What will be the focal length of a lens whose power is +2.0 D and the type of the lens is 25cm concave lens 50cm convex lens 50cm concave lens none of the above?

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Focal length of convex lens f1 = + 25 cm = + 0.25 m 

Power of convex lens`p_1=1/(f_1)=1/0.25=4D` 

Focal length of concave lens f2 = - 10 cm = - 0.10 m 

Power of concave lens  `p_2=1/(f_2)=1/-0.10=-10D` 

(a) The power of the combination of lenses is the algebraic sum of the powers of the individual lenses.
∴ Power of combination P = P1 + P2
⇒ P = 4 - 10 = - 6 D. 

(b) Suppose, the focal length of the combination of the lenses is f.
The power of a lens and the focal length are related as: 

`p=1/f` 

`-6=1/f` 

`f=1/-6=-0.167m -16.7 cm`

Therefore, the focal length of the combination of the lenses is - 16.7 cm.

(c) The focal length of the combination of the lenses is - 16.7 cm. Here, the negative sign shows that the combination of the two lenses acts like a concave lens. Therefore, this combination of lenses is diverging.