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Defects of spherical mirrors
Rays close to the axis of the mirror are brought to a focus at one point but those far from the
axis meet at a range of different points. The effective focal length varies for rays at different
distances from the axis (See Figure 1).
This is called Spherical Aberration and the locus of the focal points is known as a caustic curve.
You can see this on the surface of tea or coffee in a mug. (Figure 2). This defect means that
large spherical mirrors are not good for giving a focused image over a wide field of view.
Figure 1
caustic curve
Figure 2
For all spherical mirrors the mirror formulae only work and there is therefore only ever a
perfectly sharp image when al the rays are parallel to the axis and close to it - in fact along the
The image actually gets more and more fuzzy the further from the axis we go. This limits the
field of view and is a serious problem for astronomical telescopes where the large aperture is
essential for maximum light gathering power.
With a spherical mirror and with a wide incident beam, the light is not focused to one point but
forms a pattern, the envelope of which is called a caustic curve (see earlier).
There are three solutions:(a) use a parabolic mirror (See: Optics/Reflection/Parabolic mirrors)
(b) use a spherical mirror but with a specially shaped glass correcting plate in the path of the
incident beam (See: Optics/Optical instruments/Schmidt telescopes)
(c) reduce the aperture of the mirror by using a stop (e.g a piece of card with a hole in it)