Page 220 - Physics_Form_2

Physics for Secondary Schools

The angular magnification is maximum However, decreasing the focal length is
when the image is at the near point of the limited by the imperfection of the lens.
eye. The limitation of the simple microscope
can be overcome by using a compound
1 1 1 microscope.
Then, = +
f u 25
FOR ONLINE READING ONLY
1 1 1
Using real is positive,   Example 6.1
f u 25
A simple microscope with a focal
25 f length of 5 cm is used to read the
This implies that, u =
25+ f division of scale 1.5 mm in size. How
large will the size of the divisions be as
Since M = D , and D = , v seen through the simple microscope?
u

Solution
25 25
1
Then, M = =+ f  5 cm h  , o 1.5 mm
25 f f 25 25 cm
1
16
25+ f M  f  5 cm 
25
1
M =+ h
f M  i
h o
This is the expression for the angular h  hM
o
i
6 9 mm
magnification of the simple microscope. h  1.5 mm 
i
The eye is most relaxed when the image
is at infinity. In this case, the object has to Therefore, each division will appear
be at the focal point of the lens. Thus, to have a size of 9 mm when viewed
through the simple microscope.
h h
θ ≈ o and θ ≈ i
object 25 image f
Construction of a simple microscope
One can construct a simple microscope
Here, the magnification is said to be
minimum, given by, for various simple uses. Materials that
are important in constructing a simple
microscope include a biconvex lens and
θ θ h h h h 25cm
25cm
object
M
M = = object = = i i ÷ ÷ ÷ o o = = a hard box.
θ θ f f 25 25 f f
image
image
Angular magnification is also known as Task 6.1
the magnifying power of the microscope.
The angular magnification of a simple Construct a simple microscope
microscope may be made as large as
possible by decreasing its focal length f.

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Student’s Book Form Two

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