A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is

Options

A. `-1.5` D
B. `-6.5 D`
C. `+6.5 D `
D. `+1.5 D `

Answer

A

Explanation

Focal length of the combination,
`1/F=1/f_1 + 1/f_2`
`rArr 1/F=1/"(+40)" + 1/"(-25)"`
`F=(-200)/3` cm
`rArr` Power P=`100/F=100/"(-200/3)"`
=-1.5 D