Page 230 - Physics_Form_2

Physics for Secondary Schools

Structure of binoculars the eyepiece. The eyepiece can be thought
Binoculars can be considered as having of as a magnifying glass. It picks up the
two telescopes that are exactly the same small image formed by the objective lens
and placed beside each other, such and magnifies it so that the observer can
that they accurately point in the same clearly see the object. However, when
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direction. This allows the observer to use light is refracted through the objective,
both eyes when looking at distant objects. the light rays cross over, resulting in
The main components of binoculars an upside-down image. The eyepiece
are the eyepiece, the objective lens and simply magnifies this image so that the
prisms. The objective lens focuses a viewer sees the object upside down. This
distant object to a point near the focal problem can be solved by deploying a pair
of prisms. These prisms are essentially
point of the eyepiece, which magnifies used to rotate and reflect the image using
that image. A pair of prisms inverts the the principle of total internal reflection.
image so that it can be seen properly To rotate the image 180 degrees, prisms
upright by the eyes. Other parts are the are arranged in a way that each prism
focusing system, which allows the lenses effectively rotates the image 90 degrees.
to be moved back and forth. Figure 6.10 Normally, two types of prisms are used in
shows some parts of binoculars. binoculars: roof prisms and porro prisms.

Focus knob Figure 6.11 shows how image inversion
Eyepiece is achieved in binoculars.

90°
A

B

Objective lens B
A
Figure 6.10: Parts of binoculars

Mode of action of binoculars Figure 6.11: Image inversion by prisms

Two objective lenses are situated at
the ends of both telescopes making the Magnification of binoculars
binoculars. The purpose of the objective Since binoculars are made of two identical
lens is to collect light from the object that refracting telescopes, the magnifying
power of the binoculars can be calculated
the user is looking at and form the image as in the case of the astronomical
of the object at or near the focal point of telescope.

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Student’s Book Form Two

Physics Form 2 Final.indd 224 25/10/2025 10:28