Page 233 - Physics_Form_2

Optical instruments

Mode of action of a lens camera and so,
To take a photograph of an object, the image v  uf
of the object must be sharply focused on the u  f

film. This is done by adjusting the distance then,
1
of the lens from the film. After focusing and m  v  uf  f
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correctly setting the aperture size and shutter u u  f u u  f
time, the shutter release button is pressed. The f
shutter opens to allow light to enter and expose Hence, m  u  f .
the film to form an image of the object being
photographed. The film is then developed to Therefore, magnification produced

produce a photograph of the object. Figure by the lens camera depends on the
focal length f of the convex lens used
6.13 illustrates the image formation by a lens and the object distance u.
camera.

Lens Film Example 6.5
Shutter A lens camera of focal length
10 cm is used to take a picture of
Real a girl 1.5 m tall. Determine the
image magnification of the image if the
girl is 11 m from the camera.
Object Aperture Solution
f = 10 cm = 0.1 m, u = 11 m
Figure 6.13: Image formation by a lens camera
but f
Magnification of a lens camera m  u  f
The magnification produced by a lens camera
is given as; m  0.1 m
11 m 0.1 m
image height ()h image distance ( )v
m = i = = 0.009
object height h object distance ( ) u
( )
o
Therefore, the image is reduced by
a factor of 0.009.
Recalling the lens formula,
1 = 1 + 1 Construction of a simple lens
f u v camera
A simple lens camera can easily be
This can also be written as, constructed using local materials,
1  1  1 including the lens from unused

v f u optical instruments. Perform task 6.5
to construct your lens camera.

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