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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? |
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Answer» Solution :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 |
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