Abstract: An optical glass composition contains, in % by mole, 0% or more and 25.0% or less of SiO2, 20.0% or more and 40.0% or less of B2O3, 0% or more and 5.0% or less of Li2O, 3.0% or more and 15.0% or less of ZnO, 0% or more and 10.0% or less of ZrO2, 2.0% or more and 7.0% or less of Ta2O5, 6.0% or more and 25.0% or less of La2O3, 5.0% or more and 22.0% or less of Gd2O3, 66.5% or less of La2O3+Gd2O3+B2O3 and 26.0% or more of La2O3+Gd2O3, and has nd of 1.83 or higher and 1.86 or lower, vd of 43 or higher and 46 or lower and a liquidus temperature of 1300° C. or lower, and a preform and an optical element are formed from the optical glass composition.

Abstract: A prism (10) is in the form of a polygonal prism and has a first optical surface (11), a second optical surface (12) and a third optical surface (13) formed on different surfaces of the prism. A light entrance region (11a) is provided at the center of the first optical surface (11), a light exit region (12a) is provided at the center of the second optical surface (12) and a substantially flat light reflecting region (13a) is provided at the center of the third optical surface (13). Suppose that there is a virtual surface between the light reflecting region (13a) and the first optical surface (11), the virtual surface is positioned more inward in the prism than a peripheral region of the third optical surface (13) extending from the light reflecting region (13a) to an edge (14) between the third optical surface (13) and the first optical surface (11).

Abstract: An optical glass composition contains, in % by mole, 14.0% or more and 31.0% or less of SiO2, 19.0% or more and 40.0% or less of B2O3, 0% or more and 5.0% or less of Li2O, 0% or more and 12.0% or less of ZnO, 0% or more and 12.0% or less of ZrO2, 15.0% or more and 26.0% or less of La2O3, 22.0% or more and 38.0% or less of La2O3+ZrO2, 2.0% or more and 5.0% or less of Ta2O5 and 4.0% or more and 16.0% or less of Gd2O3, and has nd of 1.83 or higher and 1.87 or lower and ?d of 43 or higher and 47 or lower, and a preform and an optical element are formed from the optical glass composition.

Abstract: An optical glass composition contains, in % by weight, 1.0% or more and 12.0% or less of SiO2, 8.0% or more and 18.0% or less of B2O3, 0% or more and 6.0% or less of ZnO, 1.0% or more and 10.0% or less of ZrO2, 25.0% or more and 47.0% or less of La2O3, 0% or more and 5.0% or less of R2O (here, R is at least one of Li, Na and K), 0% or more and 15.0% or less of Nb2O5, 0% or more and 7.0% or less of TiO2, 0% or more and 15.0% or less of Ta2O5, 1.0% or more of Nb2O5+TiO2+Ta2O5, 0% or more and 25.5% or less of Gd2O3, 0.5% or more and 15.0% or less of WO3 and 0.5% or more and 26.0% or less of Gd2O3+WO3, and has nd of 1.88 or higher and 1.92 or lower and ?d of 33 or higher and 37 or lower, and a preform and an optical element are formed from the optical glass composition.

Abstract: A prism 30 includes a prism body 30. The prism body 31 is formed into a polygonal column. At least one of a plurality of ridges 35, 36 of the prism body 31 is formed into an R-plane.

Abstract: An optical glass composition contains, in % by mole, 0% or more and 25.0% or less of SiO2, 20.0% or more and 40.0% or less of B2O3, 0% or more and 5.0% or less of Li2O, 3.0% or more and 15.0% or less of ZnO, 0% or more and 10.0% or less of ZrO2, 2.0% or more and 7.0% or less of Ta2O5, 6.0% or more and 25.0% or less of La2O3, 5.0% or more and 22.0% or less of Gd2O3, 66.5% or less of La2O3+Gd2O3+B2O3 and 26.0% or more of La2O3+Gd2O3, and has nd of 1.83 or higher and 1.86 or lower, vd of 43 or higher and 46 or lower and a liquidus temperature of 1300° C. or lower, and a preform and an optical element are formed from the optical glass composition.

Abstract: A prism 30 includes a prism body 31 formed into a polygonal column. At least one ridge 35 of a plurality of ridges 35 of the prism body 31 is formed into a flat face 37. A joint part 38 between the flat face 37 and at least one 32, 33 of the faces 32, 33 of the prism body 31 which are adjacent to each other with the ridge 35 in the flat face 37 interposed is formed into an R-plane.

