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In these revision notes for Lenses. Equation of Lenses. Image Formation of Lenses, we cover the following key points:
Lenses are optical tools used to enlarge or reduce the size of images by means of refraction of light. A lens is a piece of transparent material, usually circular in shape, with two polished surfaces, either or both of which is/are curved.
There are two main categories of lenses: converging (concave) and diverging (convex). Converging lenses are thicker at middle and thinner in extremities, while diverging lenses are thinner at middle and thicker in extremities.
All lenses except plane-concave and plane-convex ones have two foci, because they are formed by joining two spherical parts. As a result, they have two centres (one in each side) as well.
Like in spherical mirrors, we have to use the special rays to build up the image in lenses. However, unlike in curved mirrors, two special rays are enough to build the image in lenses. They are:
Converging lenses are very similar in concept to concave mirrors. Therefore, we have again six possible cases of image formation in converging lenses depending to the position of object in respect to the lens.
Like in convex mirrors, the image formation in diverging lenses has only one case. The image is formed closer to the mirror than focus. It is erect and diminished. Since the image is obtained from rays extensions, it is virtual.
The equation of lenses is identical to that of curved mirrors. Thus, if we denote by do the position of object in respect to the lens, by dîthe position of image and by F the focus (focal length), we obtain
The sign rules are identical to those used in spherical mirrors, i.e.
The most important feature of lenses (for which they are produced) is the magnification they provide. The approach is the same as magnification in curved mirrors. This means we can use two formulae for the calculation of magnification:
where h stands for height, and
Aberration is the non-regular deviation of light rays through lenses due to non-uniform thickness, causing images of objects to be blurred.
In an ideal system, every point on the object will focus to a point of zero size on the image. However, in reality this does not occur, because lenses are not ideal optical tools. As a result, parallel rays do not converge at a single dimensionless point as assumed earlier, but in a zone around focus.
In curved mirrors, aberrations are less visible as light does not enter inside the glass but it is reflected by the mirroring surface. Therefore, curved mirrors are more preferable than lenses to be used in powerful optical systems such as telescopes and microscopes.
We can combine optical tools such as plane and curved mirror with lenses to produce new optical systems.
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