BY- K. Sai Manogna (MSIWM014)
Principle :
A single lens, historically called a loupe, consists of a simple microscope. A reading or magnifying glass is the most familiar example nowadays. Higher-magnification lenses are often made of two glass elements that create a color-corrected image. They can be worn in a cylindrical shape around the neck, which can be kept immediately in front of the eye. These are commonly referred to as eye loupes or lenses for jewelers. A single magnifying lens was used to create the standard simple microscope, which was often of good optical quality to enable the study of microscopic species, including Hydra and protists.
Magnifications
1. When one wants to study an object’s information, it is instinctive to put it as close to the eye as possible.
2. The closest the object is to the eye, the greater the angle it subtends to the eye, and thus the more extensive the object appears.
3. However, the human eye can no longer form a clear picture of an object brought too close.
4. The magnifying lens between the viewer and the object makes it possible to construct a “virtual image” that can be viewed comfortably.
5. The magnifier should be positioned in front of the eye to obtain the best possible image. To make out the object of interest, one should position the object at the microscope’s lens’s focus.
6. The highest magnification possible without lenses is when the object is carried to the closest location where a clear virtual image is observed.
7. This distance from the picture is around 25 cm for many people. The closest point of separate vision recedes to greater distances as individuals age, making a magnifier a valuable adjunct to a vision for older people.
8. The optical system’s geometry is related to magnifying force, or the degree to which the object is seen appears to be expanded, and the field of view, or the scale of the object that can be seen.
9. The working value of the magnifying power of the lens can be measured by dividing the minimum distance of separate vision by the focal length of the lens, which is the distance from the lens to the plane where the incoming light is centered
10. A lens with a minimum different vision distance of 25 cm and a focal length of 5 cms would also have a magnifying power of around 5 percent, for example.
11. If the magnifying lens diameter is adequate to fill or exceed the eye’s pupil diameter, the viewed virtual image will appear to be of significantly the same brightness as the original object.
12. As the focal length of the magnifier is increased. the field of view will be determined by the degree to which the working diameter is exceeded by the lens and the distance between the lens and the eye. The clearer the virtual picture, the more dependent it will be on the irregularities of the lens, its contours, and the conditions of its use.
Aberrations:
Various aberrations affect the picture’s sharpness or consistency.
1. Chromatic aberrations create colored fringes around the image’s high-contrast regions since longer light wavelengths (such as red) are brought to focus slightly further from the lens than shorter wavelengths in a plane (such as blue).
2. Spherical aberration creates a picture in which, while the periphery may not be, the center of the field of view is in focus and uses lenses with spherical (rather than nonspherical or aspherical) surfaces.
3. The distortion produces curved images from straight lines in the object. The apparent shape and degree of distortion are closely related to the magnifier’s possible spherical aberration and is usually the most severe in high-powered lenses.
4. As relative aperture, i.e., the working diameter divided by the lens’s focal length, increases, the aberrations of a lens increase.
5. The aberration of the lens with a smaller diameter than the focal length would be more important than that of the other lens with a greater diameter.
6. Thus, there is a conflict between a short focal length that allows for high magnifying power but a narrow field of view and a longer focal length that offers a lower magnifying power but a wider linear field of view.
7. The 1670s high-powered lenses of Leeuwenhoek had a focal length and a few millimeters’ working size. This made it hard to use them, but they produced remarkable pictures that have not been changed for a century.
Magnification:
Several kinds of magnifiers are available. The option of an optical design for a magnifier depends on the power needed and the magnifier’s intended use.
For low powers:
a. A simple double convex lens is applicable for low forces, around 2-10x. Early simple microscopes, such as the microscope of Leeuwenhoek, magnified up to 300x.
b. If the lens has unique aspherical surfaces, as can be easily obtained in a plastic molded lens, the image can be enhanced.
c. When an aspheric lens is used, a reduction in distortion is noted, and the manufacture of such low-power aspheric plastic magnifiers is an important industry.
For high powers:
There are various types of magnifiers in which the basic magnifier is replaced by a compound lens consisting of multiple lenses mounted together for higher powers of 10-50x.
Using two simple lenses, usually plano-convex, which are flat on one side, angled outward on the other side, with curved surfaces facing each other, would automatically increase the distortion expected from a loupe. This form of magnifier is based on the Huygenian telescope eyepiece, in which the chromatic lateral aberration is corrected by removing the elements from the focal length. Since two components provide and share the image properties, the magnifier’s spherical aberration and distortion are substantially reduced compared to those of a single lens of the same strength.
Coddington lens:
To choose the imaging light portion with the lowest aberrations, a Coddington lens incorporates two lens components into a single thick element, with a groove cut in the center of the element. This is a simple and inexpensive system that suffers from the requirement that the distance between the optic beam and the target of imaging is small.
More complex magnifiers:
Three or more components are used by more advanced magnifiers, such as the Steinheil or Hastings types, to achieve better correction for chromatic aberrations and distortion. In general, the use of aspherical surfaces and fewer components is a safer solution.
– Often, mirrors are used. The British physicist C.R. brought reflecting microscopes, in which the image is magnified by concave mirrors rather than convex lenses, to their height of excellence in 1947.
– Burch, who made a set of giant tools that used ultraviolet rays.
– Using a magnifying mirror, chromatic aberration will not be minimized, although, with the use of an aspheric mirror carefully contoured, distortion and spherical aberration are reduced.
– The reflecting microscopes of today are limited to analytical instruments that use infrared rays.
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