Optical lens with protrusion unit, and method for application thereof

An optical lens includes a barrel body, a plurality of optical elements, a plurality of pairs of adjacent annular surfaces, and at least one protrusion unit. The optical elements are received in an accommodation space of the barrel body and are arranged along an axis of the barrel body. The barrel body is formed with a light admittance hole coaxial with the barrel body. The pairs of adjacent annular surfaces are formed on the barrel body and the optical elements. Each pair of adjacent annular surfaces includes a front annular surface closer to the light admittance hole and facing rearward, and a rear annular surface farther from the light admittance hole and facing the front annular surface. The protrusion unit includes a plurality of protrusions formed on one of the front and rear annular surfaces of one of the pairs of adjacent annular surfaces and abutting against the other one of the front and rear annular surfaces of said pair of adjacent annular surfaces, said other one of the front and rear annular sur