Three special drawing methods of convex lens: ① the light passing through the focus will be parallel to the main optical axis after refraction by convex lens; ② The light parallel to the main optical axis will pass through the focal point after being refracted by the convex lens; ③ The propagation direction of the light passing through the optical center will not change after being refracted by the convex lens.

The drawing of three special rays of concave lens: ① The rays of the extension line passing through another focus will be parallel to the main optical axis after being refracted by concave lens; ② The light parallel to the main optical axis will pass through the focal point after being refracted by the concave lens; ③ After refraction by concave lens, the propagation direction of light passing through the optical center will not change.