There are three rules of refraction of incident rays for converging/double convex lense.

• An incident ray traveling parallel to the principal axis will refract through the lens and travel through the focal point on the other side of the lens.
• An incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.
• An incident ray that passes through the optical center of the lens will continue in the same direction (without refraction).