3. Thin lenses
Let's consider a lens or mirror, of negligible thickness compared to their focal length (or any other of their dimensions), and illuminated by rays sufficiently little inclined to the axis that the sines of the corresponding angles can be approximated by the values of the angles themselves. These conditions define the so-called Gaussian approximation, from which many properties of thin systems can be deduced.
3.1 Focal length and power of a thin lens
Let's assume, as is often the case, that the lens is bathed in air (or vacuum). If the heights and angles of incidence of the rays are small, we can consider a thin lens as a plane component which deflects an incident ray by an angle proportional to the height of its point of impact in relation to the axis (the...
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Thin lenses
Thick lenses
