Eyepieces

1. Eyepieces

2. An  eyepiece , or  ocular lens , is a type of lens that is closest to the eye when someone looks through the optical device. The eyepiece is placed near the focal point  of the objective to magnify this image . The amount of magnification depends on the focal length  of the eyepiece .

3. The lens or combination of lenses in an optical instrument (microscope, telescope, etc.) through which the observer views the image formed by the objective. The most common eyepieces are composed of two single lenses or two doublets: the lens or doublet nearer the eye is called the  eye lens  and the one nearer the objective is called the  field lens .  The role of the eyepiece is to magnify the image and to reduce the aberrations of the image formed by the objective .

4. Negative eyepiece  :Eyepiece made up of two lenses, in which the first principal focus of the eyepiece lies between the two lensessuch as in a Huygens' eyepiece. Positive eyepiece:  Eyepiece made up of two lenses in which the first principal focus of the eyepiece lies in front of the field lens, such as in a Ramsden eyepiece.

5. Types of Eyepieces 1.Ramsden Eyepiece 2.Huygens Eyepiece

6. Ramsden Eyepiece An  eyepiece consisting of two  plano -convex crown-glass lenses  of equal  focal length, placed with the convex sides facing each other andwith a separation between the lenses of about two-thirds of the  focal length  of each .

7. Huygens Eyepiece A telescope eyepiece consisting of two planoconvex lenses separated by a distance equal to half the sum of their focal lengths, which are in the ratio of three to one, and oriented so that their curved surfaces face the incident light. .

8. Comparison of Ramsden & Huygens Eyepiece

9. Ramsden Eyepiece Huygens Eyepiece Ramsden’s eyepiece is a positive eyepiece. The image formed by the objective lies in front of the field lens. Therefore , crosswires can be used. Huygen’s eyepiece is a negative eyepiece. The image formed by the objective lies in between the two lenses. Therefore crosswires cannot be used. The condition for minimum spherical aberration is not satisfied. But by spreading the deviations over four surface ,spherical aberration is minimized. The condition for minimum spherical aberration is satisfied. It does not satisfy the condition for achromatism but can be made achromatic by using an achromatic doublet as the eye lens. It satisfies the condition for achromatism. It is achromatic for only two chosen colors. It is achromatic for all colors. The other types of aberration are better eliminated. Coma is absent and distortion is 5% higher. Other aberration like pincushion distortion are not eliminated. The eye clearance is 5% higher. The eye clearance is too small and less comfortable. It is used for quantitative purpose in microscopes and telescopes. It is used for qualitative purpose in microscopes and telescopes.

10. Ramsden Eyepiece Huygens Eyepiece Its power is positive Its power is positive The two principle planes are crossed. The two principal planes are crossed It can be used as a simple microscope because the first principal plane lies to the left of the field lens and the focal plane is real. It cannot be used as simple microscope because the first focal plane lies to the right of the field lens and the focal plane is virtual. The nodal points coincide with the principal points. The nodal points coincide with the principal points.

11. Aberration

12. The departures of real images from the ideal images in respect of actual size shape and position. They are inevitable consequences of the laws of refraction at spherical surfaces.

13. Types of aberration

14. Monochromatic Aberration: The defects due to wide angle incidence and peripheral incidence. It occurs with monochromatic light. Chromatic Aberration: It occurs due to dispersion of light .

15. Types of Monochromatic aberration Spherical aberration Coma Astigmatism Curvature of field Distortion

16. Spherical Aberration Spherical aberration comes from the spherical surface of a lens. The further away the rays from the lens center, the bigger the error is. The image is improved if the image plane move closer to the lens to find optimal spot size.

17. Coma Arises from off-axis object points. The transverse magnification is a function of ray height The resulting pattern is like a comet.

18. Astigmatism In optical design, the vertical plane is general called the “tangential plane” The “sagittal plane” is the plane at right angle to the trangential plane and containing the principle ray. Astigmatism results in different focusing power to the tangential and sagittal plane.

19. Distortion All points in the object plane are imaged to points in image plane. Distortion arises when he magnification of off axis image is a function of the distance to the lens center. With distortion Corrected

20. Reference: Text book of optics Fundamentals of optics http:/aberration/wikipedia/encyclopedia http:/types of aberration/ ppt/encyclopedia/freedownload.html http:/eyepiece/wikipedia/encyclopedia.html http:/wikipedia/encyclopedia/types of eyepiece.html

21. Done By A.K.RAAHA SUTHA(13PHY040) D.DEEPIGA(13PHY010)