Explanation of why a ray parallel to the principal axis of a convex lens passes through its second focus:
Understanding the behavior of light rays passing through a convex lens can be explained using the concept of focal points and the properties of convex lenses.

1. Focus of a convex lens:
- A convex lens has two focal points, one on each side of the lens.
- The focal point on the same side as the light source is called the first focus (F1), and the focal point on the opposite side is called the second focus (F2).

2. Parallel rays and focal points:
- When a ray of light enters a convex lens parallel to the principal axis, it refracts and passes through the focal point on the opposite side of the lens.
- This phenomenon is a result of the converging nature of convex lenses, which causes parallel rays to converge at the focal point.

3. Second focus and parallel rays:
- The second focus of a convex lens is the focal point on the opposite side of the lens from where the light enters.
- When a ray of light enters the lens parallel to the principal axis, it refracts and passes through the second focus due to the focusing properties of the lens.
In conclusion, a ray parallel to the principal axis of a convex lens passes through its second focus because of the converging nature of convex lenses, which causes parallel rays to converge at the focal point on the opposite side of the lens. This phenomenon is a fundamental principle in understanding the behavior of light rays in optical systems involving convex lenses.