We use a lens to form an image of a point object that\'s located on the optic ax

Question

We use a lens to form an image of a point object that's located on the optic axis. Assume that the lens is made of two spherical surfaces, i.e. this isn't the sort of ideal aspheric lens that we talked about when we studied the Principle of Least Time. What's sort of aberrations will be present in the image of this object that is on the optic axis? (Ignore chromatic aberration.) What will the shape of the aberrated image be? How does the size of this circle depend on the lens radius r? (The answer is in the second section of the chapter.) Now we move the lens off-axis a short distance h'. If the image were affected by coma aberration, what would the shape be? What if the image is affected by astigmatism? What will the shape be?

Explanation / Answer

1) The image of a point object formed by the lens is found to have a egg like or comet like shape is called COMA. The image formed due to the lens system unable to focus the image at a point.

2) The image is like forming some circular rings due to the different lateral magnification of the different zones by the lens.

3) The size of the image depends on the radius. It is directly proportional. The radius of comatic circle increases as the radius of the zone of the lens increases. Size of the image depends up the refractive index of the lens materials as well.

4) When the object is moved far off th axis of the lens, the image formed is not properly focussed. The image formed is a two mutually perpendicular lines separated by a finite distance. Such aberration is called astigamatism.

5) Image is like two lines separated by a distance that are muthually perpendicular. Its like a irregular patch of light and never be focussed as a point image.

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