Abstract: An optical glass composition contains, in % by mole, 14.0% or more and 31.0% or less of SiO2, 19.0% or more and 40.0% or less of B2O3, 0% or more and 5.0% or less of Li2O, 0% or more and 12.0% or less of ZnO, 0% or more and 12.0% or less of ZrO2, 15.0% or more and 26.0% or less of La2O3, 22.0% or more and 38.0% or less of La2O3+ZrO2, 2.0% or more and 5.0% or less of Ta2O5 and 4.0% or more and 16.0% or less of Gd2O3, and has nd of 1.83 or higher and 1.87 or lower and ?d of 43 or higher and 47 or lower, and a preform and an optical element are formed from the optical glass composition.

Abstract: A prism (10) is in the form of a polygonal prism and has a first optical surface (11), a second optical surface (12) and a third optical surface (13) formed on different surfaces of the prism. A light entrance region (11a) is provided at the center of the first optical surface (11), a light exit region (12a) is provided at the center of the second optical surface (12) and a substantially flat light reflecting region (13a) is provided at the center of the third optical surface (13). Suppose that there is a virtual surface between the light reflecting region (13a) and the first optical surface (11), the virtual surface is positioned more inward in the prism than a peripheral region of the third optical surface (13) extending from the light reflecting region (13a) to an edge (14) between the third optical surface (13) and the first optical surface (11).

Abstract: An optical glass composition contains, in % by weight, 1.0% or more and 12.0% or less of SiO2, 8.0% or more and 18.0% or less of B2O3, 0% or more and 6.0% or less of ZnO, 1.0% or more and 10.0% or less of ZrO2, 25.0% or more and 47.0% or less of La2O3, 0% or more and 5.0% or less of R2O (here, R is at least one of Li, Na and K), 0% or more and 15.0% or less of Nb2O5, 0% or more and 7.0% or less of TiO2, 0% or more and 15.0% or less of Ta2O5, 1.0% or more of Nb2O5+TiO2+Ta2O5, 0% or more and 25.5% or less of Gd2O3, 0.5% or more and 15.0% or less of WO3 and 0.5% or more and 26.0% or less of Gd2O3+WO3, and has nd of 1.88 or higher and 1.92 or lower and ?d of 33 or higher and 37 or lower, and a preform and an optical element are formed from the optical glass composition.

Abstract: A prism 30 includes a prism body 30. The prism body 31 is formed into a polygonal column. At least one of a plurality of ridges 35, 36 of the prism body 31 is formed into an R-plane.

Abstract: A prism 30 includes a prism body 31 formed into a polygonal column. At least one ridge 35 of a plurality of ridges 35 of the prism body 31 is formed into a flat face 37. A joint part 38 between the flat face 37 and at least one 32, 33 of the faces 32, 33 of the prism body 31 which are adjacent to each other with the ridge 35 in the flat face 37 interposed is formed into an R-plane.

Abstract: A glass material (4) is placed in a forming die including a pair of upper and lower dies (1, 2) and a control member (3) for controlling the space between the upper and the lower die, and then heated, softened, and molded by pressure into a glass substrate in the shape of a parallel plate. The amount of thermal contraction of the control member is smaller than that of the glass material. Therefore, the glass substrate is released from the die when cooled after molding. The present invention can provide a glass substrate that is excellent in smoothness and form accuracy of the surface, and that is suitable for recording media such as magnetic disks that are inexpensive and suitable for mass production.

Abstract: A glass material (4) is placed in a forming die including a pair of upper and lower dies (1, 2) and a control member (3) for controlling the space between the upper and the lower die, and then heated, softened, and molded by pressure into a glass substrate in the shape of a parallel plate. The amount of thermal contraction of the control member is smaller than that of the glass material. Therefore, the glass substrate is released from the die when cooled after molding. The present invention can provide a glass substrate that is excellent in smoothness and form accuracy of the surface, and that is suitable for recording media such as magnetic disks that are inexpensive and suitable for mass production.

Abstract: A glass material (4) is placed in a forming die including a pair of upper and lower dies (1, 2) and a control member (3) for controlling the space between the upper and the lower die, and then heated, softened, and molded by pressure into a glass substrate in the shape of a parallel plate. The amount of thermal contraction of the control member is smaller than that of the glass material. Therefore, the glass substrate is released from the die when cooled after molding. The present invention can provide a glass substrate that is excellent in smoothness and form accuracy of the surface, and that is suitable for recording media such as magnetic disks that are inexpensive and suitable for mass production.