Page 218 - Physics_Form_2

Physics for Secondary Schools

Structure of a simple microscope Mode of action of a simple microscope
A simple microscope is made of When using a simple microscope, you can simply put it
a biconvex lens normally held over the object to be viewed. For the comfort of the
by a round-shaped frame with a observer’s eye, the position of the magnifying glass
handle. It can therefore be hand- with respect to the object is adjusted so that the
held and moved according to the object is at the focal point of the lens. This produces
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user’s needs. A simple microscope a virtual, upright, and magnified image of the
is sometimes called a magnifying object. Note that the largest image can be formed
glass or simply a magnifier. Figure on the retina of the naked eye when the object is at
6.1 shows a simple microscope. the near point (D ≈ 25 cm). Therefore, for the clear
and maximum size of the image to be formed on
the retina, the magnifier must have a focal length
of approximately 25 cm. The image is formed at
infinity when the position of the magnifying glass
or the object itself is adjusted so that the object is at
the focal point of the magnifying glass. In this way,
the eyes see a magnified image such that smaller
features can be observed. Figure 6.2 illustrates the
Figure 6.1: A simple microscope action of a simple microscope.

Angle subtended by
the object (θ image ) Image formed
(a) on the retina
h 0

Object at the near point

Angle subtended by
(b) the object (θ image ) Magnifier Image formed

h 1 on the retina

Image not at the near point

u
v l
L

Figure 6.2: Viewing an object (a) with a naked eye (b) with the aid of a magnifier

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Student’s Book Form Two

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