An optical system includes an aperture stop, an image surface, a reflective polarizer, a partial reflector, first and second quarter-wave plates and first, second and third lens elements. The aperture stop and the image surface are respectively at a front side and a rear side of the optical system. The reflective polarizer is between the aperture stop and the image surface. The partial reflector is between the reflective polarizer and the image surface. The first quarter-wave plate is between the reflective polarizer and the partial reflector. The second quarter-wave plate is between the partial reflector and the image surface. The first lens element is between the aperture stop and the image surface. The second lens element is between the first lens element and the image surface. The third lens element has negative refractive power and is between the second lens element and the image surface.