What is the focal length of the combination if a lens of focal length - 70 c is brought in contact with a lens of focal length 150 cm? What is the power of the combination?

Explanation

Give: focal length of first lens `f_(1) = - 70 cm`, focal length of second lens, `f_(2) = 150` cm.
Equation for focal length of lenses in contact. `((1)/(F) = (1)/(f_(1)) + (1)/(f_2))`
Substituting the values.
`(1)/(F)=(1)/(-70)+(1)/(150)=-(1)/(70)+(1)/(150)`
`(1)/(F)=(-150)/(70xx150)=(-50)/(70xx150)=-(80)/(10500)`
`F = (-1050)/(8) = - 131.25 cm`
As the focal lenght is negative, the combination of two lenes is a diverging system of lenses.
The power of combination is.
`P=(1)/(F)=(1)/(-1.3125)=-0.76` diopter