Optical system

An optical system, sequentially comprising along an optical axis from a first side to a second side:

• a first lens with positive refractive power, having a convex first side surface and a concave second side surface;
• a second lens with positive refractive power, having a planar first side surface and a convex second side surface;
• a third lens with negative refractive power, having a concave first side surface;
• and
• a fourth lens with refractive power, having a planar second side surface;
• wherein, the optical system further comprises:
• a first quarter-wave plate, arranged on the first side surface of the second lens;
• a reflective polarizing element, arranged on the first side surface of the first quarter-wave plate;
• a partially reflective element, arranged on the second side surface of the third lens;
• a second quarter-wave plate, arranged on the second side surface of the fourth lens;
• and
• a polarizer, arranged on the second side surface of the second quarter-wave plate;
• the number of lenses with refractive power in the optical system is four;
• the effective focal length f1 of the first lens and the effective focal length f2 of the second lens satisfy:
• 0.2<f1/f2≤0.5;
• the combined focal length fz1 of the first lens, the reflective polarizing element, the first quarter-wave plate and the second lens, the radius of curvature R1 of the first side surface of the first lens, and the radius of curvature R4 of the second side surface of the second lens satisfy:
• 0.4<fz1/|R1+R4|<1.2.