What Is Microscope Eyepiece Or And Its Function?

What Is Microscope Eyepiece?

An eyepiece or ocular lens is the part of the microscope that magnifies the image produced by the microscope’s objective so that it can be seen by the human eye.

It is so named because it is usually a lens located closest to the observer’s eye when someone looks through the device. Oculars, or ocular lenses, are alternative names for eyepieces. To maintain consistency during this discussion, we will refer to all oculars and ocular lenses as eyepieces.

The eyepiece is a type of lens that is attached to a variety of optical devices such as binoculars, telescopes, and microscopes.

In Microscope eyepiece work in combination with an objective lens to further magnify the intermediate image so that specimen detail can be observed.

First, the Objective lens collects light and brings it into focus, and creates an image. The eyepiece is placed near the focal point of the objective lens to magnify this image. the amount of magnification depends on the focal length of the eyepiece.

To achieve the best results in microscopy, combine objectives with eyepieces that are appropriate for the correction and objective type.

The standard eyepiece has a 24-mm-diameter field of view, whereas wide-field eyepieces for Plano-type objectives have a 30-mm-diameter field of view, which increases the usable area of the primary image. Eyepieces typically produce an additional 10X magnification, but this can vary from 1X – 30X.

Eyepiece of microscope

Construction and working principle of eyepiece.

An eyepiece is essentially a combination of lenses that are used as magnifiers. An eyepiece is commonly made up of two lenses or lens combinations.  

From that two-lens combination, the lens which is located closer to the eye is termed an eye lens. while the other located closer to the primary image formed by the objective is termed field lens.

The combination of field lens and eye lens is designed in such a way so they can minimize or reduced the aberrations according to the requirement of the purpose of hand.

Out of these two lenses, the field lens has a larger aperture and field of view so that it can catch all the rays forming the primary image. while on the other side-eye lens is of a smaller aperture, which produces rays that the eye can accept.

The primary function of the field lens is to bend the rays that form the primary image and produce the first stage of magnification. the whereafter second stage is provided by the eye lens.

In the instrument where the final image is viewed by the eye rather than captured on the recording device. the eye is to be placed at the exit pupil of the eyepiece so as to capture all rays coming out later.

The exit pupil is the bright circle that can be seen in the center of the eyepiece. For comfortable viewing, this should be at an appropriate position convenient for the placement of the eye.

Types Of Eyepiece

Here are two major types of eyepieces that are grouped according to lens and diaphragm arrangement:

  1. Negative eyepieces (or Huygenian eyepieces) with an internal diaphragm and
  2. Positive eyepieces (or Ramsden eyepieces) that have a diaphragm below the lenses of the eyepiece.

Negative eyepieces have two lenses:

  • Upper lens, which is closest to the observer’s eye, is called the eye lens
  • Lower lens (beneath the diaphragm) is often termed the field lens

In their simplest form, both the eye and field lenses are plano-convex, with convex sides facing the specimen. About midway between these lenses is a fixed circular opening or internal diaphragm. The size of the diaphragm defines the circular field of view that is observed when you look into the microscope.

What Is a Huygenian Eyepiece?

The simplest negative eyepiece design, often termed the Huygenian eyepiece, is found on most teaching and laboratory microscopes fitted with achromatic objectives. An ocular has two planoconvex lenses that are formed from similar glass and that are separated by a space equal to half the sum of their focal lengths.

The lenses are called the eye lens and the field lens. The focal plane is located between the two lenses. It was invented by Christiaan Huygens in the late 1660s and was the first compound (multi-lens) eyepiece.

Although the Huygenian eye and field lenses are not well corrected, their aberrations tend to cancel each other out. More highly corrected negative eyepieces have two or three lens elements cemented together to make the eye lens.

What Is a Ramsden Eyepiece?

The other main type of simple eyepiece is the positive eyepiece with a diaphragm below its lenses, commonly known as the Ramsden eyepiece. This eyepiece has an eye lens and field lens that is also two identical plano-convex lenses with their convex faces pointing towards each other.

They are separated by a distance of two-thirds of the focal length of either lens. It was invented by the astronomical and scientific instrument maker Jesse Ramsden in 1782.

The front focal plane of this eyepiece lies just below the field lens, at the level of the eyepiece diaphragm, making this eyepiece readily adaptable for mounting reticles. To provide better correction, the two lenses of the Ramsden eyepiece may be cemented together.

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