Editing Optics (section)

=====Lenses=====
{{main|Lens (optics)}}
[[File:lens3b.svg|class=skin-invert-image|upright=1.65|thumb|A ray tracing diagram for a converging lens]]
A device that produces converging or diverging light rays due to refraction is known as a ''lens''. Lenses are characterized by their [[focal length]]: a converging lens has positive focal length, while a diverging lens has negative focal length. Smaller focal length indicates that the lens has a stronger converging or diverging effect. The focal length of a simple lens in air is given by the [[lensmaker's equation]].{{sfnp|Hecht|2017|p=159}}

Ray tracing can be used to show how images are formed by a lens. For a [[thin lens]] in air, the location of the image is given by the simple equation

<math display="block">\frac{1}{S_1} + \frac{1}{S_2} = \frac{1}{f} ,</math>

where {{math|''S''{{sub|1}}}} is the distance from the object to the lens, {{math|''S''{{sub|2}}}} is the distance from the lens to the image, and {{mvar|f}} is the focal length of the lens. In the [[sign convention]] used here, the object and image distances are positive if the object and image are on opposite sides of the lens.{{sfnp|Hecht|2017|p=165}}

[[File:Lens1.svg|class=skin-invert-image|upright=1.65|thumb]]
Incoming parallel rays are focused by a converging lens onto a spot one focal length from the lens, on the far side of the lens. This is called the rear focal point of the lens. Rays from an object at a finite distance are focused further from the lens than the focal distance; the closer the object is to the lens, the further the image is from the lens.

With diverging lenses, incoming parallel rays diverge after going through the lens, in such a way that they seem to have originated at a spot one focal length in front of the lens. This is the lens's front focal point. Rays from an object at a finite distance are associated with a virtual image that is closer to the lens than the focal point, and on the same side of the lens as the object. The closer the object is to the lens, the closer the virtual image is to the lens. As with mirrors, upright images produced by a single lens are virtual, while inverted images are real.{{sfnp|Young|Freedman|2020|p=1157}}

Lenses suffer from [[optical aberration|aberrations]] that distort images. ''Monochromatic aberrations'' occur because the geometry of the lens does not perfectly direct rays from each object point to a single point on the image, while [[chromatic aberration]] occurs because the index of refraction of the lens varies with the wavelength of the light.{{sfnp|Young|Freedman|2020|p=1143,1163,1175}}

[[File:Thin lens images.svg|thumb|none|upright=2.25|Images of black letters in a thin convex lens of focal length {{mvar|f}} are shown in red. Selected rays are shown for letters '''E''', '''I''' and '''K''' in blue, green and orange, respectively. Note that '''E''' (at {{math|2''f''}}) has an equal-size, real and inverted image; '''I''' (at {{mvar|f}}) has its image at infinity; and '''K''' (at {{math|''f''/2}}) has a double-size, virtual and upright image.]]