Optical system and head-mounted device

A head-mounted device, comprising:

• a display unit, configured to face a user's eyes to display an image;
• a digital signal processor, in signal communication with the display unit;
• an inertial measurement unit, in signal communication with the digital signal processor;
• a support structure, configured to be worn on the user's head;
• and
• two optical systems, located on one side of the display unit, and each of the optical systems comprising:
• an image surface, located at a front side of the optical system, wherein the image surface is located on the display unit;
• an aperture stop, located at a rear side of the optical system;
• a first quarter-wave plate, located between the image surface and the aperture stop;
• a partial reflector, having an average light reflectivity of at least 35%, wherein the partial reflector is located between the first quarter-wave plate and the aperture stop;
• a second quarter-wave plate, located between the partial reflector and the aperture stop;
• a reflective polarizer, located between the second quarter-wave plate and the aperture stop;
• a first optical lens element, located between the image surface and the aperture stop, wherein the first optical lens element has a front-side surface being convex;
• and
• a second optical lens element, located between the first optical lens element and the aperture stop, wherein the second optical lens element has a rear-side surface being concave;
• wherein a curvature radius of a front-side surface of the second optical lens element is R3, a curvature radius of the rear-side surface of the second optical lens element is R4, and the following condition is satisfied:
• |R3|/|R4|>1.2.