What is Roshan's error in the ray diagram?

What is Roshan's error in the ray diagram?

• A. The ray that goes out parallel with the main axis should bend toward the axis and go through the focal point on the other side.
• B. The ray that starts out parallel with the main axis should bend toward the axis and go through the focal point on the other side.
• C. The ray that goes parallel to the main axis after passing through the lens should also be parallel from the object to the lens.
• D. The rays in the table describe how rays are drawn for a concave lens rather than for a convex lens.

Answer: (B)

In a convex lens, a ray that starts parallel to the principal axis must bend as it passes through the lens so that it goes through the focal point on the opposite side. This is why parallel rays converge toward the focal point after refraction. Roshan’s diagram didn’t follow this rule: the parallel ray should strike the lens and emerge aimed at the far-side focal point, not continue parallel or head toward the wrong point. The idea that the outgoing parallel-ray should pass through the far focal point captures the correct behavior for a converging lens. (Remember the flip side rule: a ray that passes through the focal point on the object side exits parallel to the axis.)

In a convex lens, a ray that starts parallel to the principal axis must bend as it passes through the lens so that it goes through the focal point on the opposite side. This is why parallel rays converge toward the focal point after refraction. Roshan’s diagram didn’t follow this rule: the parallel ray should strike the lens and emerge aimed at the far-side focal point, not continue parallel or head toward the wrong point. The idea that the outgoing parallel-ray should pass through the far focal point captures the correct behavior for a converging lens. (Remember the flip side rule: a ray that passes through the focal point on the object side exits parallel to the axis.)