Optical system, virtual image display device, and head-mounted display

An optical system comprising:

• a light guide to guide image light emitted from an image display element that displays an image;
• a partial reflector to:
• transmit a first image light guided in a first direction from a first side of the partial reflector in a light guide;
• and
• reflect second image light to exit outside the light guide from a light emission surface of the light guide, the second image light guided in the light guide in a second direction opposite to the first direction from a second side different from the first side of the partial reflector;
• and
• a reflector at the second side of the partial reflector to reflect the first image light, transmitted through the partial reflector in the first direction, back to the partial reflector as the second image light in the second direction,
• wherein a dimension of the light guide in a thickness direction is longer than a beam diameter of the image light guided in the light guide, the thickness direction being defined by a distance between the light emission surface and an opposing surface of the light guide opposed to the light emission surface, and
• the light emission surface has a negative power.