Optical system

An optical system, sequentially comprising along the 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 planar second side surface;
• a reflective polarizing element;
• a first quarter-wave plate;
• a second lens with positive refractive power, having a convex first side surface;
• a third lens with negative refractive power, having a concave first side surface and a planar second side surface;
• a partially reflective element;
• a second quarter-wave plate;
• a polarizer;
• and
• a fourth lens with negative refractive power;
• wherein, 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 f3 of the third lens satisfy:
• −1.5<f3/f1<−1.0;
• and
• the center thickness CT3 of the third lens on the optical axis, the center thickness CTL of the polarizer on the optical axis, the center thickness CTQ2 of the second quarter-wave plate on the optical axis, the center thickness CT1 of the first lens on the optical axis, the center thickness CTR of the reflective polarizing element on the optical axis, and the center thickness CTQ1 of the first quarter-wave plate on the optical axis satisfy:
• 2≤(CT3+CTQ2+CTL)/(CT1+CTR+CTQ1) <2.9.