Science — CBSE Class 10 board question

(i)
Given: h = 5 cm, u = −20 cm, f = −18 cm (concave lens)

Using lens formula: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$$\frac{1}{v} = \frac{1}{f} + \frac{1}{u} = \frac{1}{-18} + \frac{1}{-20} = \frac{-10 - 9}{180} = \frac{-19}{180}$$

(1) Image distance: $v = \frac{-180}{19} \approx -9.47$ cm (virtual, same side as object)

(2) Magnification: $m = \frac{v}{u} = \frac{-180/19}{-20} = \frac{180}{380} \approx +0.47$

(ii)
A concave lens always forms a virtual image with magnification m < 1 (diminished).
A convex lens forms a virtual image (object between F and O) with magnification m > 1 (enlarged).

Source: Light – Reflection and Refraction, Section 9.3.4

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• Always use sign convention: distances measured from optical centre; concave lens has negative focal length.
• For part (ii), examiners expect a direct comparison: concave → m < 1; convex → m > 1. This follows directly from Table 9.5 and Table 9.4 in the textbook.
• Show formula and substitution clearly for full marks.