Three lenses lenses of focal length 20 cm each are kept in contact with each other . Find the power of their combination.

Explanation

Data : `f_(1)=20 cm =0.2 m`,
`f_(2)=20cm =0.2 m `, P(combination )= ?
`1/f=1/(f_(1))+(1)/(f_(2))=(1)/(0.2 m) +(1)/(0.2 m)=(2)/(0.2m)=(1)/(0.1m)`
`therefore ` Focal length of the combination of the lenses. F=0.1 m.
`P=1/f =1/(0.1 m)=10D`
The power of the combination of the lenses, P=10 D.