Question

In a concave lens, a ray of light emanating from the object parallel to the principal axis of the lens, after refraction: ____.

• A. passes through the second principal focus.
• B. appears to diverge from the first principal focus.
• C. emerges parallel to the principal axis.
• D. passes through 2F, which is the radius of curvature of the lens.

Hint:

Concave lens = Diverging (appears to come from F). Convex lens = Converging (actually passes through F).

Answer 1

The Correct Option is B

Step 1: Understanding the Topic:
This question is about "Ray Optics" and the refraction rules for spherical lenses. Every lens has specific ray-tracing rules that allow us to predict where an image will form. A concave lens is a "diverging" lens, meaning it spreads light rays apart.
Step 2: Key Formulas and Approach:
The approach involves visualizing the three standard rules for a diverging lens:
Rule 1: A ray parallel to the principal axis diverges as if it came from the focus.
Rule 2: A ray aimed toward the focus emerges parallel to the axis.
Rule 3: A ray through the optical center passes straight through.

Step 3: Detailed Explanation:

Analyze the physical behavior: A concave lens is thinner at the center than at the edges. When a parallel beam of light hits it, the refractive geometry forces the rays to bend away from the principal axis.
Define the focal point: For a diverging lens, the principal focus ($F_1$) is defined as the point from which rays that were initially parallel to the principal axis seem to originate after passing through the lens.
The Result: Therefore, a ray that comes in parallel will be refracted "outward." If an observer looks at this outgoing refracted ray, their brain traces it back in a straight line, and that line will lead directly to the first principal focus.
This is why we say the ray "appears to diverge" from the focus. This behavior is responsible for the formation of virtual, upright images in concave lenses.
Step 4: Final Answer:
After refraction, the ray appears to diverge from the first principal focus.