⭐For physics Newton⭐
Ravi is given lenses with powers + 5 D, - 5 D, + 10 D,- 10 D and -20 D. Considering a pair of lenses at a time, which two lenses will he select to have a combination of total focal length when two lenses are kept in contact in each case:
i. -10 cm
ii. 20 cm
iii. -20 cm
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Total power , P= P1 + P2 and
total length f is 1/f = 1/f1 + 1/f2
When lenses of 10 D and -5 Dare taken, total power
P= P1 + P2
= 10 D-5 D = 5 D
Hence, focal length = 100/5 = 20cm