Question

If a convex lens has to form the image of an object at infinity, where should the object be placed?

• A. At pole
• B. At $C = 2f$
• C. At infinity
• D. At focus

Hint:

When an object is placed at the focus of a convex lens, the outgoing rays become parallel. This principle is used in searchlights and vehicle headlights.

Answer 1

The Correct Option is D

Concept: A convex lens forms different types of images depending on the position of the object relative to the focus and optical center.

Step 1: Recall image formation rules for a convex lens.
• Object at infinity $\rightarrow$ image at focus
• Object beyond $2f$ $\rightarrow$ image between $f$ and $2f$
• Object at focus $\rightarrow$ image at infinity

Step 2: Analyze the given condition. The question asks for the object position when image is formed at infinity. For a convex lens, when the object is placed exactly at the principal focus, the refracted rays become parallel after refraction. Parallel rays meet at infinity.

Step 3: Identify the correct position. Therefore, the object must be placed: \[ \boxed{\text{At focus}} \] Hence, the correct option is (4).