Question

Convex lens forms the image of Sun at :

• A. centre of curvature
• B. no image is formed
• C. between centre of curvature and focus
• D. focus

Hint:

Key image formation rule for a convex lens:
{Object at Infinity (like the Sun) \(\rightarrow\) Image at Focus (F)} The image will be real, inverted, and very small (point-sized if the object is a point source at infinity, or a small disc for the Sun). This is why you can use a magnifying glass (a convex lens) to focus sunlight and burn paper.

Answer 1

The Correct Option is D

Concept: The Sun is a very distant object. For optical purposes, rays of light coming from an object at infinity are considered to be parallel to each other. A convex lens (converging lens) has a specific point where parallel rays of light converge after passing through it. Step 1: Object at Infinity Since the Sun is extremely far away from the Earth (and thus from the lens), the light rays from the Sun arriving at the convex lens are effectively parallel to the principal axis (assuming the lens is pointed towards the Sun). Step 2: Action of a Convex Lens on Parallel Rays A convex lens converges parallel rays of light that are incident on it.
If the parallel rays are parallel to the principal axis, they converge at the principal focus (F) of the lens after refraction.
If the parallel rays are incident at an angle to the principal axis, they converge at a point in the focal plane (a plane passing through the principal focus and perpendicular to the principal axis). Since the Sun is a large object, even if it's at infinity, different points on the Sun will form an image in the focal plane. For simplicity, "the image of the Sun" is typically considered to be formed at the principal focus if we consider the central rays. Step 3: Image Formation When parallel rays of light (from an object at infinity like the Sun) pass through a convex lens, they refract and converge to form a real, inverted, and highly diminished image at the principal focus (F) of the lens (or in the focal plane). This is the principle used in burning glasses, where sunlight is focused by a convex lens to a small point, concentrating the energy. Step 4: Analyzing the options
(1) centre of curvature (2F): An image is formed at 2F when the object is placed at 2F.
(2) no image is formed: Incorrect, a convex lens forms images of distant objects.
(3) between centre of curvature and focus: An image is formed between F and 2F when the object is beyond 2F (but not at infinity).
(4) focus: Correct. For an object at infinity (like the Sun), the image is formed at the focus (or in the focal plane). Therefore, a convex lens forms the image of the Sun at its focus.