Abstract: Provided is an objective optical system including: a first spherical lens and a second spherical lens that are arrayed in this order from an object; and at least one of a first optical medium and a second optical medium, wherein the first optical medium is a solid or liquid disposed at an object side of the first spherical lens and is in close contact with a surface on the object side of the first spherical lens, over an entire optical path; and the second optical medium is a solid or liquid disposed at an opposite side of the second spherical lens from the object and is in close contact with a surface on the opposite side of the second spherical lens from the object, over the entire optical path.

Abstract: An objective optical system including a first lens and a second lens that are arranged in this order from an object side along a main optical axis, wherein the first lens has a partially spherical shape having a first flat surface, the second lens has a partially spherical shape having a second flat surface, the first flat surface is disposed on a side of the object and is inclined with respect to the main optical axis, and the second flat surface is disposed on an opposite side which is opposite from the side of the object and is perpendicular to the main optical axis.

Abstract: An optical unit includes a first lens, a second lens, and a holding member. The first lens is formed in a spherical segment having a first flat surface and a first convex spherical surface. The second lens is formed in a spherical segment having a second flat surface and a second convex spherical surface. A holding member has a first end portion that surrounds the first lens and a second end portion that surrounds the second lens. The holding member holds the first lens and the second lens with a frictional force such that the first convex spherical surface and the second convex spherical surface are adjacent to each other between the first end portion and the second end portion. The first end portion is located on the same surface as the first flat surface or closer to the second lens than the first flat surface. The second end portion is located on the same surface as the second flat surface or closer to the first lens than the second flat surface.

Abstract: An optical system includes a plurality of lenses. The plurality of lenses have refractive index distributions in which a refractive index changes in directions orthogonal to optical axes. The optical axes of the plurality of lenses are disposed on the same straight line as each other. The plurality of lenses are manufactured on the basis of same design values relating to an on-axis refractive index and a refractive index distribution constant. When rotation positions around the optical axes of the plurality of lenses at which a total aberration amount of the plurality of lenses reaches a maximum are set as reference rotation positions for the plurality of lenses, any one of the plurality of lenses is arranged at a position acquired by relatively rotating the reference rotation position around the optical axis.

Abstract: Provided is a scanning endoscope including: an insertion portion that has a distal-end section and a proximal-end section; a light-guide optical system that guides illumination light toward the distal-end section; a spherical lens that is disposed in the distal-end section and that radiates the illumination light guided by the light-guide optical system onto a subject; an optical waveguide that extends from the distal-end section to the proximal-end section, that receives observation light coming from the subject, and that guides the observation light; and a light detector that detects the observation light guided by the optical waveguide, wherein the optical waveguide is inclined, at the distal-end section, in such a direction as to approach an optical axis of the spherical lens toward a distal end.

Abstract: Provided is an objective optical system including: a first spherical lens and a second spherical lens that are arrayed in this order from an object; and at least one of a first optical medium and a second optical medium, wherein the first optical medium is a solid or liquid disposed at an object side of the first spherical lens and is in close contact with a surface on the object side of the first spherical lens, over an entire optical path; and the second optical medium is a solid or liquid disposed at an opposite side of the second spherical lens from the object and is in close contact with a surface on the opposite side of the second spherical lens from the object, over the entire optical path.