Two convex lenses of focal lengths 2f and f are arranged to from an eye-piece and are spaced by f. The first lens is to be used as field lens and the second one as eye-lens. Find (a) equivalent focal length. (b) See whether it is positive or negative eye-piece. (c) Whether it is free from spherical aberration and chromatic aberration? (d) What will be the disadvantage it f is used as field lens?

Two convex lenses of focal lengths 2f and f are arranged to from an ey

1.	Two convex lenses of focal lengths 2f and f are arranged to from an eye-piece and are spaced by f. The first lens is to be used as field lens and the second one as eye-lens. Find (a) equivalent focal length. (b) See whether it is positive or negative eye-piece. (c) Whether it is free from spherical aberration and chromatic aberration? (d) What will be the disadvantage it f is used as field lens?
Answer» Answer :(a) F = f, (b) positive, (c) free from spherical aberration but not CHROMATIC, (d) eye-lens position falls as the FOCUS of the field lens and so DUST PARTICLES etc on field lens will be infinitely enlarged and whole field of view will be spoiled

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