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Thermodynamics

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Oculars

Microscopes and telescopes suffer from various defects due to the following reasons:

(1) Chromatic aberration of the lenses used, and

(2) Spherical aberration.

The first defect is removed by using achromatic combination of lenses and the second defect is removed by using the more spherical surfaces towards the incoming parallel rays.

As already discussed, the eye cannot be held close to the eyepiece because the rays from the extremities of the image are refracted through the peripheral portion of the eyepiece lens. The best position for the eye is the eye ring. Therefore, to avoid this difficulty a field lens is used and usually an eyepiece consists of two lenses, the field lens and the eye lens. The angular object field and the angular image field can be increased considerably.

In figure, L₁ is the objective and the extreme ray that can be collected by the eye lens L₃ in the absence of L₂ makes and angular object field and the angular image field will be . A is the centre of the exit pupil. When a lens L₂ is placed where the image of the object due to the objective is formed, the dotted ray which was first not collected by the eye lens is now collected by it. The centre of the exit pupil shifts to B. The angular object field is where and the angular image field is where . This combination of the field lens L₂ and the eye lens L₃ forms the eyepiece or the ocular.

The functions of the field lens are: (1) It increases the angular object field. (2) It brings the centre of the exit pupil nearer the eye lens and increases the angular image field. (3) It helps to minimize spherical aberration and chromatic aberration.