For a convex lens object placed at a distance 7.5cm,its image is found at 10cm from lens.The focal length of lens will be
a) 10cm (b) 20cm
c) 30cm (d) 40cm
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Answer: 30 CM
v= -10cm
f= ?
u= - 7.5 cm
1/v -1/u = 1/f
1/-10 - 1/-7.5 = 1/ f
1/ -10 + 1/ 7.5 = 1/ f
1/-10 + 10/ 75 = 1/f
-15 + 20/150 = 1/f
5 /150 = 1/ f
f = 150 / 5
f= 30 cm
Explanation:
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