Question

⭐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​

Answer 1

(i) Lens C (+ 10D)and Lens E (-20D)

Total power = +10 + ( -20 )= -10D

f = 1/(-10)m = -10cm

Similarly,

(ii) Lens B (-5D) and Lens C (+10D)

(iii) Lens A (+5D) and Lens D(-10D)

Hope it helps

Answer 2

SOLUTION :
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