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Optical instruments
5. Fix the eyepiece lens securely at one 2. An astronomical telescope has its
end of the smaller tube using tape or two lenses 78.0 cm apart. If the
glue. objective lens of the telescope has
6. Insert the smaller tube with the a focal length of 75.5 cm, what is
eyepiece into the larger tube with the magnification produced by the
the objective lens as in Figure 6.9. telescope?
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3. A homemade telescope has an
objective lens of focal length 140 cm
and an eyepiece of focal length 4 cm.
What is the magnifying power of this
Figure 6.9 telescope? Determine the separation
distance between the objective lens
7. Adjust the length between the lenses and the eyepiece.
to about the sum of their focal lengths 4. How would the magnification of an
(f₁ + f₂) to focus the image. astronomical telescope be affected if
8. Use your telescope to observe distant the focal length of the eyepiece and
objects like the moon or buildings, the objective lens are increased?
and record your observations. 5. An observatory telescope has an
objective lens of focal length 14 m.
Questions Suppose an eyepiece of focal length
1. What did you observe when looking 2 cm is used,
through your telescope? Was the (a) What is the angular
image upside down? magnification of the telescope?
2. How did adjusting the distance (b) If this telescope is used to view
between the lenses affect the focus the moon, what is the diameter
and clarity of the image? of the moon’s image formed by
3. What challenges did you face during the objective lens? The diameter
the construction of the telescope, and of the moon is 3.48 10 m 6
how did you solve them? and the radius of the lunar orbit
4. How could you improve your 8
telescope to increase magnification is 3.8 10 m.
or image clarity?
Binoculars
Exercise 6.3 When you visit a national park, you cannot
1. An astronomical telescope in normal get very close to dangerous animals, such
adjustment has a total length of as lions and leopards, for safety reasons.
78 cm and produces an angular To closely watch such animals, one needs
magnification of 15. What is the to use a device that can help the eye
see distant objects closely. An optical
focal length of the objective and device used in such a situation is called
eyepiece lens?
binoculars.
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Physics Form 2 Final.indd 223 25/10/2025 10:28
