| 摘要 |
An optical glass having high-refractivity and low-dispersion optical properties and having a low glass transition point so that a heat-treating furnace can be operated for a long period of time. The optical glass has a refractive index nd of at least 1.875, an Abbe's number vd of at least 39.5 and a glass transition point Tg of 700° C. or lower, and contains at least one of La<SUB>2</SUB>O<SUB>3</SUB>, Gd<SUB>2</SUB>O<SUB>3</SUB>, Y<SUB>2</SUB>O<SUB>3 </SUB>or Yb<SUB>2</SUB>O<SUB>3 </SUB>and at least one of ZrO<SUB>2</SUB>, Ta<SUB>2</SUB>O<SUB>5 </SUB>or Nb<SUB>2</SUB>O<SUB>5</SUB>, with a weight ratio of the total content of La<SUB>2</SUB>O<SUB>3</SUB>, Gd<SUB>2</SUB>O<SUB>3</SUB>, Y<SUB>2</SUB>O<SUB>3 </SUB>and Yb<SUB>2</SUB>O<SUB>3 </SUB>to the total content of SiO<SUB>2 </SUB>and B<SUB>2</SUB>O<SUB>3 </SUB>of from 3.2 to 5 and the weight ratio of the total content of ZrO<SUB>2</SUB>, Ta<SUB>2</SUB>O<SUB>5 </SUB>and Nb<SUB>2</SUB>O<SUB>5 </SUB>to the total content of SiO<SUB>2 </SUB>and B<SUB>2</SUB>O<SUB>3 </SUB>is from 1.1 to 1.5.
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