281.  The lens equation ? = ? + ? is applicablef
A. when the thickness of the lens is small
B. only to conjugate distances along the principal axis
C. the computed quantities are accurate enough for geometrical optics of simple distances
D. all the above.

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282.  The defect of a lens whereby rays of white light proceeding from a point get dispersed into their components and conveyed to various foci, forming a blurred and coloured image is known as
A. chromatic aberration
B. spherical aberration
C. astigmatism
D. coma.

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283.  Flint glass
A. has slightly the greater refracting power than crown glass
B. has roughly double refracting power than that of crown
C. and crown glass proportions yield the required focal length and neutralise the dispersion produced by the convex lens at the emergence from the concave
D. all the above.

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284.  Pick up the correct statement from the following :
A. spherical aberration may be reduced by diminishing the aperture
B. spherical aberration may be minimised by replacing the single lens by a combination of the lenses.
C. in telescope objectives, a combination of convex lens and concave lens is used.
D. all the above.

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285.  A lens or combination of lenses in which the following defect is completely eliminated is called aplanatic
A. spherical aberration
B. chromatic aberration
C. coma
D. astigmatism.

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286.  The Huygen's telescope eye piece
A. is aplanatic
B. achromatic
C. both A and B
D. neither A nor B.

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287.  Pick up the correct statement from the following :
A. a refracting telescope consists optically of two lenses
B. the principal axes of both the lenses coincide the optical axis of the telescope
C. the lens nearer the object to be viewed is convex and is called objective
D. all the above.

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