A convex lens of focal length 25cm and a concave lens of focal length 10cm are placed in close focal contact with each other. Calculate the lens power of this combination.
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Focal length: the distance between the centre of a lens or curved mirror and its focus. The equivalent distance in a compound lens or telescope. Optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device, $\text{P}=\frac{1}{\text{f}}$$\text{f}=25\text{cm (Convex lens)}$
$\text{f}=-10\text{cm (Concave lens)}$
Power of convex lens $=\frac{1}{\text{f}}$$=\frac{1}{0.25\text{d}}=4.0\text{d}$
Power of concave lens $=\frac{1}{\text{f}}$$=\frac{1}{-0.10\text{d}}=-10\text{d}$
Total Power $=4.0\text{d}-10.0\text{d}$ Total Power $=-6.0\text{d}$
$\text{f}=-10\text{cm (Concave lens)}$
Power of convex lens $=\frac{1}{\text{f}}$$=\frac{1}{0.25\text{d}}=4.0\text{d}$
Power of concave lens $=\frac{1}{\text{f}}$$=\frac{1}{-0.10\text{d}}=-10\text{d}$
Total Power $=4.0\text{d}-10.0\text{d}$ Total Power $=-6.0\text{d}$
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