Spherical Aberration in Lens

The presence of spherical in the image formed by a single lens in illustrated in the fig. O is a point object on the axis of the lens and Ip and Im are the images formed by the paraxial and marginal rays respectively. It is clear from the figure that the paraxial rays of light form the image at the longer distance from the lens than the marginal rays. The image is not sharp at any point on the axis. However, if the screen is placed perpendicular to the axis at AB, the image appears to be a circular patch of diameter AB. At positions on the two sides of AB, the image patch has a larger confusion, which corresponds to the position of the best image. The distance Im Ip measures the lateral spherical aberration. When the aperture of the lens is relatively large compared to the focal length of the lens, the cones of the rays of light refracted through the different zones of the lens surface are not brought to focus at a nearer point Im and the axial rays come to the focus image extends over the length Im Ip. This effect is called spherical aberration and arises due to the fact that different annular zones have different focal lengths. The spherical aberration produced by a concave lens is illustrated in fig.

The spherical produced by a lens depends on the distance of the object point and varies approximately as the square of the distance of the object ray above the axis of the lens. The spherical aberration produced by a convex lens is positive and that produced by a concave lens is negative.

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