What happens to an incident ray that passes through the principal focus of a convex lens?

When an incident ray passes through the principal focus of a convex lens, it emerges from the lens traveling parallel to the principal axis. This behavior can be understood through the principles of optics, specifically refraction, which is the bending of light as it passes through different media.

In the case of a convex lens, rays of light that are parallel to the principal axis converge at the focus after passing through the lens. Conversely, a ray that originates from the principal focus and then travels through the lens is refracted so that it travels parallel to the axis after emerging. This characteristic is key to the functioning of a convex lens, enabling its applications in magnifying glasses, cameras, and other optical devices.

The behavior of the ray is a direct consequence of how a convex lens is shaped—thicker in the middle than at the edges—causing the light to bend inward. Thus, the option indicating that the ray travels parallel to the axis after passing through the lens is the correct understanding of this optical phenomenon.