All the rays of light parallel to the principal axis, after reflection, pass through the focus of a lens. This phenomenon is known as the "law of reflection" and is a fundamental principle in optics.

Explanation:

Law of Reflection:
The law of reflection states that the angle of incidence is equal to the angle of reflection. This means that when a ray of light strikes a surface and is reflected, the angle at which it strikes the surface will be equal to the angle at which it is reflected.

Parallel Rays:
When parallel rays of light strike a lens, they refract (bend) and converge at a specific point. This point is known as the focus of the lens.

Reflection at a Lens:
When light passes through a lens, it undergoes both refraction and reflection. The curved shape of the lens causes the light rays to bend as they pass through it. However, at the surface of the lens, some of the light rays are reflected back.

Focus:
The focus of a lens is the point where parallel rays of light converge or appear to diverge from. For a convex lens (thicker in the middle), the focus is on the opposite side of the lens from where the light is coming. For a concave lens (thinner in the middle), the focus is on the same side as the incoming light.

Passing through the Focus:
When parallel rays of light strike a lens, they refract and converge at the focus. After reflection at the surface of the lens, the rays continue to converge and pass through the focus again. This is because the angle of incidence (which is equal to the angle of reflection) at the surface of the lens causes the rays to converge towards the focus.

Therefore, all the rays of light parallel to the principal axis, after reflection, pass through the focus of a lens. This is why the correct answer is option 'B' - focus.