the best example of light rays converging on a focal point is the spectacles that most of our friends wear.
take note that the lenses in the spectacle actually are convex lenses which are thicker around the middle.
For a thin double convex lens, all parallel rays will be focused to a point referred to as the principal focal point.
the key question is why on earth do the lens actually help the light rays actually converge anyway?
the idea behind lies in the curvature of the lens
notice that the closer to the edges of the lens, the more curved the surface is.
This causes the normal to the incident ray be slanted.
and we know that refraction will cause the light ray to bend towards the normal.
So the light ray will be refracted towards a point…. which is the focal point! 🙂