This invention provides an optical glass including, by mass %: 30 to 40% of B 2 O 3 ; 2.5 to 6% of Li 2 O; 10 to 17% of ZnO; 12 to 22% of La 2 O 3 ; 15 to 25% of Gd 2 O 3 ; 0 to 5% of SiO 2 ; 0 to 5% of Bi 2 O 3 ; 0 to 15% of Al 2 O 3 ; 0 to 3% of ZrO 2 ; 0 to 5% of WO 3 ; 0 to 5% of Ta 2 O 5 ; 0 to 3% of TiO 2 ; 0 to 5% of Y 2 O 3 ; and 0 to 5% of MgO+CaO+SrO+BaO.
1. An optical glass comprising, by mass %:
30 to 40% of B 2 O 3 ;
2.5 to 6% of Li 2 O;
10 to 17% of ZnO;
12 to 22% of La 2 O 3 ;
15 to 25% of Gd 2 O 3 ;
0 to 5% of SiO 2 ;
0 to 5% of Bi 2 O 3 ;
0 to 15% of Al 2 O 3 ;
0 to 3% of ZrO 2 ;
0 to 5% of WO 3 ;
0 to 5% of Ta 2 O 5 ;
0 to 3% of TiO 2 ;
0 to 5% of Y 2 O 3 ; and
0 to 5% of MgO+CaO+SrO+BaO,
wherein an average linear thermal expansion coefficient (α) from 50 to 350° C. is 85×10 −7 /° C. or less.
2. The optical glass according to claim 1 , wherein a refractive index (nd) is 1.65 to 1.72 and an Abbe number (νd) is 47 to 57.
3. The optical glass according to claim 1 , wherein a glass transition temperature (Tg) is 580° C. or less.
4. The optical glass according to claim 1 , wherein a liquid phase temperature (L.T.) is 950° C. or less.
5. The optical glass according to claim 1 , wherein a viscosity (η L.T. ) at the liquid phase temperature (L.T.) is 10 0.5 to 10 2.4 dPa·s.
6. The optical glass according to claim 1 , wherein the B 2 O 3 is present in an amount of from 33.2 to 38%.
7. The optical glass according to claim 1 , wherein the Li 2 O is present in an amount of from 2.9 to 5.4%.
8. The optical glass according to claim 1 , wherein the ZnO is present in an amount of from 10.4 to 15.5%.
9. The optical glass according to claim 1 , wherein the La 2 O 3 is present in an amount of from 14.8 to 20.3%.
10. The optical glass according to claim 1 , wherein Gd 2 O 3 is present in an amount of 16.5 to 22.6%.
11. The optical glass according to claim 1 , wherein SiO 2 is present in an amount of 0.1% or more.
12. The optical glass according to claim 1 , wherein each of BaO, MgO, CaO and SrO or the sum of the contents thereof are present in an amount of 0.1% or more.
13. The optical glass according to claim 1 , wherein each of Na 2 O and K 2 O or a sum of the contents thereof are present in an amount of 0.1% or more.
14. The optical glass according to claim 1 , wherein each of Bi 2 O 3 , ZrO 2 , WO 3 , Ta 2 O 5 , TiO 2 and Y 2 O 3 is present in an amount of 0.1% or more.
15. The optical glass according to claim 1 , wherein Al 2 O 3 is present in an amount of 0.1% or more.
16. The optical glass according to claim 1 , wherein Sb 2 O 3 is present in an amount of 0.1% or more.
17. The optical glass according to claim 1 , wherein none of PbO, As 2 O 3 or TlO 2 are present.
18. The optical glass according to claim 2 , wherein the refractive index (Nd) is from 1.68 to 1.70 and the Abbe number (νd) is 52 to 54.
19. The optical glass according to claim 3 , wherein the glass transition temperature (Tg) is 560° C. or less.
20. The optical glass according to claim 4 , wherein the liquid phase temperature (l.t.) is 870° C. or less.