JEE Main · 2026 · 23 Jan 2026 · Shift 2 · Geometrical Optics

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The size of the images of an object, formed by a thin lens are equal when the object is placed at two different positions 8 cm and 24 cm from the lens. The focal length of the lens is

____

cm.

Answer: 16

Solution

For a thin lens the linear magnification is

m = \frac{f}{f + u}

, with

u

negative for a real object. Equal-sized images at the two positions require equal magnitudes with opposite signs,

m_{1} = - m_{2}

\frac{f}{f - 8} = - \frac{f}{f - 24}

. This gives

f - 8 = - (f - 24)

, so

2 f = 32

and

f = 16 cm

JEE Main · 2026 · 23 Jan 2026 · Shift 2 · Integer answer

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