Page 65 - Fundamentals of Light Microscopy and Electronic Imaging
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48 LENSES AND GEOMETRICAL OPTICS
a < f
F F
Virtual
Image Object magnified
image
a
(a)
No image F
F a = f
Image
Object distance ∞
a
(b)
F F Image 2f > a > f
Real magnified
Object image
a
(c)
F 2F a = 2f
Real,
2F
F magnified = 1x
a
(d)
F Image a > 2f
Real,
Object F demagnified
a
(e)
Figure 4-6
Object-image relationships. (a) a f; (b) a f; (c) 2f a f; (d) a 2f; (e) a 2f.
the image may be real or virtual, and either magnified or demagnified (Fig. 4-6;
Spencer, 1982). It is good practice to work through the following relationships using any
handheld magnifier or simple biconvex lens such as the front lens of a binocular (10–50
mm is convenient), whose focal length has been determined by the procedure just
described. The object should be self-luminous, moderately bright, and well defined. A
5–10 mm diameter hole cut in a large opaque cardboard sheet placed in front of a lamp
works well as an object. Work in a partially darkened room. It is useful to remember the
principal conditions describing object-image relationships for a simple positive lens:
