscope should form the final image at
the near point (the least distance of distinct vision). |
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A telescope should form the final image at
the far point (infinity). |
0.5 |
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| b. |
Draw
a diagram to show the passage of light rays through a simple astronomical
refracting telescope when it is used in normal adjustment. Derive an expression
for its angular magnification. |
4
marks |
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2 |
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Without the telescope, the visual angle of the distant
object is |
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With the telescope, the final image is at infinity. The
visual angle of the final image is |
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Thus, the angular magnification is |
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| c. |
A
reflecting telescope is similar to a refracting telescope, except that
the objective lens of the refracting telescope is replaced by a concave
mirror. Draw a diagram to show how the final image could be viewed when
the reflecting telescope is used in normal adjustment. Explain the advantages
of using a reflecting telescope over a refracting telescope. |
4
marks |
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1 |
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Advantages of using a reflecting
telescope: |
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The objective mirror does not reduce as much light intensity as objective
lens.
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In reflecting telescope, there is no problem of chromatic aberrration.
Lens produces a fringe of false colors around the image star, as different
colors of light have different refractive indices.
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Spherical aberration can be eliminated by making the reflecting surface
paraboloial.
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A large mirror can be built easily and inexpensively, compared to a large
lens.
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A mirror can be supported from beneath over all area, while a lens can
only be held over its rim.
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any 3 |
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| d. |
What
is the eye ring of an optical instrument? Show that, in a refracting telescope,
the eye ring is the position of the image of the objective lens produced
by the eyepiece. |
3
marks |
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Eye ring is the position
of the eye, when using an optical instrument, at which most light enters
the eye. |
1 |
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From the diagram, all the light rays from the distant object
entering the telescope pass through the circular region EE'. Thus,
EE' is the position of the eye ring. |
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Consider the two light rays that pass through
the optical center. They will converge to a point on the line EE'
after refraction by eyepiece. P is the position that the eyepiece
will produce a real image at E. Thus, the eye ring is the image
position of the objective lens by the eyepiece. |
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| e. |
Draw
a simple diagram for a spectrometer. Assume that the turntable of a spectrometer
has been levelled. Describe other adjustments that have to be made before
the instrument can be used. |
3
marks |
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Adjustments before use |
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Adjust the telescope to receive parallel light. Turn the telescope to view
a distant object and then adjust the eyepiece until the image is focused
at the cross wire.
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Adjust the collimator to transmit parallel light. Align the telescope with
the collimator. Change the position of the slit of the collimator until
it is seen clearly by the telescope.
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