Optics 101

Spherical Aberration - What is Spherical Aberration?

Spherical Aberration is a condition that occurs in lenses with spherical surfaces, such as the lenses used in small microscopes. Spherical aberration is caused by the failure of light rays to converge to the same point when the light rays are parallel to the optic axis but at different lengths from the optic axis.

Spherical aberration causes the image of a specimen or an objective to appear blurred and slightly out of focus. When waves of light passes through a convex lens, light waves which are passed through or near the centre of the lens are only slightly refracted. However, for the waves of light which passes near the edge of the lens experiences a greater degree of refraction. This leads to the production of a number of different points of focal along the optic axis.

For an ideal aberration-free lens, all the waves of light are converted to a spherical wavefront from a plane wavefront. This causes the waves of light to refract to a single common focal point in the sphere center. As a result, a perfect image is produced.

As stated earlier, light waves that comes through the edge of the lens will produce a shorter focus than compared to waves of light that passes through the center of the lens or the axial area. This is commonly referred to as axial spherical aberration or longitudinal spherical aberration. Axial spherical aberration is the result of non-spherical wavefronts generated by the objective.

Spherical aberration is very crucial for lens resolution. This can lead to the sharpness and clarity of the image of the specimen to be affected. Spherical aberration can be corrected by cementing glass elements with various convexity and/or concavity together. An example of this is a parabolic lens. This ensures the light rays refracted near the edge of the lens and the light rays that are refracted in the centre of the lens into a common focal point.

Source: AZoOptics

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