42.Three lenses are in contact have a combined focal length of 12cm.when the third lens is removed the combined focal length is 60/7 cm The third lens is -----------

1/f = 1/f1 + 1/f2 + 1/f3
 1/12 = 1/f1 + 1/f2 + 1/f3
7/60 = 1/f1 + 1/f2
1/12 = 7/60 + 1/f3
1/12 - 7/60 = 1/f3
-2/60 = 1/f3
f3 = -60/2
therefore f= -30 (answer)
this means that the lens is diverging
 
  • 62
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