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प्रश्न
If a spherical lens has a power of, −0.25 D, the focal length of this lens will be :
(a) −4 cm
(b) −400 mm
(c) −4 m
(d) −40 cm
उत्तर
-4 m
Focal length = `1/"power"=1/-0.25=-100/25=-4m`
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