Question:medium

A convex lens of focal length $f$ is used to form an image whose size is one fourth that of size of the object. Then the object distance is

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For real images formed by a convex lens, magnification is negative. A quick shortcut formula relating object distance, focal length, and magnification is $m = \frac{f}{f + u}$. Setting $-\frac{1}{4} = \frac{f}{f + u}$ instantly gives $u = -5f$.
Updated On: Jun 4, 2026
  • $2f$
  • $5f$
  • $4f$
  • $3f$
Show Solution

The Correct Option is B

Solution and Explanation

Step 1: Understand the situation.
A convex lens forms an image that is one fourth the size of the object. We must find the object distance in terms of the focal length $f$.

Step 2: Use the magnification rule.
Magnification is $m=\frac{v}{u}$. A convex lens making a small image gives a real, inverted image, so we take $m=-\frac{1}{4}$.

Step 3: Express $v$ in terms of $u$.
From $\frac{v}{u}=-\frac{1}{4}$ we get $v=-\frac{u}{4}$.

Step 4: Write the lens formula.
The lens equation is \[ \frac{1}{v}-\frac{1}{u}=\frac{1}{f}. \]

Step 5: Substitute $v=-\frac{u}{4}$.
\[ \frac{1}{-\frac{u}{4}}-\frac{1}{u}=\frac{1}{f}\Rightarrow -\frac{4}{u}-\frac{1}{u}=\frac{1}{f}. \]

Step 6: Combine and solve.
\[ -\frac{5}{u}=\frac{1}{f}\Rightarrow u=-5f. \]

Step 7: Read the size.
The minus sign just means the object sits in front of the lens. The distance is $5f$, which is option (2).
\[ \boxed{5f} \]
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