An optician makes spectacles having a combination of a convex lens of focal length 40 cm in contact with a…

An optician makes spectacles having a combination of a convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm . The power of this combination of lenses in dioptre is
  1. -6.67
  2. -1.5
  3. +6.67
  4. +1.5

Solution

$P=\frac{1}{f}$
For combination of lenses, $\mathrm{P}=\mathrm{P}_1+\mathrm{P}_2$
Given: $\mathrm{P}_1=2.5 \mathrm{D}$ and $\mathrm{P}_2=-4 \mathrm{D}$ $\therefore \quad P=-1.5 D$

Asked in: MHT CET 2024 (16 May Shift 2)

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