274.  The apparent error on reversal is
A. equal to the actual error
B. twice the actual error
C. thrice the actual error
D. none of these.

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275.  Pick up the correct statement from the following :
A. the apparent error on reversal is twice the actual error
B. the correction may be made equal to half the observed discrepancy.
C. the good results may be obtained from a defective instrument by reversing and taking the mean of two erroneous results
D. all the above.

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276.  Pick up the correct statement from the following :
A. it is difficult to eliminate an error completely at first trial
B. instability of the instrument makes it almost impossible to adjust it satisfactorily
C. adjustment screws must be left bearing firmly but should never be forced
D. all the above.

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277. The power of a lens
A. is reciprocal of its focal length
B. is positive if it is a convex lens
C. is negative if it is a concave lens
D. is measured in diopter.

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278.  Diopter is the power of a lens having a focal length of
A. 25 cm
B. 50 cm
C. 75 cm
D. 100 cm

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279.  Pick up the correct statement from the following :
A. the power of a lens is the reciprocal of its focal length
B. the unit of power of the lens is diopter
C. the power of two or more thin lenses in contact is the power of the combination of the lenses
D. all the above.

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280.  Two concave lenses of 60 cm focal length are cemented on either side of a convex lens of 15 cm focal length. The focal length of the combination is
A. 10 cm
B. 20 cm
C. 30 cm
D. 40 cm.

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