Question

Two thin lenses are in contact and the focal length of combination is 80 cm. If the focal length of one of the lens is 20 cm. The power of the other lens is

• A. 1.66 D
• B. 4 D
• C. – 1D
• D. – 3.75 D

Answer 1

Answer: (D)

– 3.75 D

Answer 2

$\frac { 1 } { 80 }$ = $\frac { 1 } { 20 }$ + $\frac { 1 } { \mathrm { f } }$

\ f = $\frac { - 80 } { 3 } \mathrm {~cm}$ = $- \frac { 4 } { 15 } \mathrm {~m}$

\ p = = – 3.75 D