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Thermodynamics

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Ramsden Eyepiece

It consists of two plano-convex lenses of equal focal length separated by the distance equal to two-thirds the focal length of either. The convex faces are towards each other and the eyepiece is placed beyond the image formed by the objective. In this eyepiece cross wires are provided and it is used in optical instruments where accurate quantitative measurements are made.

Let F be the focal length of the equivalent lens.

∴ F = + ¾ ƒ

The equivalent lens must be placed at a distance ¾ ƒ behind the field lens at a distance from it.

Thus, the equivalent lens is in between the field lens and the eye lens.

As the focal length of the eyepiece (equivalent lens) is ¾ ƒ, the image of the object due to the objective must be formed at a distance ¾ ƒ – ½ ƒ in front of the field lens. This image will act as an object for the eyepiece and the final image will be formed at infinity. The cross wires must be placed at the position where the image due to the objective is formed, i.e. at a distance of ½ ƒ in front of the field lens. This is the advantage of Ramsden eyepiece over the Huygens eyepiece.

The chromatic aberration in a Ramsden eyepiece is small. In some cases, both the lenses of the eyepiece are made of a combination of crown and flint glass and chromatic aberration is eliminated. As both the lenses are plano-convex with their convex surfaces facing such other the spherical aberration produced is small.