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Science · 2023 · 3 marks
CBSE 2023 · Region 4 · Set 2 · Q30
(A) An object is kept at a distance of 1m from a lens of power+2D: (i) Identify the type of lens. (ii) Calculate its focal length and distance of the…
(A)
An object is kept at a distance of 1m from a lens of power+2D:
(i)
Identify the type of lens.
(ii)
Calculate its focal length and distance of the image formed.
OR
(B)
Define the following terms in the context of a diverging lens:
(i)
Principal focus,
(ii)
Focal length. Draw a labelled ray diagram to illustrate your answer.
Marking-scheme solution
(A)(i)
Convex lens / Converging lens
(ii) Power, \(\displaystyle \mathrm{P}=+2 \mathrm{D}\)
\[\begin{aligned}
& f(m)=\frac{1}{P} \\
& f=\frac{1}{2}=0.5 \mathrm{~m}=50 \mathrm{~cm} \\
& u=-100 \mathrm{~cm}
\end{aligned}
\]
\[\begin{aligned}
& \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\
& \frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{50}-\frac{1}{100}=\frac{1}{100} \\
& v=100 \mathrm{~cm}=1 \mathrm{~m}
\end{aligned}
\]
Alternate answer only for image distance Since object is at 2f, image will be formed at 2f. Therefore, image distance is $\displaystyle 100$ cm. as \(\displaystyle \mathrm{f}=50 \mathrm{~cm}, \mathrm{u}=1 \mathrm{~m}\) or \(\displaystyle 100 \mathrm{~cm}=2 \mathrm{f}\)
\[\therefore \mathrm{u}=\mathrm{v}=1 \mathrm{~m}
\]
(B) (i) It is a point on the principal axis of a diverging lens from where the rays parallel to principal axis appear to diverge.
(ii) The distance between the optical centre and the principal focus of the lens.
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CBSE Class 10 Science past-paper question from the 2023 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.