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Science · 2026 · 4 marks
CBSE 2026 · Region 3 · Set 1 · Q38
The variation of image distance (v) with the object distance (u) for a convex lens is given in the following observation table. Analyse it and answer…
The variation of image distance $\displaystyle (v)$ with the object distance $\displaystyle (u)$ for a convex lens is given in the following observation table. Analyse it and answer the questions given below :
| S.No. | Object distance $\displaystyle (u) \mathrm{cm}$ | Image distance (v) cm |
| $\displaystyle 1$ | - $\displaystyle 150$ | + $\displaystyle 30$ |
| $\displaystyle 2$ | -$\displaystyle 75$ | + $\displaystyle 37$•$\displaystyle 5$ |
| $\displaystyle 3$ | -$\displaystyle 50$ | +$\displaystyle 50$ |
| $\displaystyle 4$ | - $\displaystyle 37$•$\displaystyle 5$ | +$\displaystyle 75$ |
| $\displaystyle 5$ | -$\displaystyle 30$ | + $\displaystyle 150$ |
| $\displaystyle 6$ | -$\displaystyle 15$ | +$\displaystyle 37$•$\displaystyle 5$ |
(a)
Without calculation, find out the focal length of the given convex lens. Justify your answer.
(b)
Which one of the observations given in the table is not correct and why ?
(c)
(i)
Find out the value of magnification for $\displaystyle u=-30 \mathrm{~cm}$. Write the nature of the image formed. Give reason for your answer.
OR
(ii)
Analyse the data given in the table above and draw the labelled ray diagram for $\displaystyle u=-30 \mathrm{~cm}$ and $\displaystyle v=150 \mathrm{~cm}$.
Marking-scheme solution
(a)
In observation no.3. The object distance is equal to image distance,
which is only possible when the object is at $\displaystyle 2 \mathrm{~F}(\mathrm{C})=50 \mathrm{~cm}$. Therefore, the
focal length is $\displaystyle 25$ cm.
(b)
Observation no. $\displaystyle 6$
Because when object is kept between F and O, image formed by
lens will be on same side of object. Hence, image distance(v)
should be negative.
\[\begin{aligned}
\mathrm{m} & =\frac{\mathrm{v}}{\mathrm{u}} \\
& =\frac{150}{-30} \\
\mathrm{~m} & =-5
\end{aligned}
\]
Nature of image: Image will be real and inverted
Reason: because the value of m is negative.
(ii)
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CBSE Class 10 Science past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.