# Solve this: Q.25. A convex lens of power + 2.5 D is in contact with a concave lens of focal length 25 cm. The power of combination is (1) - 1.5 D (2) 0 D (3) + 1.5 D (4) + 6.5 D

Dear student,

so option (1) is correct.
Regards.

• 1
For combination of lenses you should apply the formula: 1/F =1/f+ 1/f' with proper signs!

• 6
for the combinations of lenses we use this formula i/F=1/f1+1/f2
given that , power of convex lens =+2.5D
we know that ,power =100/focal length(in cm)
focal length =100/2.5(in cm)
f1=40 cm
and given that ,focal length of concave lens is 25 cm
f2=-25 cm (because focal length is negative in concave lens
therefore 1/F=1/f1+1/f2
1/F=1/40-1/25
1/F=-3/200
therefore,F=-200/3 cm
power=100/(-200/3)=-3/2=-1.5D
• 2
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