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Optical instruments

Since the astronomical telescope The larger the diameter of the objective lens,
uses a lens as its objective glass, it the brighter the image observed. The resolving
is also called a refracting telescope. power, which is the ability to observe two close
Figure 6.7 shows some parts of the objects distinctly, also depends on the diameter
astronomical telescope. of the objective lens. So, the desire is to make
astronomical telescopes with an objective
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Eyepiece lens of larger diameter. Nevertheless, lenses
with big diameters tend to be very heavy and
therefore, difficult to make and to be supported
by their edges. Further, it is rather difficult and
Objective lens expensive to make such large-sized lenses that
may form images that are free from any kind of
Focusing knob distortions.

Tripod
L
f 0 Eyepiece
f
Figure 6.7: Parts of an astronomical e
telescope E
α B'
Mode of action of an astronomical B'' α h β
telescope A'
A telescope is used to provide
angular magnification of distant Objective lens
objects. Light from a distant object
enters the objective lens and a real
image is formed in the tube at the A''
second focal point of the objective
lens. This image becomes the Figure 6.8: Image formation by an astronomical
object for the eyepiece. The telescope
position of the eyepiece lens is Magnification of an astronomical telescope
adjusted until the image from the
objective lens falls at the focal point The magnifying power, ,m of the astronomical
of the eyepiece. The image of the telescope is given by the ratio of the angle β
eyepiece is inverted and magnified subtended by the final image at the eye and the
as illustrated in Figure 6.8. Note angle α subtended by the object at the objective
that the main considerations with β
an astronomical telescope are lens. That is; m = α
its light-gathering power and its Assuming that the angles are very small, we
resolution or resolving power. The have,
light-gathering power depends
on the area of the objective lens. tanα ≈ α and tanβ ≈ β

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Physics Form 2 Final.indd 221 25/10/2025 10:28