Page 235 - Photonics Essentials an introduction with experiments

Measurements in Photonics

Measurements in Photonics 229

Figure 10.1. A convex lens will focus a parallel beam of light to a point at a distance
from the lens that is equal to the focal length of the lens.

To use a convex lens in the context of optical characterization, there
are only two simple rules to remember:

1. A parallel beam of light is focused to a point by a convex lens at a
distance equal to the focal length (Fig. 10.1).
2. A point source of light at distance 2 times the focal length from a
convex lens is focused to a point at 2 times the focal length on the
other side of the lens (Fig. 10.2).

These features are conveniently summarized in the lens equation:
1 1 1
+ = (10.1)
x object x image focal length

Figure 10.2. A convex lens will focus a point source to a point of the same size when the
lens is placed at a distance equal to twice the focal length, as shown. The image is
formed on the other side of the lens at twice the focal length.

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